long essay on skeletal system

Chapter 10. The Skeletal System

Priscilla Stewart

Unit outline

A. Bone Tissue and the Skeletal System

Part 1: the functions of the skeletal system.

• Support, Movement, and Protection
• Mineral Storage, Energy Storage, and Hematopoiesis

Part 2: Bone Classification

Part 3: bone structure, gross anatomy of bone.

• Bone Cells and Tissues

Compact and Spongy Bone

Blood and nerve supply, part 4: bone formation and development, cartilage templates, intramembranous ossification, endochondral ossification, how bones grow in length, how bones grow in diameter, bone remodeling, part 5: fractures.

B. Skeletal Anatomy

Part 1: The Axial Skeleton

The vertebral column, the thoracic cage.

Part 2: The Appendicular Skeleton

The Pectoral Girdle

Bones of the upper limb.

• The Pelvic Girdle and Pelvis
• Bones of the Lower Limb

Part 3: The Pelvic Girdle and Pelvic

Part 4: Bones of the Lower Limbs

Learning Objectives

At the end of this unit, you should be able to:

 I. Describe the functions of the skeletal system and the five basic shapes of human bones.

II. Describe the structure and histology of the skeletal system.

III. Define and identify the following parts of a long bone: diaphysis, epiphysis, metaphysis, articular cartilage, periosteum, medullary cavity, and endosteum.

IV. Compare the composition and function of compact bone versus spongy bone.

V. Define ossification, compare intramembranous ossification with endochondral ossification, describe how a long bone grows in length and width, and specify how various factors might affect the rate of ossification and, by extension, the height of a mature individual.

VI. Describe four types of bone fractures.

VII. Specify the components of the axial and appendicular skeletons; describe the general function of each skeleton.

VIII. Describe the structure and function of a typical vertebra and explain how these differ in the case of the atlas and axis.

IX. Describe the components and functions of the pectoral girdle and the pelvic girdle.

X. Specify all bones and structures in the human skeleton covered in this unit.

XI. Describe the differences between the pelvis of a human female and that of a human male.

XII. Describe the major differences between the skeleton of an infant and that of an adult.

Your skeleton is a structure of living tissue that grows, repairs, and renews itself. The bones within it are dynamic and complex organs that serve a number of important functions, including some necessary to maintain homeostasis.

The skeletal system forms the rigid internal framework of the body. It consists of the bones, cartilages, and ligaments. Bones support the weight of the body, allow for body movements, and protect internal organs. Cartilage provides flexible strength and support for body structures such as the thoracic cage, the external ear, and the trachea and larynx . At joints of the body, cartilage can also unite adjacent bones or provide cushioning between them. Ligaments are the strong connective tissue bands that hold the bones at a moveable joint together and serve to prevent excessive movements of the joint that would result in injury. Providing movement of the skeleton are the muscles of the body, which are firmly attached to the skeleton via connective tissue structures called tendons . As muscles contract, they pull on the bones to produce movements of the body. Thus, without a skeleton, you would not be able to stand, run, or even feed yourself!

Each bone of the body serves a particular function, and therefore, bones vary in size, shape, and strength based on these functions. For example, the bones of the lower back and lower limb are thick and strong to support your body weight. Similarly, the size of a bony landmark that serves as a muscle attachment site on an individual bone is related to the strength of this muscle. Muscles can apply very strong pulling forces to the bones of the skeleton. To resist these forces, bones have enlarged bony landmarks at sites where powerful muscles attach. This means that not only the size of a bone, but also its shape, is related to its function. Bones are also dynamic organs that can modify their strength and thickness in response to changes in muscle strength or body weight. Thus, muscle attachment sites on bones will thicken if you begin a workout program that increases muscle strength. Similarly, the walls of weight-bearing bones will thicken if you gain body weight or begin pounding the pavement as part of a new running regimen. In contrast, a reduction in muscle strength or body weight will cause bones to become thinner. This may happen during a prolonged hospital stay, following limb immobilization in a cast, or going into the weightlessness of outer space. Even a change in diet, such as eating only soft food due to the loss of teeth, will result in a noticeable decrease in the size and thickness of the jaw bones.

Bone, or osseous tissue, is a hard, dense connective tissue that forms most of the adult skeleton, the support structure of the body. In the areas of the skeleton where bones move (for example, the rib cage and joints), cartilage, a semi-rigid form of connective tissue, provides flexibility and smooth surfaces for movement. The skeletal system is the body system composed of bones and cartilage and performs the following critical functions for the human body:

• supports the body
• facilitates movement
• protects internal organs
• produces blood cells
• stores and releases minerals and fat

Support, Movement and Protection

The most apparent functions of the skeletal system are the gross functions—those visible by observation. Simply by looking at a person, you can see how the bones support, facilitate movement, and protect the human body.

Just as the steel beams of a building provide a scaffold to support its weight, the bones and cartilage of your skeletal system compose the scaffold that supports the rest of your body. Without the skeletal system, you would be a limp mass of organs, muscle, and skin.

Bones also facilitate movement by serving as points of attachment for your muscles. While some bones only serve as a support for the muscles, others also transmit the forces produced when your muscles contract. From a mechanical point of view, bones act as levers , and joints serve as fulcrums (Figure 10.1). Unless a muscle spans a joint and contracts, a bone is not going to move.

Bones also protect internal organs from injury by covering or surrounding them. For example, your ribs protect your lungs and heart; the bones of your vertebral column (spine) protect your spinal cord; and the bones of your cranium (skull) protect your brain (Figure10.2).

Mineral Storage, Energy Storage, and Hematopoiesis: On a metabolic level, bone tissue performs several critical functions. For one, the bone matrix ( ground substance ) acts as a reservoir for a number of minerals important to the functioning of the body, especially calcium, and phosphorus. These minerals, incorporated into bone tissue, can be released back into the bloodstream to maintain levels needed to support physiological processes. Calcium ions, for example, are essential for muscle contractions and controlling the flow of other ions involved in the transmission of nerve impulses.

Bone also serves as a site for fat storage and blood cell production. The softer connective tissue that fills the interior of most bone is referred to as bone marrow (Figure 10.3). There are two types of bone marrow: yellow marrow and red marrow. Yellow marrow contains adipose tissue; the triglycerides stored in the adipocytes of the tissue can serve as a source of energy. Red marrow is where hematopoiesis —the production of blood cells—takes place. Red blood cells, white blood cells, and cell fragments called platelets are all produced in the red marrow.

The 206 bones that compose the adult skeleton can be divided into five categories based on their shapes (Figure 10.4). Their shapes and their functions are related such that each categorical shape of bone has a distinct function.

Long Bones: A long bone is cylindrical in shape, with a diameter smaller than its height. Keep in mind, however, that the term describes the shape of a bone, not its size. Long bones are found in the arms ( humerus , ulna , and radius ) and legs ( femur , tibia , and fibula ), as well as in the fingers ( metacarpals and phalanges) and toes ( metatarsals and phalanges ). Long bones function as levers; they move when muscles contract.

Short Bones: A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. The only short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles. Short bones provide stability and support as well as some limited motion.

Flat Bones: The term “flat bone” is somewhat of a misnomer because, although a flat bone is typically thin, it is also often curved. Examples include the cranial bones of the skull, the scapulae (shoulder blades), the sternum (breastbone), and the ribs. Flat bones serve as points of attachment for muscles and often protect internal organs.

Irregular Bones: An irregular bone is one that does not have any easily characterized shape and therefore does not fit any other classification. These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones, particularly the ones containing sinuses, are classified as irregular bones.

Sesamoid Bones: A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form in tendons (the sheaths of tissue that connect bones to muscles) where a great deal of pressure is generated in a joint. The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement from person to person but are typically found in tendons associated with the feet, hands, and knees. The patellae (singular = patella) are the only sesamoid bones found in common with every person.

Table 10.1 reviews bone classifications with their associated features, functions, and examples.

Bone tissue (osseous tissue) differs greatly from other tissues in the body. Bone is hard and many of its functions depend on that characteristic hardness. Later discussions in this chapter will show that bone is also dynamic in that its shape adjusts to accommodate stresses. This section will examine the gross anatomy of bone first and then move on to its histology.

The structure of a long bone allows for the best visualization of all of the parts of a bone (Figure 10.5). A long bone has two parts: the diaphysis and the epiphysis . The diaphysis is the tubular shaft that runs between the proximal and distal ends of the bone. The hollow region in the diaphysis is called the medullary cavity , which is filled with yellow marrow . The walls of the diaphysis are composed of dense and hard compact bone . The wider section at each end of the bone is called the epiphysis (plural = epiphyses), which is filled with spongy bone . Red marrow fills the spaces in the spongy bone.

Each epiphysis meets the diaphysis at the metaphysis, the narrow area that contains the epiphyseal plate (growth plate), a layer of hyaline (transparent) cartilage in a growing bone. When the bone stops growing in early adulthood (approximately 18–21 years), the cartilage is replaced by osseous tissue and the epiphyseal plate becomes an epiphyseal line .

The medullary cavity has a delicate membranous lining called the endosteum (end- = “inside”; oste- = “bone”), where bone growth, repair, and remodeling occur. The outer surface of the bone is covered with a fibrous membrane called the periosteum (peri– = “around” or “surrounding”). The periosteum contains blood vessels, nerves, and lymphatic vessels that nourish compact bone. Tendons and ligaments also attach to bones at the periosteum. The periosteum covers the entire outer surface except where the epiphyses meet other bones to form joints (Figure 10.6). In this region, the epiphyses are covered with articular cartilage , a thin layer of cartilage that reduces friction and acts as a shock absorber.

Flat bones, like those of the cranium, consist of a layer of diploë (spongy bone), lined on either side by a layer of compact bone (Figure 10.7). The two layers of compact bone and the interior spongy bone work together to protect the internal organs. If the outer layer of a cranial bone fractures, the brain is still protected by the intact inner layer.

Bone Cells and Tissue

Bone contains a relatively small number of cells entrenched in a matrix of collagen fibers that provide a surface for inorganic salt crystals to adhere. These salt crystals, made of a substance called hydroxyapatite , form when calcium phosphate and calcium carbonate combine with other inorganic salts and solidify (i.e., calcify) on the collagen fibers. The crystals give bones their hardness and strength, while the collagen fibers give them flexibility.

Although bone cells compose a small amount of the bone volume, they are crucial to the function of bones. Four types of cells are found within bone tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts (Figure 10.8).

The osteoblast is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast become trapped within it; as a result, it changes in structure and becomes an osteocyte , the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a space called a lacuna and is surrounded by bone matrix. Osteocytes maintain the mineral concentration of the matrix. Like osteoblasts, osteocytes lack mitotic activity. They can communicate with each other and receive nutrients via long cytoplasmic processes that extend through canaliculi (singular = canaliculus), channels within the bone matrix.

If osteoblasts and osteocytes are incapable of mitosis , then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cells—the osteogenic cell . These cells are undifferentiated with high mitotic activity; and they are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. They differentiate and develop into osteoblasts.

The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the osteoclast . They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages , two types of white blood cells, rather than from osteogenic cells. Osteoclasts are continually breaking down old bone, while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone. Table 10.2 reviews the bone cells, their functions, and locations.

The differences between compact and spongy bone are best explored via their histology. Most bones contain compact and spongy osseous tissue, but their distribution and concentration vary based on the bone’s overall function. Compact bone is dense so that it can withstand compressive forces, while spongy (cancellous) bone has open spaces and supports shifts in weight distribution.

1. Compact Bone: Compact bone is the denser, stronger of the two types of bone tissue (Figure 10.9). It can be found deep to the periosteum and in the diaphyses of long bones, where it provides support and protection.

The microscopic structural unit of compact bone is called an osteon , or Haversian system. Each osteon is composed of concentric rings of calcified matrix called lamellae (singular = lamella). Running down the center of each osteon is the central canal , or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. These vessels and nerves branch off at right angles through a perforating canal , also known as Volkmann’s canals, to extend to the periosteum and endosteum .

The osteocytes are located inside spaces called lacunae (singular = lacuna), found at the borders of adjacent lamellae . As described earlier, canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This system allows nutrients to be transported to the osteocytes and wastes to be removed from them.

2. Spongy (Cancellous) Bone: Like compact bone, spongy bone , also known as cancellous bone, contains osteocytes housed in lacunae, but they are not arranged in concentric circles. Instead, the lacunae and osteocytes are found in a lattice-like network of matrix spikes called trabeculae (singular = trabecula; Figure 10.10). The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone. The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where hematopoiesis occurs.

The spongy bone and medullary cavity receive nourishment from arteries that pass through the compact bone. The arteries enter through the nutrient foramen (plural = foramina), a small opening in the diaphysis (Figure 10.11). The osteocytes in spongy bone are nourished by blood vessels of the periosteum that penetrate spongy bone and blood that circulates in the marrow cavities. As the blood passes through the marrow cavities, it is collected by veins, which then pass out of the bone through the foramen.

In addition to the blood vessels, nerves follow the same paths into the bone where they tend to concentrate in the more metabolically active regions of the bone. The nerves sense pain, and it appears they also play roles in regulating blood supplies and in bone growth, hence their concentrations in metabolically active sites of the bone.

In the early stages of embryonic development, the embryo’s skeleton consists of fibrous membranes and hyaline cartilage . By the sixth or seventh week of embryonic life, the actual process of bone development, ossification (osteogenesis), begins. There are two osteogenic pathways— intramembranous ossification and endochondral ossification —but bone is the same regardless of the pathway that produces it.

Bone is a replacement tissue; that is, it uses a model tissue on which to lay down its mineral matrix. For skeletal development, the most common template is cartilage. During fetal development, a framework is laid down that determines where bones will form. This framework is a flexible, semi-solid cartilage matrix produced by chondroblasts. As the matrix surrounds and isolates chondroblasts , they mature into cells called chondrocytes. Unlike most connective tissues, cartilage is avascular , meaning that it has no blood vessels supplying nutrients and removing metabolic wastes. All of these functions are carried on by diffusion through the matrix. This is why damaged cartilage does not repair itself as readily as most tissues do.

Throughout fetal development and into childhood growth and development, bone forms on the cartilaginous matrix. By the time a fetus is born, most of the cartilage has been replaced with bone. Some additional cartilage will be replaced throughout childhood, and some cartilage remains in the adult skeleton.

During intramembranous ossification , compact and spongy bone develops directly from sheets of mesenchymal (undifferentiated) connective tissue. The flat bones of the face, most of the cranial bones, and the clavicles (collarbones) are initially formed via intramembranous ossification.

The process begins when mesenchymal cells in the embryonic skeleton gather together and begin to differentiate into specialized cells (Figure 10.12a). Some of these cells will form capillaries, while others will become osteogenic cells and then osteoblasts. Although they will ultimately be spread out by the formation of bone tissue, early osteoblasts appear in a cluster called an ossification center .

The osteoblasts secrete osteoid , uncalcified matrix, which calcifies (hardens) within a few days as mineral salts are deposited on it, thereby entrapping the osteoblasts within. Once entrapped, the osteoblasts become osteocytes (Figure 10.12b). As osteoblasts transform into osteocytes, osteogenic cells in the surrounding connective tissue differentiate into new osteoblasts .

Osteoid (unmineralized bone matrix) secreted around the capillaries results in a trabecular matrix, while osteoblasts on the surface of the spongy bone become the periosteum (Figure 10.12c). The periosteum then creates a protective layer of compact bone superficial to the trabecular bone. The trabecular bone crowds nearby blood vessels, which eventually condense into red marrow (Figure 10.12d).

Intramembranous ossification begins in utero during fetal development and continues on into adolescence. At birth, the skeleton is not fully ossified. Most joints of the skull, for example, are more mobile in an infant than in an adult to allow the skull to deform during passage through the birth canal. The flat bones of the cranium continue to grow throughout childhood, ultimately being separated by narrow immobile joints called sutures. Each clavicle also initially (at about six weeks of embryonic age) forms by intramembranous ossification from two primary ossification centres that fuse together in utero to form a single bone with cartilage at both ends. This cartilage later ossifies to form the mature clavicles with articular cartilage on either end (usually in an individual’s early twenties). The last bones to ossify via intramembranous ossification are the flat bones of the face, which reach their adult size at the end of the adolescent growth spurt. The mandible in an infant, for example, consists of two separate bones (left and right), connected by a joint called a symphysis . This mandibular symphysis is fully ossified within the first year of life, permanently fusing the left and right bones to form the mandible.

In endochondral ossification , bone develops by replacing hyaline cartilage. Cartilage does not become bone but instead serves as a template to be completely replaced by new bone. Endochondral ossification takes much longer than intramembranous ossification . Bones at the base of the skull and long bones form via endochondral ossification.

In a long bone, for example, at about six to eight weeks after conception, some of the mesenchymal cells differentiate into chondroblasts (cells that secrete the organic components of cartilage matrix) that form the cartilaginous skeletal precursor of the bones (Figure 10.13a). Soon after, the perichondrium , a membrane that covers the cartilage, appears (Figure 10.13b).

As more matrix is produced, the chondrocytes in the center of the cartilaginous model grow in size. As the matrix calcifies, nutrients can no longer reach the chondrocytes. This results in their death and the disintegration of the surrounding cartilage. Blood vessels invade the resulting spaces, not only enlarging the cavities but also carrying osteogenic cells with them, many of which will become osteoblasts (Figure 10.13c). These enlarging spaces eventually combine to become the medullary cavity (Figure 10.13d).

As the cartilage grows, capillaries penetrate it. This penetration initiates the transformation of the perichondrium into the bone-producing periosteum. Here, the osteoblasts form a periosteal collar of compact bone around the cartilage of the diaphysis . By the second or third month of fetal life, bone cell development and ossification ramps up and creates the primary ossification center , a region deep in the periosteal collar where ossification begins (Figure 10.13c).

While these deep changes are occurring, chondrocytes and cartilage continue to grow at the ends of the bone (the future epiphyses ), which increases the bone’s length at the same time bone is replacing cartilage in the diaphyses. By the time the fetal skeleton is fully formed, cartilage only remains at the joint surface as articular cartilage and between the diaphysis and epiphysis as the epiphyseal plate , the latter of which is responsible for the longitudinal growth of bones (Figure 10.13f). After birth, this same sequence of events (matrix mineralization, death of chondrocytes, invasion of blood vessels from the periosteum , and seeding with osteogenic cells that become osteoblasts) occurs in the epiphyseal regions, and each of these centers of activity is referred to as a secondary ossification center  (Figure 10.13e).

The epiphyseal plate is the area of growth in a long bone. It is a layer of hyaline cartilage where ossification occurs in immature bones. On the epiphyseal side of the epiphyseal plate, cartilage is formed. On the diaphyseal side, cartilage is ossified, and the diaphysis grows in length. The epiphyseal plate is composed of four zones of cells and activity (Figure 10.14). The reserve zone is the region closest to the epiphyseal end of the plate and contains small chondrocytes within the matrix. These chondrocytes do not participate in bone growth but secure the epiphyseal plate to the osseous tissue of the epiphysis.

The proliferative zone is the next layer toward the diaphysis and contains stacks of slightly larger chondrocytes . It makes new chondrocytes (via mitosis) to replace those that die at the diaphyseal end of the plate. Chondrocytes in the next layer, the zone of maturation and hypertrophy , are older and larger than those in the proliferative zone. The more mature cells are situated closer to the diaphyseal end of the plate. The longitudinal growth of bone is a result of cellular division in the proliferative zone and the maturation of cells in the zone of maturation and hypertrophy.

Most of the chondrocytes in the zone of calcified matrix , the zone closest to the diaphysis, are dead because the matrix around them has calcified. Capillaries and osteoblasts from the diaphysis penetrate this zone, and the osteoblasts secrete bone tissue on the remaining calcified cartilage.

Thus, the zone of calcified matrix connects the epiphyseal plate to the diaphysis. A bone grows in length when osseous tissue is added to the diaphysis.

Bones continue to grow in length until early adulthood. The rate of growth is controlled by hormones, which will be discussed later. When the chondrocytes in the epiphyseal plate cease their proliferation and bone replaces the cartilage, longitudinal growth stops. All that remains of the epiphyseal plate is the now fully ossified epiphyseal line (Figure 10.15).

While bones are increasing in length, they are also increasing in diameter; growth in diameter can continue even after longitudinal growth ceases. This is called appositional growth. Osteoclasts resorb old bone that lines the medullary cavity, while osteoblasts, via intramembranous ossification, produce new bone tissue beneath the periosteum. The erosion of old bone along the medullary cavity and the deposition of new bone beneath the periosteum not only increase the diameter of the diaphysis but also increase the diameter of the medullary cavity. This process is called modeling .

The process in which matrix is resorbed on one surface of a bone and deposited on another is known as bone modeling. Modeling primarily takes place during a bone’s growth. However, in adult life, bone undergoes remodeling , in which resorption of old or damaged bone takes place on the same surface where osteoblasts lay new bone to replace that which is resorbed. Injury, exercise, and other activities lead to remodeling. Those influences are discussed later in the unit, but even without injury or exercise, about 5% to 10% of the skeleton is remodeled annually just by destroying old bone and renewing it with fresh bone.

A fracture is a broken bone. It will heal whether or not a physician resets it in its anatomical position. If the bone is not reset correctly, the healing process will keep the bone in its deformed position.

Types of Fractures: Fractures are classified by their complexity, location, and other features (Figure 10.16). Table 10.3 outlines common types of fractures. Some fractures may be described using more than one term because it may have the features of more than one type (e.g., an open transverse fracture). Of the types pictured in Figure 10.16 and Table 10.3, you are only required to understand the details of closed, open, comminuted, and greenstick fractures.

Test Your Knowledge

I. Describe the functions of the skeletal system and the five basic shapes of human bones.

• Specify the ways that the skeletal system functions in the human body.
• The main common functions of all components of the human skeletal system are “protection” and “support”
• Name, describe, and provide one example of each of the five different shapes of human bones.
• Where in the body, and from which type of cell, it arose.
• Where it normally resides in the body, as specifically as possible.
• What its main function is, and how (briefly) it serves that function.
• What happens to the cell if the matrix that surrounds it calcifies.
• Compare and contrast the components of cartilage matrix and bone matrix, explaining the differences in the physical characteristics of cartilage and bone.
• From what tissue type do bones and cartilage arise during early development? What other mature tissues arise from the same fetal tissue type?
• Identify a typical long bone, showing the main external and internal features and identifying all the main tissue types found in a long bone.
• The tissue type and cell type found in each type of bone.
• The arrangement of tissue and/or cells in each type of bone.
• The location of each within a bone.
• The function of each type of bone.
• Intramembranous ossification
• Endochondral ossification
• Growth in length of a long bone
• Growth in width of a long bone
• Compare and contrast the processes of intramembranous ossification and endochondral ossification.
• Compare and contrast the processes of endochondral ossification and lengthwise growth of a long bone.
• Compare and contrast the processes of intramembranous ossification and widthwise growth of a long bone.
• The height of an individual is largely determined by the rate of ossification prior to physical maturity. Briefly explain why this is so.
• Hypersecretion of growth hormone during development
• Hyposecretion of growth hormone during development
• Premature onset of puberty
• Late onset of puberty
• Shorter than average parents
• Taller than average parents
• Excessive caloric intake during development
• Malnutrition during development
• Closed (simple) vs. open (compound) fractures
• Comminuted vs. closed fractures
• Greenstick vs. closed fractures
• Comminuted vs. greenstick fractures
• A closed open fracture
• A closed comminuted fracture
• A closed greenstick fracture
• An open comminuted fracture
•    Specify the ways that the skeletal system functions in the human body.
•    The main common functions of all components of the human skeletal system are “protection” and “support.” Based on the material that follows in this unit, select several examples of individual components (including both bone and cartilage examples) and describe how they serve each of these two functions.
•    Name, describe, and provide one example of each of the five different shapes of human bones

B. Divisions of the Skeletal System

The skeletal system includes all of the bones, cartilages, and ligaments of the body that support and give shape to the body and body structures. The skeleton consists of the bones of the body. For adults, there are 206 bones in the skeleton. Younger individuals have higher numbers of bones because some bones fuse together during childhood and adolescence to form an adult bone. The skeleton is subdivided into two major divisions—the axial skeleton and the appendicular skeleton.

The Axial Skeleton: The skeleton is subdivided into two major divisions—the axial skeleton and appendicular skeleton. The axial skeleton forms the vertical central axis of the body and includes all bones of the head, neck, chest, and back (Figure 10.17). It serves to protect the brain, spinal cord, heart, and lungs. It also serves as the attachment site for muscles that move the head, neck, and back, and for muscles that act across the shoulder and hip joints to move their corresponding limbs.

The axial skeleton of the adult consists of 80 bones, including the skull , the vertebral column , and the thoracic cage . The skull is formed by 22 bones. Also associated with the head are an additional seven bones, including the hyoid bone and the ear ossicles  (three small bones found in each middle ear). The vertebral column consists of 24 bones, each called a vertebra , plus the sacrum and coccyx . The thoracic cage includes the 12 pairs of ribs , and the sternum , the flattened bone of the anterior chest.

The Appendicular Skeleton: The appendicular skeleton includes all bones of the upper and lower limbs, plus the bones that attach each limb to the axial skeleton (Figure 10.17). There are 126 bones in the appendicular skeleton of an adult. The bones of the appendicular skeleton are covered later in the unit.

The cranium (skull) is the skeletal structure of the head that supports the face and protects the brain. It is subdivided into the facial bones and the brain case , or cranial vault (Figure 10.19). The facial bones underlie the facial structures, form the nasal cavity, enclose the eyeballs, and support the teeth of the upper and lower jaws. The rounded brain case surrounds and protects the brain and houses the middle and inner ear structures.

In the adult, the skull consists of 22 individual bones, 21 of which are immobile and united into a single unit. The 22nd bone is the mandible (lower jaw), which is the only moveable bone of the skull.

Development of the Skull: As the brain case bones grow in the fetal skull, they remain separated from each other by large areas of dense connective tissue, each of which is called a fontanelle (Figure 10.18). The fontanelles are the soft spots on an infant’s head. They are important during birth because these areas allow the skull to change shape as it squeezes through the birth canal. After birth, the fontanelles allow for continued growth and expansion of the skull as the brain enlarges. The largest fontanelle is located on the anterior head, at the junction of the frontal and parietal bones. The fontanelles decrease in size and disappear by age 2. However, the skull bones remained separated from each other at the sutures , which contain dense fibrous connective tissue that unites the adjacent bones. The connective tissue of the sutures allows for continued growth of the skull bones as the brain enlarges during childhood growth. This structure also means that, although the size of the cranium increases from birth to adulthood, proportionately, it does so less than other parts of the skeleton. The relative size of the cranium in proportion to the rest of the body therefore decreases with age from birth to adulthood.

Bones of the Brain Case: The brain case contains and protects the brain (Figure 10.19). The interior space that is almost completely occupied by the brain is called the cranial cavity .

The brain case consists of eight bones (Figures 10.20 & 10.21). These include the paired parietal and temporal bones, plus the unpaired frontal , occipital , sphenoid , and ethmoid bones. For our purposes, we will not be specifying the details of the sphenoid and ethmoid bones.

1. Parietal Bone: The parietal bone forms most of the upper lateral side of the skull (Figures 10.21 & 10.22). These are paired bones, with the right and left parietal bones joining together at the top of the skull. Each parietal bone is also bounded anteriorly by the frontal bone, inferiorly by the temporal bone, and posteriorly by the occipital bone.

2. Temporal Bone: The temporal bone forms the lower lateral side of the skull (Figure 10.21). Common wisdom has it that the temporal bone (temporal = “time”) is so named because this area of the head (the temple) is where hair typically first turns gray, indicating the passage of time.

3. Frontal Bone: The frontal bone is the single bone that forms the forehead (Figure 10.20).

4. Occipital Bone: The occipital bone is the single bone that forms the posterior skull and posterior base of the cranial cavity (Figures 10.21 & 10.22). On its outside surface, at the posterior midline, is a small protrusion called the external occipital protuberance , which serves as an attachment site for a ligament of the posterior neck.

Facial Bones of the Skull: The facial bones of the skull form the upper and lower jaws, the nose, nasal cavity and nasal septum, and the orbit. The facial bones include 14 bones, with six paired bones and two unpaired bones (Figures 10.20 & 10.21). We will focus on the maxillary bones and the mandible bone.

1. Maxillary Bone: The maxillary bone , often referred to simply as the maxilla (plural = maxillae), is one of a pair that together form the upper jaw, much of the hard palate, the medial floor of the orbit, and the lateral base of the nose (Figures 10.20 & 10.21).

2. Mandible: The mandible forms the lower jaw and is the only moveable bone of the skull. At the time of birth, the mandible consists of paired right and left bones, but these fuse together during the first year to form the single U-shaped mandible of the adult skull (Figures 10.20 & 10.21).

The Bones of the Middle Ear: Three small bones ( ossicles ) are found on either side of the head in the middle ear. These are the malleus, incus, and stapes, and they function in transferring the vibrations from the eardrum (tympanic membrane) to the inner ear.

The Hyoid Bone: The hyoid bone is an independent bone that does not contact any other bone and thus is not part of the skull (Figure 10.23). It is a small U-shaped bone located in the upper neck near the level of the inferior mandible, with the tips of the “U” pointing posteriorly. The hyoid serves as the base for the tongue above and is attached to the larynx below and the pharynx posteriorly. The hyoid is held in position by a series of small muscles that attach to it either from above or below. These muscles act to move the hyoid up/down or forward/back. Movements of the hyoid are coordinated with movements of the tongue, larynx, and pharynx during swallowing and speaking.

The vertebral column is also known as the spinal column or spine (Figure 10.24). It consists of a sequence of vertebrae (singular = vertebra), each of which is separated and united by an intervertebral disc . Together, the vertebrae and intervertebral discs form the vertebral column. It is a flexible column that supports the head, neck, and body and allows for their movements. It also protects the spinal cord, which passes down the back through openings in the vertebra.

Regions of the Vertebral Column : The vertebral column originally develops as a series of 33 vertebrae, but this number is eventually reduced to 24 vertebrae, plus the sacrum and coccyx. The vertebral column is subdivided into five regions, with the vertebrae in each area named for that region and numbered in descending order. In the neck, there are seven cervical vertebrae, each designated with the letter “C” followed by its number. Superiorly, the C1 vertebra articulates (forms a joint) with the occipital condyles of the skull. Inferiorly, C1 articulates with the C2 vertebra, and so on. Below these are the 12 thoracic vertebrae, designated T1–T12. The lower back contains the L1–L5 lumbar vertebrae. The single sacrum, which is also part of the pelvis, is formed by the fusion of five sacral vertebrae. Similarly, the coccyx, or tailbone, results from the fusion of four small coccygeal vertebrae. However, the sacral and coccygeal fusions do not start until age 20 and are not completed until middle age.

Curvatures of the Vertebral Column: The adult vertebral column does not form a straight line but instead has four curvatures along its length (see Figure 10.24). These curves increase the vertebral column’s strength, flexibility, and ability to absorb shock.

During fetal development, the body is flexed anteriorly into the fetal position, giving the entire vertebral column a single curvature that is concave anteriorly. In the adult, this fetal curvature is retained in two regions of the vertebral column as the thoracic curve , which involves the thoracic vertebrae, and the sacrococcygeal curve , formed by the sacrum and coccyx.

General Structure of a Vertebra: Within the different regions of the vertebral column, vertebrae vary in size and shape, but they all follow a similar structural pattern. A typical vertebra will consist of a body, a vertebral arch, and seven processes (Figure 10.25).

The body is the anterior portion of each vertebra and is the part that supports the body weight. Because of this, the vertebral bodies progressively increase in size and thickness going down the vertebral column. The bodies of adjacent vertebrae are separated and strongly united by an intervertebral disc.

The vertebral arch forms the posterior portion of each vertebra.

The large opening between the vertebral arch and body is the vertebral foramen , which contains the spinal cord. In the intact vertebral column, the vertebral foramina of all of the vertebrae align to form the vertebral (spinal) canal , which serves as the bony protection and passageway for the spinal cord down the back. When the vertebrae are aligned together in the vertebral column, notches in the margins of the pedicles of adjacent vertebrae together form an intervertebral foramen , the opening through which a spinal nerve exits from the vertebral column (Figure 10.26).

Seven processes arise from the vertebral arch. Each paired transverse process projects laterally and arises from the junction point between the pedicle and lamina. The single spinous process (vertebral spine) projects posteriorly at the midline of the back. The vertebral spines can easily be felt as a series of bumps just under the skin down the middle of the back. The transverse and spinous processes serve as important muscle attachment sites. A superior articular process extends or faces upward, and an inferior articular process faces or projects downward on each side of a vertebrae. The paired superior articular processes of one vertebra join with the corresponding paired inferior articular processes from the next higher vertebra. These junctions form slightly moveable joints between the adjacent vertebrae. The shape and orientation of the articular processes vary in different regions of the vertebral column and play a major role in determining the type and range of motion available in each region.

Regional Modifications of Vertebrae: In addition to the general characteristics of a typical vertebra described above, vertebrae also display characteristic size and structural features that vary between the different vertebral column regions. Thus, cervical vertebrae are smaller than lumbar vertebrae due to differences in the proportion of body weight that each will support. Thoracic vertebrae have sites for rib attachment, and the vertebrae that give rise to the sacrum and coccyx have fused together into single bones. We will focus on the anatomically distinct natures of the first two cervical vertebrae, the atlas and the axis.

Cervical Vertebrae: Typical cervical vertebrae , such as C4 or C5, have several characteristic features that differentiate them from thoracic or lumbar vertebrae (Figure 10.27). Cervical vertebrae have a small body, reflecting the fact that they carry the least amount of body weight. Cervical vertebrae usually have a bifid (Y-shaped) spinous process . The transverse processes of the cervical vertebrae are sharply curved (U-shaped) to allow for passage of the cervical spinal nerves. Each transverse process also has an opening called the transverse foramen .

The first and second cervical vertebrae are further modified, giving each a distinctive appearance. The first cervical (C1) vertebra is also called the atlas , because this is the vertebra that supports the skull on top of the vertebral column (in Greek mythology, Atlas was the god who supported the heavens on his shoulders). The C1 vertebra does not have a body or spinous process. Instead, it is ring-shaped, consisting of an anterior arch and a posterior arch . The transverse processes of the atlas are longer and extend more laterally than do the transverse processes of any other cervical vertebrae. The superior articular processes face upward and are deeply curved for articulation with the occipital condyles on the base of the skull. The inferior articular processes are flat and face downward to join with the superior articular processes of the C2 vertebra.

The second cervical (C2) vertebra is called the axis , because it serves as the axis for rotation when turning the head right or left. The axis resembles typical cervical vertebrae in most respects but is easily distinguished by the dens (odontoid process), a bony projection that extends upward from the vertebral body. The dens joins with the inner aspect of the anterior arch of the atlas, where it is held in place by transverse ligament.

The thoracic cage (rib cage) forms the thorax (chest) portion of the body. It consists of the 12 pairs of ribs with their costal cartilages and the sternum (Figure 10.28). The ribs are anchored posteriorly to the 12 thoracic vertebrae (T1–T12). The thoracic cage protects the heart and lungs.

Sternum: The sternum is the elongated bony structure that anchors the anterior thoracic cage. It consists of three parts: the manubrium , body, and xiphoid process .

Ribs: Each rib is a curved, flattened bone that contributes to the wall of the thorax. The ribs articulate posteriorly with the T1–T12 thoracic vertebrae, and most attach anteriorly via their costal cartilages to the sternum . There are 12 pairs of ribs. The ribs are numbered 1–12 in accordance with the thoracic vertebrae.

The bony ribs do not extend anteriorly completely around to the sternum. Instead, each rib ends in a costal cartilage. These cartilages are made of hyaline cartilage and can extend for several inches. Most ribs are then attached, either directly or indirectly, to the sternum via their costal cartilage (Figure 10.28). The ribs are classified into three groups based on their relationship to the sternum.

Ribs 1–7 are classified as true ribs (vertebrosternal ribs). The costal cartilage from each of these ribs attaches directly to the sternum. Ribs 8–12 are called false ribs (vertebrochondral ribs). The costal cartilages from these ribs do not attach directly to the sternum. For ribs 8–10, the costal cartilages are attached to the cartilage of the next higher rib. Thus, the cartilage of rib 10 attaches to the cartilage of rib 9, rib 9 then attaches to rib 8, and rib 8 is attached to rib 7. The last two false ribs (11–12) are also called floating ribs (vertebral ribs). These are short ribs that do not attach to the sternum at all. Instead, their small costal cartilages terminate within the musculature of the lateral abdominal wall.

Part 2. The Appendicular Skeleton

Attached to the axial skeleton are the limbs, whose 126 bones constitute the appendicular skeleton (Figure 10.29) These bones are divided into two groups: the bones that are located within the limbs themselves and the girdle bones that attach the limbs to the axial skeleton. The bones of the shoulder region form the pectoral girdle , which anchors the upper limb to the thoracic cage of the axial skeleton. The lower limb is attached to the vertebral column by the pelvic girdle .

Because of our upright stance, different functional demands are placed upon the upper and lower limbs. Thus, the bones of the lower limbs are adapted for weight-bearing support and stability, as well as for body locomotion via walking or running. In contrast, our upper limbs are not required for these functions. Instead, our upper limbs are highly mobile and can be utilized for a wide variety of activities. The large range of upper limb movements, coupled with the ability to easily manipulate objects with our hands and opposable thumbs, has allowed humans to construct the modern world in which we live.

The bones that attach each upper limb to the axial skeleton form the pectoral girdle (shoulder girdle). This consists of two bones, the scapula and clavicle (Figure 10.30).

The clavicle (collarbone) is an S-shaped bone located on the anterior side of the shoulder. It is attached on its medial end to the sternum of the thoracic cage, which is part of the axial skeleton. The lateral end of the clavicle articulates (joins) with the scapula just above the shoulder joint. You can easily palpate, or feel with your fingers, the entire length of your clavicle.

The scapula (shoulder blade) lies on the posterior aspect of the shoulder. It is supported by the clavicle, which also articulates with the humerus (upper arm bone) to form the shoulder joint. The scapula is a flat, triangular-shaped bone with a prominent ridge running across its posterior surface. This ridge extends out laterally, where it forms the bony tip of the shoulder and joins with the lateral end of the clavicle. By following along the clavicle, you can palpate out to the bony tip of the shoulder, and from there, you can move back across your posterior shoulder to follow the ridge of the scapula. Move your shoulder around and feel how the clavicle and scapula move together as a unit. Both of these bones serve as important attachment sites for muscles that aid with movements of the shoulder and arm (Figures 10.30 & 10.31).

The right and left pectoral girdles are not joined to each other, allowing each to operate independently. In addition, the clavicle of each pectoral girdle is anchored to the axial skeleton by a single, highly mobile joint. This allows for the extensive mobility of the entire pectoral girdle, which in turn enhances movements of the shoulder and upper limb.

The upper limb is divided into three regions. These consist of the arm , located between the shoulder and elbow joints; the forearm , which is between the elbow and wrist joints; and the hand , which is located distal to the wrist. There are 30 bones in each upper limb. The humerus is the single bone of the upper arm, and the ulna (medially) and the radius (laterally) are the paired bones of the forearm. The base of the hand contains eight bones, each called a carpal bone , and the palm of the hand is formed by five bones, each called a metacarpal bone. The fingers and thumb contain a total of 14 bones, each of which is a phalanx bone of the hand  (Figure 10.29).

Humerus: The humerus is the single bone of the upper arm region (Figure 10.32). At its proximal end is the head of the humerus. This is the large, round, smooth region that faces medially. The head articulates with the glenoid cavity of the scapula to form the glenohumeral (shoulder) joint. Distally, the humerus becomes flattened and has two articulation areas, which join the ulna and radius bones of the forearm to form the elbow joint.

Ulna: The ulna is the medial bone of the forearm. It runs parallel to the radius, which is the lateral bone of the forearm (Figure 10.33). The proximal end of the ulna articulates with the humerus as part of the elbow joint.

Radius: The radius runs parallel to the ulna, on the lateral (thumb) side of the forearm (Figure 10.33). The head of the radius is a disc-shaped structure that forms the proximal end. The distal end of the radius has a smooth surface for articulation with two carpal bones to form the radiocarpal joint or wrist joint (Figures 10.34 & 10.35).

Carpal Bones: The wrist and base of the hand are formed by a series of eight small carpal bones (Figure 10.34). The carpal bones are arranged in two rows, forming a proximal row of four carpal bones and a distal row of four carpal bones.

The carpal bones form the base of the hand. This can be seen in the radiograph (X-ray image) of the hand that shows the relationships of the hand bones to the skin creases of the hand (Figure 10.35).

Metacarpal Bones: The palm of the hand contains five elongated metacarpal bones. These bones lie between the carpal bones of the wrist and the bones of the fingers and thumb (Figure 10.34). The proximal end of each metacarpal bone articulates with one of the distal carpal bones. Each of these articulations is a carpometacarpal joint (Figure 10.35). The expanded distal end of each metacarpal bone articulates at the metacarpophalangeal joint with the proximal phalanx bone of the thumb or one of the fingers. The distal end also forms the knuckles of the hand, at the base of the fingers. The metacarpal bones are numbered 1–5, beginning at the thumb.

Phalanx Bones: The fingers and thumb contain 14 bones, each of which is called a phalanx bone (plural = phalanges), named after the ancient Greek phalanx (a rectangular block of soldiers). The thumb ( pollex ) is digit number 1 and has two phalanges, a proximal phalanx, and a distal phalanx bone (Figure 10.34). Digits 2 (index finger) through 5 (little finger) have three phalanges each, called the proximal, middle, and distal phalanx bones. An interphalangeal joint is one of the articulations between adjacent phalanges of the digits (Figure 10.35).

Part 3: The Pelvic Girdle and Pelvis

The pelvic girdle (hip girdle) is formed by a single bone, the hip bone or coxal bone (coxal = “hip”), which serves as the attachment point for each lower limb. Each hip bone, in turn, is firmly joined to the axial skeleton via its attachment to the sacrum of the vertebral column. The right and left hip bones also converge anteriorly to attach to each other. The bony pelvis is the entire structure formed by the two hip bones, the sacrum and the coccyx , which is attached inferiorly to the sacrum (Figure 10.36).

Unlike the bones of the pectoral girdle, which are highly mobile to enhance the range of upper limb movements, the bones of the pelvis are strongly united to each other to form a largely immobile, weight-bearing structure. This is important for stability because it enables the weight of the body to be easily transferred laterally from the vertebral column, through the pelvic girdle and hip joints, and into either lower limb whenever the other limb is not bearing weight. Thus, the immobility of the pelvis provides a strong foundation for the upper body as it rests on top of the mobile lower limbs.

Hip Bone: The hip bone, or coxal bone, forms the pelvic girdle portion of the pelvis. The paired hip bones are the large, curved bones that form the lateral and anterior aspects of the pelvis. Each adult hip bone is formed by three separate bones that fuse together during the late teenage years. These bony components are the ilium , ischium , and pubis (Figure 10.37). These names are retained and used to define the three regions of the adult hip bone.

The ilium is the fan-like, superior region that forms the largest part of the hip bone. It is firmly united to the sacrum at the largely immobile sacroiliac joint (Figure 10.36). The ischium forms the posteroinferior region of each hip bone. It supports the body when sitting. The pubis forms the anterior portion of the hip bone. The pubis curves medially, where it joins to the pubis of the opposite hip bone at a specialized joint called the pubic symphysis .

Pelvis: The pelvis consists of four bones: the right and left hip bones, the sacrum, and the coccyx (Figure 10.36). The pelvis has several important functions. Its primary role is to support the weight of the upper body when sitting and to transfer this weight to the lower limbs when standing. It serves as an attachment point for trunk and lower limb muscles and also protects the internal pelvic organs.

Comparison of the Female and Male Pelvis: The differences between the adult female and male pelvis relate to function and body size. In general, the bones of the male pelvis are thicker and heavier, adapted for support of the male’s heavier physical build and stronger muscles; this average size difference is generally true of other bones of the skeleton as well. The pelvis does show more robust differences between males and females due to its functional relationship to bipedal movement (requiring a relatively narrow pelvis) and birth of infants with large brains (requiring a relatively broad pelvis). Because the female pelvis is adapted for childbirth, it is wider than the male pelvis, as evidenced by the distance between the anterior superior iliac spines (Figure 10.38). The ischial tuberosities of females are also farther apart, which increases the size of the pelvic outlet. Because of this increased pelvic width, the subpubic angle is larger in females (greater than 80 degrees) than it is in males (less than 70 degrees). The female sacrum is wider, shorter, and less curved, and the sacral promontory projects less into the pelvic cavity, thus giving the female pelvic inlet (pelvic brim) a more rounded or oval shape compared to males. The pelvic cavity of females is also wider and shallower than the narrower, deeper, and tapering lesser pelvis of males. The greater sciatic notch of the male hip bone is narrower and deeper than the broader notch of females. Because of the obvious differences between female and male hip bones, this is the one bone of the body that allows for the most accurate sex determination. Table 10.4 provides an overview of the general differences between the female and male pelvis.

Part 4: Bones of the Lower Limb

Like the upper limb, the lower limb is divided into three regions. The thigh is the portion of the lower limb located between the hip joint and knee joint. The leg is specifically the region between the knee joint and the ankle joint. Distal to the ankle is the foot . The lower limb contains 30 bones. These are the femur , patella , tibia , fibula , tarsal bones, metatarsal bones, and phalanges (Figure 10.29). The femur is the single bone of the thigh. The patella is the kneecap and articulates with the distal femur. The tibia is the larger, weight-bearing bone located on the medial side of the leg, and the fibula is the thin bone of the lateral leg. The bones of the foot are divided into three groups. The posterior portion of the foot is formed by a group of seven bones, each of which is known as a tarsal bone , whereas the mid-foot contains five elongated bones, each of which is a metatarsal bone . The toes contain 14 small bones, each of which is a phalanx bone of the foot .

Femur: The femur, or thigh bone, is the single bone of the thigh region (Figure 10.39). It is the longest and strongest bone of the body and accounts for approximately one-quarter of a person’s total height. The rounded, proximal end is the head of the femur, which articulates with the acetabulum of the hip bone to form the hip joint .

Patella: The patella (kneecap) is the largest sesamoid bone of the body (see Figure 10.39). A sesamoid bone is a bone that is incorporated into the tendon of a muscle where that tendon crosses a joint. The sesamoid bone articulates with the underlying bones to prevent damage to the muscle tendon due to rubbing against the bones during movements of the joint. The patella is found in the tendon of the quadriceps femoris muscle, the large muscle of the anterior thigh that passes across the anterior knee to attach to the tibia. The patella articulates with the patellar surface of the femur and thus prevents rubbing of the muscle tendon against the distal femur. The patella also lifts the tendon away from the knee joint, which increases the leverage power of the quadriceps femoris muscle as it acts across the knee. The patella does not articulate with the tibia.

Tibia: The tibia (shin bone) is the medial bone of the leg and is larger than the fibula, with which it is paired (Figure 10.40). The tibia is the main weight-bearing bone of the lower leg and the second longest bone of the body, after the femur . The medial side of the tibia is located immediately under the skin, allowing it to be easily palpated down the entire length of the medial leg.

Fibula: The fibula is the slender bone located on the lateral side of the leg (Figure 10.40). The fibula does not bear weight. It serves primarily for muscle attachments and thus is largely surrounded by muscles. Only the proximal and distal ends of the fibula can be palpated.

Tarsal Bones: The posterior half of the foot is formed by seven tarsal bones (Figure 10.43). The most superior tarsal bone, the talus , articulates with the tibia and fibula to form the ankle joint . Inferiorly, the talus articulates with the calcaneus (heel bone), the largest bone of the foot, which forms the heel. Body weight is transferred from the tibia to the talus to the calcaneus, which rests on the ground.

Metatarsal Bones: The anterior half of the foot is formed by the five metatarsal bones, which are located between the tarsal bones of the posterior foot and the phalanges of the toes (Figure 10.41). These elongated bones are numbered 1–5, starting with the medial side of the foot.

Phalanx bones: The toes contain a total of 14 phalanx bones (phalanges), arranged in a similar manner as the phalanges of the fingers (Figure 10.41). The toes are numbered 1–5, starting with the big toe ( hallux ). The big toe has two phalanx bones, the proximal and distal phalanges. The remaining toes all have proximal, middle, and distal phalanges. A joint between adjacent phalanx bones is called an interphalangeal joint.

Test Your Knowledge: Guiding Questions

    I. Divisions of the Skeletal System

• What bones are apart of the axial skeleton?
• What bones are apart of the appendicular skeleton ?

  II. Describe the structure and histology of the skeletal system.

• Based on previously covered material and the information in this unit, compare and contrast the components of cartilage matrix and bone matrix, explaining the differences in the physical characteristics of cartilage and bone.

  III. Define and identify the following parts of a long bone: diaphysis, epiphysis, metaphysis, articular cartilage, periosteum, medullary cavity, and endosteum.

• Create a fully-labeled diagram of a typical long bone, showing the main external and internal features and identifying all the main tissue types found in a long bone.

  IV. Compare the composition and function of compact bone versus spongy bone.

• Use annotated diagrams to compare and contrast the internal structure of an osteon with that of a trabecula.

  V. Define ossification, compare intramembranous ossification with endochondral ossification, describe how a long bone grows in length and width, and specify how various factors might affect the rate of ossification and, by extension, the height of a mature individual.

  VI. Describe four types of bone fractures.

• Comminuted vs closed fractures
• Greenstick vs closed fractures
• Comminuted vs greenstick fractures

For the activity below, drag the structure names to the correct empty boxes on the figure.

Image Descriptions

Tube composed of cartilaginous rings and supporting tissue that connects the lung bronchi and the larynx; provides a route for air to enter and exit the lung.

cartilaginous structure that produces the voice, prevents food and beverages from entering the trachea, and regulates the volume of air that enters and leaves the lungs

Strong connective tissue bands that hold the bones at a moveable joint together.

Dense regular connective tissue that attaches skeletal muscle to bone.

Simple machine consisting of a beam or rigid rod (bone) pivoted at a fixed hinge, or fulcrum (joint).

Site at which two or more bones or bone and cartilage come together (articulate).

Fluid or semi-fluid portion of the matrix.

Connective tissue in the interior cavity of a bone where fat is stored.

Specialized areolar tissue rich in stored fat.

Connective tissue in the interior cavity of a bone where hematopoiesis takes place.

(Also, hematopoiesis) production of the formed elements of blood.

(Also, thrombocytes) one of the formed elements of blood that consists of cell fragments broken off from megakaryocytes.

Single bone of the upper arm.

Bone located on the medial side of the forearm.

Bone located on the lateral side of the forearm.

Thigh bone; the single bone of the thigh.

Shin bone; the large, weight-bearing bone located on the medial side of the leg.

Thin, non-weight-bearing bone found on the lateral side of the leg.

One of the five long bones that form the palm of the hand; numbered 1–5, starting on the lateral (thumb) side of the hand.

One of the five elongated bones that forms the anterior half of the foot; numbered 1–5, starting on the medial side of the foot.

(plural = phalanges) one of the bones that form the fingers or toes.

One of the eight small bones that form the wrist and base of the hand; these are grouped as a proximal row consisting of (from lateral to medial) the scaphoid, lunate, triquetrum, and pisiform bones, and a distal row containing (from lateral to medial) the trapezium, trapezoid, capitate, and hamate bones.

One of the seven bones that make up the posterior foot; includes the calcaneus, talus, navicular, cuboid, medial cuneiform, intermediate cuneiform, and lateral cuneiform bones.

Shoulder blade bone located on the posterior side of the shoulder.

Flattened bone located at the center of the anterior chest.

Tubular shaft that runs between the proximal and distal ends of a long bone.

Wide section at each end of a long bone; filled with spongy bone and red marrow.

Hollow region of the diaphysis; filled with yellow marrow.

Dense osseous tissue that can withstand compressive forces.

(Also, cancellous bone) trabeculated osseous tissue that supports shifts in weight distribution.

(Also, growth plate) sheet of hyaline cartilage in the metaphysis of an immature bone; replaced by bone tissue as the organ grows in length.

Most common type of cartilage, smooth and made of short collagen fibers embedded in a chondroitin sulfate ground substance.

completely ossified remnant of the epiphyseal plate

Delicate membranous lining of a bone’s medullary cavity.

Fibrous membrane covering the outer surface of bone and continuous with ligaments.

Thin layer of cartilage covering an epiphysis; reduces friction and acts as a shock absorber.

Layer of spongy bone, that is sandwiched between two the layers of compact bone found in flat bones.

The most abundant of three protein fibres found in the extracellular matrix of connective tissues.

A form of calcium phosphate mineral found in bones (also hydroxylapatite)

Cell responsible for forming new bone.

Primary cell in mature bone; responsible for maintaining the matrix.

(Plural= lacunae) small spaces in bone or cartilage tissue that cells occupy.

(Singular = canaliculus) channels within the bone matrix that house one of an osteocyte’s many cytoplasmic extensions that it uses to communicate and receive nutrients.

Division of genetic material, during which the cell nucleus breaks down and two new, fully functional, nuclei are formed. Usually immediately followed by cytokinesis (cell division).

Undifferentiated cell with high mitotic activity; the only bone cells that divide; they differentiate and develop into osteoblasts.

Cell responsible for resorbing bone.

Precursor to macrophages and dendritic cells seen in the blood.

Ameboid (irregular outline with peripheral projections) phagocyte found in several tissues throughout the body.

(Also, Haversian system) basic structural unit of compact bone; made of concentric layers of calcified matrix.

Concentric rings of calcified matrix that form an osteon.

(Also Haversian canal) longitudinal channel in the center of each osteon; contains blood vessels, nerves, and lymphatic vessels.

(Also, Volkmann’s canal) channel that branches off from the central canal and houses vessels and nerves that extend to the periosteum and endosteum.

(Singular= trabecula) spikes or sections of the lattice-like matrix in spongy bone.

Small opening in the middle of the external surface of the diaphysis, through which an artery enters the bone to provide nourishment.

(Also, osteogenesis) bone formation.

Process by which bone forms directly from mesenchymal tissue.

Process in which bone forms by replacing hyaline cartilage.

Cell responsible for forming new cartilage.

Lacking blood vessels.

Embryonic tissue from which connective tissue cells derive.

Collarbone; elongated bone that articulates with the manubrium of the sternum medially and the acromion of the scapula laterally.

Cluster of osteoblasts found in the early stages of intramembranous ossification.

Uncalcified bone matrix secreted by osteoblasts.

Region, deep in the periosteal collar, where bone development starts during endochondral ossification.

Unit Outline

Part 1: The Anatomical and Functional Organization of the Nervous System

Anatomical Divisions

Functional divisions, part 2: nervous tissue, glial cells.

Part 3: The Central Nervous System

The Cerebrum

The Diencephalon

• The Brainstem

The Cerebellum

The spinal cord, the meninges, the ventricular system and cerebrospinal fluid circulation, part 4: the peripheral nervous system, the somatic nervous system, the autonomic nervous system, part 5: neuronal signaling, ion channels and the resting membrane potential, generation of an action potential, propagation of action potentials, neurotransmission.

I. Describe the organization of the nervous system and explain the functions of its principal components.

II. Describe the structure of the following: neuron, glia, ganglion, nerve, gray matter, tract, white matter, sensory neuron, motor neuron.

III. Name, locate, and describe the functions of the main areas of the human brain.

IV. Describe the structure and functions of the spinal cord.

V. Describe the components of a reflex arc and explain how a reflex arc works.

V I. Describe the function of the autonomic nervous system (ANS) and compare the specific functions of the parasympathetic and sympathetic divisions of the ANS.

VII. Describe the resting membrane potential of a neuron and explain how it is maintained.

VIII . Explain how a neuronal action potential is generated.

IX. Explain how neuronal action potentials propagate down the axon.

X. Explain the process of neurotransmission and name three different neurotransmitters.

Part 1: Anatomical and Functional Organization of the Nervous System

The picture you have in your mind of the nervous system probably includes the brain , the nervous tissue contained within the cranium, and the spinal cord , the extension of nervous tissue within the vertebral column. That suggests it is made of two organs—and you may not even think of the spinal cord as an organ—but the nervous system is a very complex structure. Within the brain, many different and separate regions are responsible for many different and separate functions. It is as if the nervous system is composed of many organs that all look similar and can only be differentiated using techniques such as microscopy or electrophysiology. In comparison, it is easy to see that the stomach is different than the esophagus or the liver, so you can imagine the digestive system as a collection of specific organs.

The nervous system can be divided into two major regions: the central and peripheral nervous systems. The central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is everything else (Figures 15.1 and 15.2). The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the central nervous system is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. In actuality, there are some elements of the peripheral nervous system that are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery—meaning beyond the brain and spinal cord. Depending on different aspects of the nervous system, the dividing line between central and peripheral is not necessarily universal.

Nervous tissue, present in both the central and peripheral nervous system, contains two basic types of cells: neurons and glial (or neuroglial) cells. A glial cell is one of a variety of cells that provide a framework of tissue that supports the neurons and their activities. The neuron is the more functionally important of the two, in terms of the communicative function of the nervous system. To describe the functional divisions of the nervous system, it is important to understand the structure of a neuron. Neurons are cells and therefore have a soma , or cell body, but they also have extensions of the cell; each extension is generally referred to as a process . There is one important process that every neuron has called an axon , which is the fiber that functionally connects a neuron with its target. Another type of process that branches off from the soma is the dendrite .

Dendrites are responsible for receiving most of the input from other neurons. Looking at nervous tissue, there are regions that predominantly contain cell bodies and regions that are largely composed of just axons.

These two regions within nervous system structures are often referred to as gray matter (the regions with many cell bodies and dendrites) or white matter (the regions with many axons). The colors ascribed to these regions are what would be seen in “fresh,” or unstained, nervous tissue (Figure 15.3). Gray matter is not necessarily gray. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved. But white matter is white because axons are insulated by a lipid-rich substance called myelin . Lipids can appear as white (“fatty”) material, much like the fat on a raw piece of chicken or beef. Gray matter may have that color ascribed to it because next to the white matter, it is just darker—hence, gray.

The distinction between gray matter and white matter is most often applied to central nervous tissue, which has large regions that can be seen with the unaided eye. When looking at peripheral structures, often a microscope is used and the tissue is stained with artificial colors. That is not to say that central nervous tissue cannot be stained and viewed under a microscope, but unstained tissue is most likely from the central nervous system —for example, a frontal section of the brain or cross section of the spinal cord.

Regardless of the appearance of stained or unstained tissue, the cell bodies of neurons or axons can be located in discrete anatomical structures that require a name. Those names are specific to whether the structure is central or peripheral. A localized collection of neuron cell bodies in the central nervous system is referred to as a nucleus . In the peripheral nervous system, a cluster of neuron cell bodies is referred to as a ganglion . The term nucleus has a few different meanings within anatomy and physiology. It is the center of an atom, where protons and neutrons are found; it is the center of a cell, where genomic DNA is found; and it is a center of some function in the central nervous system (Figure 15.4). There is also a potentially confusing use of the word ganglion (plural = ganglia) that has a historical explanation. In the central nervous system, there is a group of nuclei that are connected together and were once called the basal ganglia before “ganglion” became accepted as a description for a peripheral structure. Some sources refer to this group of nuclei as the “basal nuclei” to avoid confusion.

Terminology applied to bundles of axons also differs depending on location. A bundle of axons, or fibers, found in the central nervous system is called a tract whereas the same thing in the peripheral nervous system would be called a nerve . There is an important point to make about these terms, which is that they can both be used to refer to the same bundle of axons . When those axons are located in the peripheral nervous system, the term is nerve, but if they are central nervous system, the term is tract . The most obvious example of this is the axons that project from the retina into the brain. Those axons are called the optic nerve as they leave the eye, but when they are inside the cranium, they are referred to as the optic tract. There is a specific place where the name changes, which is the optic chiasm, but they are still the same axons (Figure 15.5). A similar situation outside of science can be described for some roads. For example, you might know of a street named Canada Way in the city of Burnaby. If you travel south long enough on this road, eventually you will leave Burnaby and enter the city of New Westminster. In New Westminster, Canada Way changes its name to Eighth Street. That is the idea behind the naming of the retinal axons. In the peripheral nervous system, they are called the optic nerve, and in the central nervous system, they are the optic tract. Table 15.1 helps to clarify which of these terms apply to the central or peripheral nervous systems.

There are two ways to consider how the nervous system is divided functionally. First, the basic functions of the nervous system are sensation, integration, and response. Secondly, control of the body can be somatic or autonomic —divisions that are largely defined by the structures that are involved in the response (Figure 15.6). There is also a region of the peripheral nervous system that is called the enteric nervous system that is responsible for a specific set of the functions within the realm of autonomic control related to gastrointestinal functions.

Basic Functions: Sensation, Integration, and Response

The nervous system is involved in receiving information about the environment around us (sensation) and generating responses to that information (motor responses). The nervous system can be divided into regions that are responsible for sensation (sensory functions) and for the response (motor functions). But there is a third function that needs to be included. Sensory input needs to be integrated with other sensations, as well as with memories, emotional state, or learning (cognition). Some regions of the nervous system are termed integration or association areas. The process of integration combines sensory perceptions and higher cognitive functions such as memories, learning, and emotion to produce a response.

The first major function of the nervous system is sensation —receiving information about the environment to gain input about what is happening outside the body (or, sometimes, within the body). The sensory functions of the nervous system register the presence of a particular event in the external or internal environment, known as a stimulus . The senses we think of most are the “big five”: taste, smell, touch, sight, and hearing. The stimuli for taste and smell are both chemical substances ( molecules , compounds , ions , etc.), touch is physical or mechanical stimuli that interact with the skin, sight is light stimuli, and hearing is the perception of sound, which is a physical stimulus similar to some aspects of touch. There are actually more senses than just those, but that list represents the major senses. These five are all senses that receive stimuli from the outside world, and of which there is conscious perception. Additional sensory stimuli might be from the internal environment (inside the body), such as the stretch of an organ wall or the concentration of certain ions in the blood.

Stimuli that are received by sensory structures are communicated to the nervous system where that information is processed. This is called integration . Stimuli are compared with, or integrated with, other stimuli, memories of previous stimuli, or the state of a person at a particular time. This leads to the specific response that will be generated. Seeing a baseball pitched to a batter will not automatically cause the batter to swing. The trajectory of the ball and its speed will need to be considered. Maybe the count is three balls and no strikes, and the batter wants to let this pitch go by in the hope of getting a walk to first base. Or maybe the batter’s team is so far ahead, it would be fun to just swing away.

The nervous system produces a response on the basis of the stimuli perceived by sensory structures. An obvious response would be the movement of muscles, such as withdrawing a hand from a hot stove, but there are broader uses of the term. The nervous system can cause the contraction of all three types of muscle tissue. For example, skeletal muscle contracts to move the skeleton, cardiac muscle is stimulated as heart rate increases during exercise, and smooth muscle contracts as the digestive system moves food along the digestive tract. Responses also include the neural control of glands in the body as well, such as the production and secretion of sweat by the eccrine and apocrine sweat glands found in the skin to lower body temperature.

Responses can be divided into those that are voluntary or conscious (contraction of skeletal muscle) and those that are involuntary (contraction of smooth muscle, regulation of cardiac muscle, activation of glands). Voluntary responses are governed by the somatic nervous system and involuntary responses are governed by the autonomic nervous system , which are discussed in the next section.

Somatic, Autonomic and Enteric Nervous Systems

The nervous system can be divided into two parts mostly on the basis of a functional difference in responses. The somatic nervous system (SNS) is responsible for conscious perception and voluntary motor responses. Voluntary motor response means the contraction of skeletal muscle, but those contractions are not always voluntary in the sense that you have to want to perform them. Some somatic motor responses are reflexes, and often happen without a conscious decision to perform them. If your friend jumps out from behind a corner and yells “Boo!” you will be startled and you might scream or leap back. You didn’t decide to do that, and you may not have wanted to give your friend a reason to laugh at your expense, but it is a reflex involving skeletal muscle contractions. Other motor responses become automatic (in other words, unconscious) as a person learns motor skills (referred to as “habit learning” or “procedural memory”).

The autonomic nervous system (ANS) is responsible for involuntary control of the body, usually for the sake of homeostasis (regulation of the internal environment). Sensory input for autonomic functions can be from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue. The role of the autonomic system is to regulate the organ systems of the body, which usually means to control homeostasis. Sweat glands, for example, are controlled by the autonomic system. When you are hot, sweating helps cool your body down. That is a homeostatic mechanism. But when you are nervous, you might start sweating also. That is not homeostatic, it is the physiological response to an emotional state.

There is another division of the nervous system that describes functional responses. The enteric nervous system (ENS) is responsible for controlling the smooth muscle and glandular tissue in your digestive system. It is a large part of the peripheral nervous system, and is not dependent on the central nervous system. It is sometimes valid, however, to consider the enteric system to be a part of the autonomic system because the neural structures that make up the enteric system are a component of the autonomic output that regulates digestion (Figure 15.7). There are some differences between the two, but for our purposes there will be a good bit of overlap.

Nervous tissue is composed of two types of cells, neurons and glial cells. Neurons are the primary type of cell that most anyone associates with the nervous system. They are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells. Glial cells, or glia, are known to play a supporting role for nervous tissue. Ongoing research pursues an expanded role that glial cells might play in signaling, but neurons are still considered the basis of this function. Neurons are important, but without glial support they would not be able to perform their function.

Neurons are the cells considered to be the basis of nervous tissue. They are responsible for the electrical signals that communicate information about sensations, and that produce movements in response to those stimuli, along with inducing thought processes within the brain. An important part of the function of neurons is in their structure, or shape. The three-dimensional shape of these cells makes the immense numbers of connections within the nervous system possible.

Parts of a Neuron

As you learned in the first section, the main part of a neuron is the cell body, which is also known as the soma (soma = “body”). The cell body contains the and most of the major organelles. But what makes neurons special is that they have many extensions of their cell membranes, which are generally referred to as . Neurons are usually described as having one, and only one, axon—a fibre that emerges from the cell body and projects to target cells (Figure 8). That single can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse, which is communicated to one or more cells. The other processes of the neuron are (Figure 8), which receive information from other neurons at specialized areas of contact called . The dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. This gives the neuron a

—meaning that information flows in this one direction.

Where the axon emerges from the cell body, there is a special region referred to as the

. This is a tapering of the cell body toward the axon fibre. Within the axon hillock, the cytoplasm changes to a solution of limited components called axoplasm. Because the axon hillock represents the beginning of the axon, it is also referred to as the initial segment.

Many axons are wrapped by an insulating substance called myelin, which is actually made from

. acts as insulation much like the plastic or rubber that is used to insulate electrical wires. A key difference between myelin and the insulation on a wire is that there are gaps in the myelin covering of an . Each gap is called a node of Ranvier and is important to the way that electrical signals travel down the axon. The length of the axon between each gap, which is wrapped in myelin, is referred to as an . At the end of the axon is the , where there are usually several branches extending toward the target cell, each of which ends in an enlargement called a . These bulbs are what make the connection with the target cell at the synapse .

Types of Neurons

There are many neurons in the nervous system—a number in the trillions. And there are many different types of neurons. They can be classified by many different criteria. The first way to classify them is by the number of

attached to the cell body. Using the standard model of neurons, one of these processes is the axon, and the rest are dendrites. Because information flows through the neuron from dendrites or cell bodies toward the axon, these names are based on the neuron’s (Figure 9).

Neurons can also be classified on the basis of where they are found, who found them, what they do, or even what chemicals they use to communicate with each other. Some neurons referred to in this section on the nervous system are named on the basis of those sorts of classifications (Figure 10). For example, a

neuron that has a very important role to play in a part of the brain called the

is known as a Purkinje (commonly pronounced per-KIN-gee) cell. It is named after the anatomist who discovered it (Jan Evangilista Purkinje, 1787–1869).

Glial cells, or or simply glia, are the other type of cell found in nervous tissue. They are considered to be supporting cells, and many functions are directed at helping neurons complete their function for communication. The name glia comes from the Greek word that means “glue,” and was coined by the German pathologist Rudolph Virchow, who wrote in 1856: “This connective substance, which is in the brain, the spinal cord, and the special sense nerves, is a kind of glue (neuroglia) in which the nervous elements are planted.” Today, research into nervous tissue has shown that there are many deeper roles that these cells play. And research may find much more about them in the future.

There are six types of glial cells (Table 2). Four of them are found in the central nervous system (Figure 11) and two are found in the peripheral nervous system (Figure 12). For reference, Table 2 outlines some common characteristics and functions of the various glial cell types, but the specific names and roles of the glial cell types are not examinable material in this course.

The insulation for axons in the nervous system is provided by glial cells: in the central nervous system, and in the peripheral nervous system. Whereas the manner in which either cell is associated with the , or segments, that it insulates is different, the means of myelinating an axon segment is mostly the same in the two situations. is a -rich sheath that surrounds the and by doing so creates a that facilitates the transmission of electrical signals along the axon. The lipids are essentially the phospholipids of the glial cell membrane. Myelin, however, is more than just the membrane of the glial cell. It also includes important proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together.

Figure 12. Glial Cells of the Peripheral Nervous System. The peripheral nervous system has satellite cells and Schwann cells.

The brain and the spinal cord are the central nervous system, and they represent the main organs of the nervous system. The spinal cord is a single structure, whereas the adult brain is described in terms of four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. A person’s conscious experiences are based on neural activity in the brain. The regulation of homeostasis is governed by a specialized region in the brain. The coordination of reflexes depends on the integration of sensory and motor pathways in the spinal cord.

The iconic gray mantle of the human brain, which appears to make up most of the mass of the brain, is the

with two distinct halves, a right and left (Figure 13). Many of the higher neurological functions, such as memory, emotion, and consciousness, are the result of cerebral function. The cerebrum comprises of a continuous, wrinkled and thin layer of that wraps around both hemispheres, the , and several deep . A (plural = gyri) is the ridge of one of those wrinkles, and a (plural = sulci) is the groove between two gyri. The pattern of these folds of tissue indicates specific regions of the

(Figure 14).

Different regions of the cerebral cortex can be associated with particular functions, a concept known as localization of function. In the early 1900s, a German neuroscientist named Korbinian Brodmann performed an extensive study of the microscopic anatomy (cytoarchitecture) of the cerebral cortex and divided the cortex into 52 separate regions on the basis of the histology of the cortex. His work resulted in a system of classification known as

, which is still used today to describe the anatomical distinctions within the cortex The results from Brodmann’s work on the anatomy align very well with the functional differences within the cortex. For example, Areas 17 and 18 in the occipital lobe are responsible for primary visual perception. That visual information is complex, so it is processed in the temporal and parietal lobes as well.

Beneath the cerebral cortex are sets of nuclei known as

that augment cortical processes (Figure 15). Some of the basal nuclei in the forebrain, for example, serve as the primary location for

production, which modulates the overall activity of the cortex, possibly leading to greater attention to sensory stimuli. Alzheimer’s disease is associated with a loss of neurons in the cholinergic basal forebrain nuclei. Some other basal nuclei control the initiation of movement. For example, while a student is sitting in a classroom listening to a lecture, the basal nuclei will keep an urge to jump up and scream from actually happening. (The basal nuclei are also referred to as the basal ganglia, although that is potentially confusing because the term ganglia is typically used for peripheral structures.)

Figure 15. Frontal Section of Cerebral Cortex and Basal Nuclei. The major components of the basal nuclei, shown in a frontal section of the brain, are the caudate (just lateral to the lateral ventricle), the putamen (inferior to the caudate and separated by the large white-matter structure called the internal capsule), and the globus pallidus (medial to the putamen). (The names of these nuclei are not required as examinable material in this course.)

translates to “through brain.” It is the connection between the cerebrum and the rest of the nervous system, with one exception. The rest of the brain, the spinal cord, and the peripheral nervous system all send information to the cerebrum through the diencephalon. Output from the cerebrum passes through the diencephalon. The single exception is the system associated with , or the sense of smell, which connects directly with the

The diencephalon is deep beneath the cerebrum and constitutes the walls of the

. The diencephalon can be described as any region of the brain with “thalamus” in its name. The two major regions of the diencephalon are the itself and the hypothalamus (Figure 16). There are other structures, such as the , which contains the pineal gland, and the

, which includes the subthalamic nucleus, one of the basal nuclei.

The thalamus is a collection of nuclei that relay information between the

and the periphery, spinal cord, or brain stem. All sensory information, except for the sense of smell, passes through the thalamus before processing by the cortex. from the peripheral sensory organs, or intermediate nuclei, in the thalamus, and thalamic neurons project directly to the . It is a requisite synapse in any sensory pathway, except for olfaction. The thalamus does not just pass the information on, it also processes that information. For example, the portion of the thalamus that receives visual information will influence what visual stimuli are important, or what receives attention. The also sends information down to the

, which usually communicates motor commands.

Hypothalamus

Inferior and slightly anterior to the thalamus is the

, the other major region of the . The hypothalamus is a collection of nuclei that are largely involved in regulating . The hypothalamus is the executive region in charge of the and the system through its regulation of the anterior . Other parts of the hypothalamus are involved in memory and emotion as part of the

The Brain Stem

and (composed of the pons and the medulla) are collectively referred to as the brain stem (Figure 17). The structure emerges from the ventral surface of the forebrain as a tapering cone that connects the brain to the spinal cord. Attached to the brain stem, but considered a separate region of the adult brain, is the . The midbrain coordinates sensory representations of the visual, auditory, and somatosensory perceptual spaces. The pons is the main connection with the cerebellum. The and the

regulate several crucial functions, including the cardiovascular and respiratory systems.

The cranial nerves connect through the brain stem and provide the brain with the sensory input and motor output associated with the head and neck, including most of the special senses. The major ascending and descending pathways between the spinal cord and brain, specifically the cerebrum, pass through the brain stem.

One of the original regions of the embryonic brain, the midbrain is a small region between the

and . The passes through the center of the

, such that these regions are the roof and floor of that canal.

The midbrain includes four bumps known as the colliculi (singular = colliculus), which means “little hill” in Latin. The

is the inferior pair of these enlargements and is part of the auditory brain stem pathway. Neurons of the inferior colliculus project to the thalamus, which then sends auditory information to the cerebrum for the conscious perception of sound. The

is the superior pair and combines sensory information about visual space, auditory space, and somatosensory space. Activity in the superior colliculus is related to orienting the eyes to a sound or touch stimulus. If you are walking along the sidewalk on campus and you hear chirping, the superior colliculus coordinates that information with your awareness of the visual location of the tree right above you. That is the correlation of auditory and visual maps. If you suddenly feel something wet fall on your head, your superior colliculus integrates that with the auditory and visual maps and you know that the chirping bird just relieved itself on you. You want to look up to see the culprit, but do not.

comes from the Latin word for bridge. It is visible on the anterior surface of the brain stem as the thick bundle of attached to the

. The pons is the main connection between the cerebellum and the brain stem.

of the midbrain and pons continues into the , also known as medulla oblongata. This diffuse region of gray matter throughout the brain stem, known as the , is related to sleep and wakefulness, general brain activity and attention. The medulla contains

nuclei with motor neurons that control the rate and force of heart contraction, the diameter of blood vessels and the rate and depth of breathing, among other essential physiological processes.

The cerebellum, as the name suggests, is the “little brain.” It is covered in

like the cerebrum, and looks like a miniature version of that part of the brain (Figure 18). The cerebellum integrates motor commands from the cerebral cortex with sensory feedback from the periphery, allowing for the coordination and precise execution of motor activities, such as walking, cycling, writing or playing a musical instrument.

Whereas the brain develops out of expansions of the neural tube into primary and then secondary vesicles, the spinal cord maintains the tube structure and is only specialized into certain regions.

The length of the spinal cord is divided into regions that correspond to the regions of the vertebral column. The name of a spinal cord region corresponds to the level at which spinal nerves pass through the intervertebral

. Immediately adjacent to the brain stem is the region, followed by the , then the

, and finally the sacral region (Figures 24 and 25).

In cross-section, the

of the spinal cord has the appearance of an ink-blot test, with the spread of the gray matter on one side replicated on the other—a shape reminiscent of a bulbous capital “H.” As shown in Figure 19, the gray matter is subdivided into regions that are referred to as horns.

is responsible for sensory processing. The sends out motor signals to the skeletal muscles. The , which is only found in the thoracic, upper lumbar, and regions, is the central component of the of the

Some of the largest neurons of the spinal cord are the

motor neurons in the anterior horn. The fibres that cause contraction of skeletal muscles are the axons of these neurons. The motor neuron that causes contraction of the big toe, for example, is located in the sacral spinal cord. The axon that has to reach all the way to the belly of that muscle may be a metre in length. The neuronal cell body that maintains that long fiber must be quite large, possibly several hundred micrometres in diameter, making it one of the largest cells in the body.

White Columns

Just as the gray matter is separated into horns, the white matter of the spinal cord is separated into columns.

of nervous system fibres in these columns carry sensory information up to the brain, whereas

carry motor commands from the brain.

The outer surface of the central nervous system is covered by a series of membranes composed of connective tissue called the meninges , which protect the brain. The dura mater is a thick fibrous layer and a strong protective sheath over the entire brain and spinal cord. It is anchored to the inner surface of the cranium and vertebral cavity. The arachnoid mater is a membrane of thin fibrous tissue that forms a loose sac around the central nervous system. Beneath the arachnoid is a thin, filamentous mesh called the arachnoid trabeculae , which looks like a spider web, giving this layer its name. Directly adjacent to the surface of the central nervous system is the pia mater , a thin fibrous membrane that follows the convolutions of gyri and sulci in the cerebral cortex and fits into other grooves and indentations (Figures 15.18).

Cerebrospinal fluid (CSF) circulates throughout and around the central nervous system. Cerebrospinal fluid is produced in special structures to perfuse through the nervous tissue of the central nervous system and is continuous with the interstitial fluid . Specifically, cerebrospinal fluid circulates to remove metabolic wastes from the interstitial fluids of nervous tissues and return them to the blood stream. The ventricles are the open spaces within the brain where cerebrospinal fluid circulates. In some of these spaces, cerebrospinal fluid is produced by filtering of the blood that is performed by a specialized membrane known as a choroid plexus . The cerebrospinal fluid circulates through all of the ventricles to eventually emerge into the subarachnoid space where it is reabsorbed into the blood.

There are four ventricles within the brain, all of which developed from the original hollow space within the neural tube, the central canal. The first two are named the lateral ventricles and are deep within the cerebrum . These ventricles are connected to the third ventricle by two openings called the interventricular foramina. The third ventricle is the space between the left and right sides of the diencephalon , which opens into the cerebral aqueduct that passes through the midbrain . The aqueduct opens into the fourth ventricle , which is the space between the cerebellum and the pons and upper medulla (Figure 15.19).

The ventricular system opens up to the subarachnoid space from the fourth ventricle. The single median aperture and the pair of lateral apertures connect to the subarachnoid space so that cerebrospinal fluid can flow through the ventricles and around the outside of the central nervous system. Cerebrospinal fluid is produced within the ventricles by a type of specialized membrane called a choroid plexus . Ependymal cells (a type of glial cell; see Figure 15.11) surround blood capillaries and filter the blood to make cerebrospinal fluid. The fluid is a clear solution with a limited amount of the constituents of blood. It is essentially water, small molecules, and electrolytes . Oxygen and carbon dioxide are dissolved into the cerebrospinal fluid, as they are in blood, and can diffuse between the fluid and the nervous tissue.

Cerebrospinal Fluid Circulation

The choroid plexuses are found in all four ventricles . Observed in dissection, they appear as soft, fuzzy structures that may still be pink, depending on how well the circulatory system is cleared in preparation of the tissue. The CSF is produced from components extracted from the blood, so its flow out of the ventricles is tied to the pulse of cardiovascular circulation.

From the lateral ventricles , the CSF flows into the third ventricle , where more CSF is produced, and then through the cerebral aqueduct into the fourth ventricle where even more CSF is produced. A very small amount of CSF is filtered at any one of the plexuses, for a total of about 500 milliliters daily, but it is continuously made and pulses through the ventricular system, keeping the fluid moving. From the fourth ventricle, CSF can continue down the central canal of the spinal cord, but this is essentially a cul-de-sac, so more of the fluid leaves the ventricular system and moves into the subarachnoid space through the median and lateral apertures.

Within the subarachnoid space, the cerebrospinal fluid flows around all of the central nervous system, providing two important functions. As with elsewhere in its circulation, the cerebrospinal fluid picks up metabolic wastes from the nervous tissue and moves it out of the central nervous system. It also acts as a liquid cushion for the brain and spinal cord. By surrounding the entire system in the subarachnoid space, it provides a thin buffer around the organs within the strong, protective dura mater . The arachnoid granulations are outpocketings of the arachnoid membrane into the dural sinuses so that cerebrospinal fluid can be reabsorbed into the blood, along with the metabolic wastes. From the dural sinuses, blood drains out of the head and neck through the jugular veins , along with the rest of the circulation for blood, to be re-oxygenated by the lungs and wastes to be filtered out by the kidneys (Table 15.3).

The peripheral nervous system is not as contained as the central nervous system because it is defined as everything that is not the central nervous system. Some peripheral structures are incorporated into the other organs of the body. In describing the anatomy of the peripheral nervous system, it is necessary to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many of the neural structures that are incorporated into other organs are features of the digestive system; these structures are known as the enteric nervous system and are a special subset of the peripheral nervous system.

A ganglion is a group of neuron cell bodies in the periphery. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of sensory ganglion is a dorsal root ganglion . These ganglia are the cell bodies of neurons with axons that are sensory endings in the periphery, such as in the skin, and that extend into the central nervous system through the dorsal nerve root.

The other major category of ganglia, those of the autonomic nervous system , will be examined later in this chapter.

Bundles of axons in the peripheral nervous system are referred to as nerves . These structures in the periphery are different than their central counterpart, called a tract . Nerves are composed of more than just nervous tissue. They have connective tissues invested in their structure, as well as blood vessels supplying the tissues with nourishment. Nerves are associated with the region of the central nervous system to which they are connected, either as cranial nerves (12 pairs) connected to the brain or spinal nerves (31 pairs) connected to the spinal cord.

The cranial nerves are primarily responsible for the sensory and motor functions of the head and neck, although one of these nerves, the vagus , targets organs in the thoracic  and abdominal cavities as part of the parasympathetic nervous system. They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the axons in these nerves originate out of sensory ganglia external to the cranium or motor nuclei within the brain stem.

All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibres, both somatic and autonomic , emerge as the ventral nerve root. The dorsal root ganglion for each nerve is an enlargement of the spinal nerve.

The somatic nervous system is traditionally considered a division within the peripheral nervous system. However, this misses an important point: somatic refers to a functional division, whereas peripheral refers to an anatomic division. The somatic nervous system is responsible for our conscious perception of the environment and for our voluntary responses to that perception by means of skeletal muscles . Peripheral sensory neurons receive input from environmental stimuli, but the neurons that produce motor responses originate in the central nervous system. The distinction between the structures of the peripheral and central nervous systems and the functions of the somatic and autonomic systems can most easily be demonstrated through a simple reflex , an automatic response that the nervous system produces in response to specific stimuli. The neurons and neural pathways responsible for a reflex action constitute the reflex arc . One of the simplest reflex acts is the stretch reflex , by which the nervous system responds to the stretching of a muscle (the stimulus) with contraction of that same muscle (the response). This response protects the muscle from over-stretching, but more importantly, it has a crucial role in maintaining posture and balance. The patellar reflex (or knee-jerk reflex) is an example of stretch reflex and it occurs through the following steps (Figure 15.21):

• Tapping of the patellar tendon with a hammer causes the stretching of muscle fibers in the quadriceps muscle, which stimulates sensory neurons innervating those fibers.
• In the sensory neuron, a nerve impulse ( action potential ) is generated, which propagates along the sensory nerve fiber from the muscle, through the dorsal root ganglion, to the spinal cord.
• The sensory neuron stimulates a motor neuron in the ventral horn of the spinal cord.
• That motor neuron sends a nerve impulse (action potential) along its axon .
• This impulse reaches the quadriceps muscle, causing its contraction and the extension of the leg (a kick).

The sensory neuron can also activate an interneuron (e.g., Figure 15.21), which inhibits the motor neuron responsible for the contraction of the antagonistic muscle to quadriceps (i.e. hamstring ).

Another example of a simple spinal reflex is the withdrawal reflex , which occurs, for example, when you touch a hot stove and pull your hand away. This reflex occurs through a similar sequence of steps:

• Sensory receptors in the skin sense extreme temperature and the early signs of tissue damage.
• In a sensory neuron, a nerve impulse ( action potential ) is generated, which propagates along the sensory nerve  fibrer from the skin, through the dorsal root ganglion , to the spinal cord.
• The sensory neuron stimulates a motor neuron in the ventral horn motor of the spinal cord.
• That motor neuron sends a nerve impulse (action potential) along its axon.
• This impulse reaches the biceps brachii , causing contraction of the muscle and flexion of the forearm at the elbow to withdraw the hand from the hot stove.

The basic withdrawal reflex includes sensory input (the painful stimulus), central processing (the synapse in the spinal cord), and motor output (activation of a ventral motor neuron that causes contraction of the biceps brachii ). As seen for the patellar reflex, the withdrawal reflex can also include inhibition of the antagonistic muscle ( triceps brachii in our example). Another possible motor output of the withdrawal reflex is cross extension: counterbalancing movement on the other side of the body by stimulation of the extensor muscles in the contralateral limb.

The somatic nervous system also controls voluntary movement and more complex motor functions. For example, reading of this text starts with visual sensory input to the retina, which then projects to the thalamus , and on to the cerebral cortex . A sequence of regions of the cerebral cortex process the visual information, starting in the primary visual cortex of the occipital lobe, and resulting in the conscious perception of these letters. Subsequent cognitive processing results in understanding of the content. As you continue reading, regions of the cerebral cortex in the frontal lobe plan how to move the eyes to follow the lines of text. The output from the cortex causes activity in motor neurons in the brain stem that cause movement of the extraocular muscles through the third, fourth, and sixth cranial nerves. This example also includes sensory input (the retinal projection to the thalamus), central processing (the thalamus and subsequent cortical activity), and motor output (activation of neurons in the brain stem that lead to coordinated contraction of extraocular muscles).

The autonomic nervous system is often associated with the “fight-or-flight response,” which refers to the preparation of the body to either run away from a threat or to stand and fight in the face of that threat. To suggest what this means, consider the (very unlikely) situation of seeing a lioness hunting out on the savannah. Though this is not a common threat that humans deal with in the modern world, it represents the type of environment in which the human species adapted and thrived. The spread of humans around the world to the present state of the modern age occurred much more quickly than any species would adapt to environmental pressures such as predators. However, the reactions humans have in the modern world are based on these prehistoric situations. If your boss is walking down the hallway on Friday afternoon looking for “volunteers” to come in on the weekend, your response is the same as the prehistoric human seeing the lioness running across the savannah: fight or flight.

Most likely, your response to your boss—not to mention the lioness—would be flight. Run away! The autonomic system is responsible for the physiological response to make that possible, and hopefully successful. Adrenaline starts to flood your circulatory system. Your heart rate increases. Sweat glands become active. The bronchi of the lungs dilate to allow more air exchange. Pupils dilate to increase visual information. Blood pressure increases in general, and blood vessels dilate in skeletal muscles. Time to run. Similar physiological responses would occur in preparation for fighting off the threat.

This response should sound a bit familiar. The autonomic nervous system is tied into emotional responses as well, and the fight-or-flight response probably sounds like a panic attack. In the modern world, these sorts of reactions are associated with anxiety as much as with response to a threat. It is engrained in the nervous system to respond this way. In fact, the adaptations of the autonomic nervous system probably pre-date the human species and are likely to be common to all mammals. That lioness might herself be threatened in some other situation

However, the autonomic nervous system is not just about responding to threats. Besides the fight-or-flight response, there are the responses referred to as “rest and digest.” If that lioness is successful in her hunting, then she is going to rest from the exertion. Her heart rate will slow. Breathing will return to normal. The digestive system has a big job to do. Much of the function of the autonomic system is based on the connections within an autonomic, or visceral, reflex.

As we have seen, the nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system. The major differences between the two systems are evident in the responses that each produces. The somatic nervous system causes contraction of skeletal muscles. The autonomic nervous system controls cardiac and smooth muscle, as well as glandular tissue. The somatic nervous system is associated with voluntary responses (though many can happen without conscious awareness, like breathing), and the autonomic nervous system is associated with involuntary responses, such as those related to homeostasis.

The autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions. In addition to the endocrine system, the autonomic nervous system is instrumental in homeostatic mechanisms in the body. The two divisions of the autonomic nervous system are the sympathetic division and the parasympathetic division . The sympathetic system is associated with the fight-or-flight response , and parasympathetic activity is referred to by the epithet of rest and digest . At each target effector , dual innervation determines activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions. One causes heart rate to increase, whereas the other causes heart rate to decrease.

Sympathetic Division of the Autonomic Nervous System

To respond to a threat—to fight or to run away—the sympathetic system causes divergent effects as many different effector organs are activated together for a common purpose. More oxygen needs to be inhaled and delivered to skeletal muscle. The respiratory, cardiovascular, and musculoskeletal systems are all activated together. Additionally, sweating keeps the excess heat that comes from muscle contraction from causing the body to overheat. The digestive system shuts down so that blood is not absorbing nutrients when it should be delivering oxygen to skeletal muscles. To coordinate all these responses, the connections in the sympathetic system diverge from a limited region of the central nervous system to a wide array of ganglia that project to the many effector organs simultaneously. The complex set of structures that compose the output of the sympathetic system make it possible for these disparate effectors to come together in a coordinated, systemic change.

The sympathetic division of the autonomic nervous system influences the various organ systems of the body through connections emerging from the thoracic and upper lumbar spinal cord. It is referred to as the thoracolumbar system to reflect this anatomical basis. A central neuron in the lateral horn of any of these spinal regions projects to ganglia adjacent to the vertebral column through the ventral spinal roots. The majority of ganglia of the sympathetic system belong to a network of sympathetic chain ganglia that runs alongside the vertebral column. The ganglia appear as a series of clusters of neurons linked by axonal bridges. A diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices (Figure 15.24, wherein the “circuits” of the sympathetic system are intentionally simplified).

An axon from the central neuron that projects to a sympathetic ganglion is referred to as a preganglionic fiber or neuron, and represents the output from the central nervous system to the ganglion. Because the sympathetic ganglia are adjacent to the vertebral column, preganglionic sympathetic fibers are relatively short, and are myelinated. A postganglionic fiber —the axon from a ganglionic neuron that projects to the target effector—represents the output of a ganglion that directly influences the organ. Compared with the preganglionic fibers, postganglionic sympathetic fibers are long because of the relatively greater distance from the ganglion to the target effector . These fibers are unmyelinated. (Note that the term “postganglionic neuron” may be used to describe the projection from a ganglion to the target. The problem with that usage is that the cell body is in the ganglion, and only the fiber is postganglionic. Typically, the term neuron applies to the entire cell.)

One type of preganglionic sympathetic fiber does not terminate in a ganglion. These are the axons from central sympathetic neurons that project to the adrenal medulla , the interior portion of the adrenal gland . These axons are still referred to as preganglionic fibers, but the target is not a ganglion . The adrenal medulla releases signaling molecules into the bloodstream, rather than using axons to communicate with target structures.

The projections of the sympathetic division of the autonomic nervous system diverge widely, resulting in a broad influence of the system throughout the body. As a response to a threat, the sympathetic system would increase heart rate and breathing rate and cause blood flow to the skeletal muscle to increase and blood flow to the digestive system to decrease. Sweat gland secretion should also increase as part of an integrated response. All of those physiological changes are going to be required to occur together to run away from the hunting lioness, or the modern equivalent. This divergence is seen in the branching patterns of preganglionic sympathetic neurons—a single preganglionic sympathetic neuron may have 10–20 targets. An axon that leaves a central neuron of the lateral horn in the thoracolumbar spinal cord will pass through the white ramus communicans and enter the sympathetic chain, where it will branch toward a variety of targets. At the level of the spinal cord at which the preganglionic sympathetic fiber exits the spinal cord, a branch will synapse on a neuron in the adjacent chain ganglion. Some branches will extend up or down to a different level of the chain ganglia. Other branches will pass through the chain ganglia and project through one of the splanchnic nerves to a collateral ganglion. Finally, some branches may project through the splanchnic nerves to the adrenal medulla. All of these branches mean that one preganglionic neuron can influence different regions of the sympathetic system very broadly, by acting on widely distributed organs.

Parasympathetic Division of the Autonomic Nervous System

When not responding to an immediate threat, the parasympathetic system is generally more active than the sympathetic system.  Many of the same effectors in the body are innervated by both divisions of the autonomic nervous system, but activation of each division tends to have opposing effects.  Sympathetic system activation tends to increase activity in the respiratory, cardiovascular, and musculoskeletal systems while reducing activity in the digestive system.  Parasympathetic system activation on the other hand tends to decrease activity in the respiratory, cardiovascular, and musculoskeletal systems while increasing activity in the digestive, urinary, and reproductive systems.  Generally speaking, the activity of the many organs that receive input from both systems is dependent on whether neurons of the parasympathetic or sympathetic system are releasing more of their neurotransmitter onto each organ at a given time.

The parasympathetic division of the autonomic nervous system is named because its central neurons are located on either side of the thoracolumbar region of the spinal cord (para- = “beside” or “near”). The parasympathetic system can also be referred to as the craniosacral system (or outflow) because the preganglionic neurons are located in nuclei of the brain stem and the lateral horn of the sacral spinal cord.

The connections, or “circuits,” of the parasympathetic division are similar to the general layout of the sympathetic division with a few specific differences (Figure 15.23). The preganglionic fibers from the cranial region travel in cranial nerves, whereas preganglionic fibers from the sacral region travel in spinal nerves. The targets of these fibers are terminal ganglia, which are located near - or even within - the target organ. The postganglionic fiber projects from the terminal ganglia a short distance to the effector. These ganglia are often referred to as intramural ganglia when they are found within the walls of the target effector, or to the specific target tissue within the organ. Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibers are long and the postganglionic fibers are short because the ganglia are close to - and sometimes within - the target effectors.

Chemical Signaling i n the Autonomic Nervous System

Where an autonomic neuron innervates a target, there is a synapse . The electrical signal of the action potential causes the release of a signaling molecule, which will bind to receptor proteins on the target cell. Synapses of the autonomic system are classified as either cholinergic , meaning that acetylcholine ( ACh ) is released, or adrenergic , meaning that norepinephrine is released. The terms cholinergic and adrenergic refer not only to the signaling molecule that is released but also to the class of receptors that each binds.

The term adrenergic should remind you of the word adrenaline, which is associated with the fight-or-flight response described at the beginning of the chapter. Adrenaline and epinephrine are two names for the same molecule. The adrenal gland (in Latin, ad- = “on top of”; renal = “kidney”) secretes adrenaline. The ending “-ine” refers to the chemical being derived, or extracted, from the adrenal gland . A similar construction from Greek instead of Latin results in the word epinephrine (epi- = “above”; nephr- = “kidney”). In scientific usage, epinephrine is preferred in the United States, whereas adrenaline is preferred in Great Britain, because “adrenalin” was once a registered, proprietary drug name in the United States. Though the drug is no longer sold, the convention of referring to this molecule by the two different names persists. Similarly, norepinephrine and noradrenaline are two names for the same molecule.

All preganglionic fibers , both sympathetic and parasympathetic , release ACh. The postganglionic parasympathetic fibers also release ACh. Postganglionic sympathetic fibers release norepinephrine, except for fibers that project to sweat glands and to blood vessels associated with skeletal muscles, which release ACh.

Signaling molecules can belong to two broad groups. Neurotransmitters are released at synapses, whereas hormones are released into the bloodstream. These are simplistic definitions, but they can help to clarify this point. Acetylcholine can be considered a neurotransmitter because it is released by axons at synapses. The adrenergic system, however, presents a challenge. Postganglionic sympathetic fibers release norepinephrine , which can be considered a neurotransmitter. But the adrenal medulla releases epinephrine and norepinephrine into circulation, so they should be considered hormones .

Having looked at the components of nervous tissue, and the basic anatomy of the nervous system, next comes an understanding of how nervous tissue is capable of communicating within the nervous system. Before getting to the nuts and bolts of how this works, an illustration of how the components come together will be helpful (summarized in Figure 15.24).

Imagine you are about to take a shower. You have turned on the faucet to start the water as you prepare to get in the shower. After a few minutes, you expect the water to be a temperature that will be comfortable to enter. So you put your hand out into the spray of water. What happens next depends on how your nervous system interacts with the stimulus of the water temperature and what you do in response to that stimulus.

Found in the skin of your fingers or toes is a type of sensory receptor that is sensitive to temperature, called a thermoreceptor . When you place your hand under the shower (Figure 15.27), the cell membrane of the thermoreceptor changes its electrical state (voltage). The amount of change is dependent on the strength of the stimulus (how hot the water is). This is called a graded potential . If the stimulus is strong, the voltage of the cell membrane will change enough to generate an electrical signal that will propagate down the axon .

The voltage at which such a signal is generated is called the threshold , and the resulting electrical signal is called an action potential . In this example, the action potential travels—a process known as propagation —along the axon from the axon hillock to the axon terminals and into the synaptic end bulbs . Propagation is a process by which the action potential is constantly re-created along small stretches of membrane, appearing to "travel". When this signal reaches the end bulbs, it causes the release of a signaling molecule called a neurotransmitter .

The neurotransmitter diffuses across the short distance of the synapse and binds to a receptor protein on the target neuron. When the molecular signal binds to the receptor, the cell membrane of the target neuron changes its electrical state and a new graded potential begins. If that graded potential is strong enough to reach threshold, the second neuron generates an action potential at its axon hillock. The target of this neuron is another neuron in the thalamus of the brain, the part of the central nervous system that acts as a relay for sensory information. At another synapse, neurotransmitter is released and binds to its receptor. The thalamus then sends the sensory information to the cerebral cortex , the outermost layer of gray matter in the brain, where conscious perception of the water temperature begins. Within the cerebral cortex, information is processed among many neurons, integrating the stimulus of the water temperature with other sensory stimuli, with your emotional state (you just aren’t ready to wake up; the bed is calling to you), memories (perhaps of the lab notes you have to study before a quiz). Finally, a plan is developed about what to do, whether that is to turn the temperature up, turn the whole shower off and go back to bed, or step into the shower. To do any of these things, the cerebral cortex has to send a command out to your body to move muscles (Figure 15.26).

A region of the cortex is specialized for sending signals down to the spinal cord for movement. The upper motor neuron is in this region, called the primary motor cortex , which has an axon that extends all the way down the spinal cord. At the level of the spinal cord at which this axon makes a synapse, a graded potential occurs in the cell membrane of a lower motor neuron . This second motor neuron is responsible for causing muscle fibers to contract. In the manner described in the chapter on muscle tissue, an action potential propagates along the motor neuron axon into the periphery. The axon terminates on muscle fibers at the neuromuscular junction . Acetylcholine is released at this specialized synapse, which causes the muscle action potential to begin, following a large potential known as an end plate potential. When the lower motor neuron excites the muscle fiber, it contracts. All of this occurs in a fraction of a second, but this story is the basis of how the nervous system functions.

The functions of the nervous system—sensation, integration, and response—depend on the functions of the neurons underlying these pathways. To understand how neurons are able to communicate, it is necessary to describe the role of an excitable membrane in generating these signals. The basis of this communication is the action potential , which demonstrates how changes in the membrane can constitute a signal. (The way these signals work in more variable circumstances involves graded potentials.)

Cells in the body make use of charged particles, ions , to build up a charge across the cell membrane. The cell membrane regulates ion movement between the extracellular fluid and cytosol. As you learned in Chapter 6, the cell membrane is primarily responsible for regulating what can cross the membrane. The cell membrane is a phospholipid bilayer, so only substances that can pass directly through the hydrophobic core can diffuse through without assistance. Charged particles, which are hydrophilic by definition, cannot pass through the cell membrane without assistance (Figure 15.29). Transmembrane proteins, specifically channel proteins , make this possible. Several passive ion channels, as well as active transport pumps, are necessary to generate a membrane potential and an action potential. Ion channels are pores that allow specific charged particles to cross the membrane in response to an existing concentration gradient .

Of special interest is the carrier protein referred to as the sodium/potassium pump that actively moves sodium ions (Na + ) out of a cell and potassium ions (K + ) into a cell, thus regulating ion concentration on both sides of the cell membrane. The sodium/potassium pump requires energy in the form of adenosine triphosphate (ATP), so it is also referred to as an ATPase. As was explained in Chapter 7, the concentration of Na + is higher outside the cell than inside, and the concentration of K + is higher inside the cell than outside. That means that this pump is moving the ions against the concentration gradients for sodium and potassium, which is why it requires energy. The only purpose of this pump is to maintain these concentration gradients.

Ion channels typically do not allow ions to diffuse across the membrane. Most are opened by certain events, meaning the channels are gated .

A ligand-gated channel opens because a signaling molecule, a ligand, binds to the extracellular region of the channel. This type of channel is also known as an ionotropic receptor because when the ligand, typically a  neurotransmitter in the nervous system, binds to the protein, ions cross the membrane, changing membrane potential. (Figure 15.28).

A mechanically gated channel opens because of a physical distortion of the cell membrane. Many channels associated with the sense of touch ( somatosensation ) are mechanically gated. For example, as pressure is applied to the skin, these channels open and allow ions to enter the cell. Similar to this type of channel would be the channel that opens on the basis of temperature change, as in testing the water in the shower (Figure 15.29).

A voltage-gated channel is a channel that responds to changes in the electrical properties of the membrane in which it is embedded. Normally, the inner portion of the membrane is at a negative voltage. When that voltage becomes less negative, the channel begins to allow ions to cross the membrane (Figure 15.30).

A leakage channel is randomly gated, meaning that it opens and closes at random, hence the reference to leaking. There is no actual event that opens the channel; instead, it has an intrinsic rate of switching between the open and closed states. Leakage channels contribute to the resting voltage of the excitable membrane (Figure 15.31).

The electrical state of the cell membrane can have several variations. These are all variations in the membrane potential . A potential is a distribution of charge across the cell membrane, measured in millivolts (mV). The standard is to compare the inside of the cell relative to the outside, so the membrane potential is a value representing the charge on the intracellular side of the membrane relative to the outside. (Figure 15.32).

The concentration of ions in extracellular and intracellular fluids is largely balanced, with a net neutral charge. However, a slight difference in charge occurs right at the membrane surface, both internally and externally. It is the difference in this very limited region that has all the power in neurons (and muscle cells) to generate electrical signals, including action potentials.

Before these electrical signals can be described, the resting state of the membrane must be explained. When the cell is at rest, and the ion channels are closed (except for leakage channels which randomly open), ions are distributed across the membrane in a very predictable way. The concentration of Na + outside the cell is 10 times greater than the concentration inside. Also, the concentration of K + inside the cell is greater than outside. The cytosol contains a high concentration of anions , in the form of phosphate ions and negatively charged proteins. Large anions are a component of the inner cell membrane, including specialized phospholipids and proteins associated with the inner leaflet of the membrane (leaflet is a term used for one side of the lipid bilayer membrane). The negative charge is localized in the large anions.

With the ions distributed across the membrane at these concentrations, the difference in charge is measured at -70 mV for most cells, the value described as the resting membrane potential . The exact value measured for the resting membrane potential varies between cells, but -70 mV is the most commonly recorded value. This voltage would actually be much lower except for the contributions of some important proteins in the membrane. Leakage K + channels allow K + to slowly move out of the cells. To a much lesser extent, leakage Na + channels allow Na + to slowly move into the cell. The constant activity of the Na + /K + pump maintains the ion gradients. This may appear to be a waste of energy, but this pump plays a vital role in maintaining the membrane potential.

Resting membrane potential describes the steady state of the cell, which is a dynamic process that is balanced by ion leakage and ion pumping. Without any outside influence, it will not change. To get an electrical signal started, the membrane potential has to change.

This starts with a channel opening for Na + in the membrane. Because the concentration of Na + is higher outside the cell than inside the cell by a factor of 10, ions will rush into the cell that are driven largely by the concentration gradient. Because sodium is a positively charged ion, it will change the relative voltage immediately inside the cell relative to immediately outside. The resting potential is the state of the membrane at a voltage of -70 mV, so the sodium cation entering the cell will cause it to become less negative. This is known as depolarization , meaning the membrane potential moves toward zero.

The concentration gradient for Na + is so strong that it will continue to enter the cell even after the membrane potential has become zero, so that the internal voltage immediately around the pore begins to become positive. The electrical gradient also plays a role, as negative proteins below the membrane attract the sodium ion. The membrane potential will reach +30 mV as a result of sodium entering the cell.

As the membrane potential reaches +30 mV, other voltage-gated channels are opening in the membrane. These channels are specific for the potassium ion. A concentration gradient acts on K + , as well. As K + starts to leave the cell, taking a positive charge with it, the membrane potential begins to move back toward its resting voltage. This is called repolarization , meaning that the membrane voltage moves back toward the -70 mV value of the resting membrane potential .

Repolarization returns the membrane potential to the -70 mV value that indicates the resting potential, but it actually overshoots that value. Potassium ions reach equilibrium when the membrane voltage is below -70 mV, so a period of hyperpolarization occurs while the K + channels are open. Those K + channels are slightly delayed in closing, accounting for this short overshoot.

What has been described above is the action potential, which is presented as a graph of voltage over time (Figure 15.35). It is the electrical signal that nervous tissue generates for communication. The change in the membrane voltage from -70 mV at rest to +30 mV at the end of depolarization is a 100-mV change. That can also be written as a 0.1-V change. To put that value in perspective, think about a battery. An AA battery that you might find in a television remote has a voltage of 1.5 V, or a 9-V battery (the rectangular battery with two posts on one end) is, obviously, 9 V. The change seen in the action potential is one or two orders of magnitude less than the charge in these batteries. In fact, the membrane potential can be described as a battery. A charge is stored across the membrane that can be released under the correct conditions. A battery in your remote has stored a charge that is “released” when you push a button.

The question is, now, what initiates the action potential ? The description above conveniently glosses over that point. But it is vital to understanding what is happening. The membrane potential will stay at the resting voltage until something changes. The description above just says that a Na + channel opens. Now, to say “a channel opens” does not mean that one individual transmembrane protein changes. Instead, it means that one kind of channel opens. There are a few different types of channels that allow Na + to cross the membrane. A ligand-gated Na + channel will open when a neurotransmitter binds to it and a mechanically gated Na + channel will open when a physical stimulus affects a sensory receptor (like pressure applied to the skin compresses a touch receptor). Whether it is a neurotransmitter binding to its receptor protein or a sensory stimulus activating a sensory receptor cell, some stimulus gets the process started. Sodium starts to enter the cell and the membrane becomes more positive.

A third type of channel that is an important part of depolarization in the action potential is the voltage-gated Na + channel. The channels that start depolarizing the membrane because of a stimulus help the cell to depolarize from -70 mV to -55 mV. A membrane potential of -55 mV is known as "threshold" for most cells. Once the membrane reaches that voltage, the voltage-gated Na + channels open. Any depolarization that does not change the membrane potential to -55 mV or higher will not reach threshold and thus will not result in an action potential. Also, any stimulus that depolarizes the membrane to -55 mV or beyond will cause a large number of channels to open and an action potential will be initiated.

Because of the threshold, the action potential can be likened to a digital event—it either happens or it does not. If the threshold is not reached, then no action potential occurs. If depolarization reaches -55 mV, then the action potential continues and runs all the way to +30 mV, at which point K + causes repolarization , including the hyperpolarizing overshoot. Also, those changes are the same for every action potential, which means that once the threshold is reached, the exact same thing happens. A stronger stimulus, which might depolarize the membrane well past threshold, will not make a “bigger” action potential. Action potentials are “all or none.” Either the membrane reaches the threshold and everything occurs as described above, or the membrane does not reach the threshold and nothing else happens. All action potentials peak at the same voltage (+30 mV), so one action potential is not bigger than another. Stronger stimuli will initiate multiple action potentials more quickly, but the individual signals are not bigger. Thus, for example, you will not feel a greater sensation of pain, or have a stronger muscle contraction, because of the size of the action potential because they are not different sizes. Only a change in action potential frequency allows us to perceive stimuli in terms of magnitude.

As we have seen, the depolarization and repolarization of an action potential are dependent on two types of channels (the voltage-gated Na + channel and the voltage-gated K + channel). The voltage-gated Na + channel actually has two gates. One is the activation gate, which opens when the membrane potential reaches -55 mV. The other gate is the inactivation gate, which closes after a specific period of time—on the order of a fraction of a millisecond. When a cell is at rest, the activation gate is closed and the inactivation gate is open. However, when the threshold is reached, the activation gate opens, allowing Na+ to rush into the cell. Timed with the peak of depolarization , the inactivation gate closes. Once this occurs, the channel is said to be "inactivated". Think of this as a locked door. During repolarization, no more sodium can enter the cell. When the membrane potential returns to -55 mV, the activation gate closes. After that, the inactivation gate re-opens, making the channel ready to start the whole process over again. Think of this as a door being closed, but unlocked.

The voltage-gated K + channel has only one gate, which is sensitive to a membrane voltage of -50 mV. However, it does not open or close as quickly as the voltage-gated Na + channel does. It might take a fraction of a millisecond for the channel to open once that voltage has been reached. The timing of this coincides exactly with when the Na + flow peaks, so voltage-gated K + channels open just as the voltage-gated Na + channels are being inactivated. As the membrane potential repolarizes and the voltage passes -50 mV again, the channel closes—again, with a little delay. Potassium continues to leave the cell for a short while and the membrane potential becomes more negative than it was at rest, resulting in the hyperpolarizing overshoot. Then the K + channel closes again and the membrane can return to the resting potential due to the ongoing activity of the non-gated channels and the Na + /K + pump. All of this takes place within approximately 2 milliseconds (Figure 15.36). While an action potential is in progress, another one cannot be initiated. That effect is referred to as the refractory period .

The action potential is initiated at the beginning of the axon, at what is called the initial segment, or axon hillock. There is a high density of voltage-gated Na + channels so that rapid depolarization can take place here. Going down the length of the axon, the action potential is propagated because more voltage-gated Na + channels are opened as the depolarization spreads. This spreading occurs because Na + enters through its channel and passively spreads along the inside of the cell membrane. As the Na+ moves, or flows, a short distance along the cell membrane, its positive charge depolarizes a little more of the cell membrane. As that depolarization spreads, new voltage-gated Na + channels open and more ions rush into the cell, spreading the depolarization a little farther (Figure 15.37).

Because voltage-gated Na + channels are inactivated at the peak of the depolarization, they cannot be opened again for a brief time—the absolute refractory period. Because of this, depolarization spreading back toward previously opened channels has no effect. The action potential must propagate toward the axon terminals; as a result, the polarity of the neuron is maintained, as mentioned above. Thus, the absolute refractory period ensures that the action potential can only propagate towards the axon terminal.

Propagation, as described above, applies to unmyelinated axons. When myelination is present, the action potential propagates differently (Figure 15.38). Sodium ions that enter the cell at the initial segment start to spread along the length of the axon segment , but there are no voltage-gated Na + channels until the first node of Ranvier . Because there is not constant opening of these channels along the axon segment, the depolarization spreads at an optimal speed. The distance between nodes (1-3 mm) is the optimal distance to keep the membrane still depolarized above threshold at the next node. As Na + spreads along the inside of the membrane of the axon segment, the charge starts to dissipate. If the node were any farther down the axon, that depolarization would have fallen off too much for voltage-gated Na + channels to be activated at the next node of Ranvier . If the nodes were any closer together, the speed of propagation would be slower.

Propagation along an unmyelinated axon is referred to as continuous conduction ; along the length of a myelinated axon, it is saltatory conduction . Continuous conduction is slow because there are always voltage-gated Na + channels opening, and more and more Na + is rushing into the cell. Saltatory conduction is faster because the action potential basically jumps from one node to the next (saltare = “to leap”), and the new influx of Na + renews the depolarized membrane. Along with the myelination of the axon, the diameter of the axon can influence the speed of conduction. Consider the speed of water through a fire hose compared to that of an ordinary garden hose. Similarly, Na + -based depolarization spreads faster down a wide axon than down a narrow one. This concept is known as internal resistance and is generally true for electrical wires or plumbing, just as it is true for axons.

The electrical changes taking place within a neuron, as described in the previous section, are similar to a light switch being turned on. A stimulus starts the depolarization, but the action potential runs on its own once a threshold has been reached. The question is now, “What flips the light switch on?” Temporary changes to the cell membrane voltage can result from neurons receiving information from the environment, or from the action of one neuron to another. These special types of potentials influence a neuron and determine whether an action potential will occur. Many of these transient signals originate at the synapse , the connection between electrically active cells.

There are two types of synapses: chemical synapses and electrical synapses. In a chemical synapse, a chemical signal—namely, a neurotransmitter —is released from one cell and it causes a change in the other cell. In an electrical synapse, there is a direct connection between the two cells so that ions can pass directly from one cell to the next. If one cell is depolarized in an electrical synapse, the joined cell also depolarizes because the ions pass between the cells. Think about two rooms of a hotel suite connected by a door that is always open. Chemical synapses involve the transmission of chemical information from one cell to the next. This section will concentrate on the chemical synapse.

An example of a chemical synapse is the neuromuscular junction described in Chapter 14, Muscle Physiology. In the nervous system, there are many more synapses that are essentially the same as the neuromuscular junction. All synapses have common characteristics, which can be summarized in this list:

• presynaptic element
• neurotransmitter (packaged in vesicles)
• synaptic cleft
• receptor proteins
• postsynaptic element
• neurotransmitter elimination or re-uptake

Synaptic transmission (or neurotransmission) takes place through the following steps (Figure 15.39):

• An action potential reaches the axon terminal .
• The change in voltage causes voltage-gated Ca 2+ channels in the membrane of the synaptic end bulb to open.
• The concentration of Ca 2+ increases inside the end bulb, and Ca 2+ ions associate with proteins on the outer surface of neurotransmitter vesicles, facilitating the merging of the vesicles with the presynaptic membrane. The neurotransmitter is then released through exocytosis into the small gap between the cells, known as the synaptic cleft .
• Once in the synaptic cleft, the neurotransmitter diffuses the short distance to the postsynaptic membrane and can interact with neurotransmitter receptors. Receptors are specific for the neurotransmitter, and the two fit together like a key and lock. Each neurotransmitter binds to its receptor and will not bind to receptors for other neurotransmitters, making the binding a specific chemical event.
• The interaction of the neurotransmitter with the receptor can result in depolarization or hyperpolarization of the postsynaptic cell membrane, leading to excitation of the postsynaptic cell (and possibly the generation of a new action potential) or inhibition, respectively. The terms excitation and inhibition describe the likelihood that the change in membrane potential will produce an action potential.  Thus, positive changes bring the membrane potential closer to threshold, making an action potential more likely, and are referred to as excitatory. Inhibitory changes do the opposite.
• The neurotransmitter is removed from the synaptic cleft by diffusion , due to the action of enzymes that chemically break it down, or by transporters in the presynaptic cell membrane.

Neurotransmitter Systems

There are several systems of neurotransmitters that are found at various synapses in the nervous system (Figure 15.38).  In this course, you are not required to know all the neurotransmitters, but only to be able to provide one example of a neurotransmitter from each of the systems below.

• Amino acids : This includes glutamate (Glu), GABA (gamma-aminobutyric acid, a derivative of glutamate), and glycine (Gly).
• B iogenic amines : This is a group of neurotransmitters that are enzymatically made from amino acids. For example, the neurotransmitter serotonin is made from tryptophan. Other biogenic amines are made from tyrosine, and include dopamine, norepinephrine , and epinephrine . The chemical epinephrine (epi- = “on”; “-nephrine” = kidney) is also known as adrenaline (renal = “kidney”), and norepinephrine is sometimes referred to as noradrenaline. The adrenal gland produces epinephrine and norepinephrine to be released into the blood stream as hormones.
• C holinergic system : This is the system based on acetylcholine . This includes the neuromuscular junction as an example of a cholinergic synapse, but cholinergic synapses are also found in other parts of the nervous system. They are located in the autonomic nervous system , as well as distributed throughout the brain.
• N europeptide s: These are neurotransmitter molecules made up of chains of amino acids connected by peptide bonds . This is what a protein is, but the term protein implies a certain length to the molecule. Some neuropeptides are quite short, such as met-enkephalin, which is five amino acids long. Others are long, such as beta-endorphin, which is 31 amino acids long. Neuropeptides are often released at synapses in combination with another neurotransmitter, and they often act as hormones in other systems of the body, such as vasoactive intestinal peptide (VIP) or substance P.

The effect of a neurotransmitter on the postsynaptic element is entirely dependent on the receptor protein. First, if there is no receptor protein in the membrane of the postsynaptic element, then the neurotransmitter has no effect. The depolarizing or hyperpolarizing effect is also dependent on the receptor. For example, when acetylcholine binds to a type of receptor called nicotinic receptor, the postsynaptic cell is depolarized. This is because the receptor is a cation channel and positively charged Na + will rush into the cell. However, when acetylcholine binds to another type of receptor called muscarinic receptor, of which there are several variants, it might cause depolarization or hyperpolarization of the target cell.

On the other hand, the amino acid neurotransmitters, glutamate, glycine, and GABA, are almost exclusively associated with just one effect. Glutamate is considered an excitatory amino acid, but only because Glu receptors in the adult cause depolarization of the postsynaptic cell. Glycine and GABA are considered inhibitory amino acids, again because their receptors cause hyperpolarization. .

For the following questions, click the correct answer choice.

Figure 15.1 Image description:

The nervous system is comprised of two major parts, or subdivisions, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord. The brain is the body’s “control center.” The CNS has various centers located within it that carry out the sensory, motor and integration of data. These centers can be subdivided to Lower Centers (including the spinal cord and brain stem) and Higher centers communicating with the brain via effectors.

The PNS is a vast network of spinal and cranial nerves that are linked to the brain and the spinal cord. It contains sensory receptors which help in processing changes in the internal and external environment. This information is sent to the CNS via afferent sensory nerves. The PNS is then subdivided into the autonomic nervous system and the somatic nervous system. The autonomic has involuntary control of internal organs, blood vessels, smooth and cardiac muscles. The somatic has voluntary control of skin, bones, joints, and skeletal muscle. The two systems function together, by way of nerves from the PNS entering and becoming part of the CNS, and vice versa.

Figure 15.5 Image description: Brain (CNS) is responsible for perception and processing of sensory stimuli (sensory/autonomic), execution of voluntary motor responses (sensory), and regulation of homeostatic mechanisms (autonomic). Nerves (PNS) contain fibers of sensory and motor neurons (sensory/autonomic). The enteric system, located in the digestive tract, (ENS) is responsible for autonomous functions and can operate independently of the brain and spinal cord. Spinal Cord (CNS) contains initiation of reflexes from the ventral horn (somatic) and lateral horn (autonomic) gray matter and pathways for sensory and motor functions between periphery and brain (somatic/autonomic). Ganglia (PNS) is responsible for reception of sensory stimuli by dorsal and cranial ganglia (sensory/somatic) and relay of motor responses by autonomic ganglia (autonomic).

Figure 15.6 Image description: Meningeal Layers of Superior Sagittal Sinus  The layers of the meninges in the longitudinal fissure of the superior sagittal sinus are shown, with the dura mater adjacent to the inner surface of the cranium, the pia mater adjacent to the surface of the brain, and the arachnoid and subarachnoid space between them. An arachnoid villus is shown emerging into the dural sinus to allow CSF to filter back into the blood for drainage .

Figure 15.7 Image description: Cerebrospinal fluid (CSF) is a clear, colorless bodily fluid that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord. The CSF occupies the space between the arachnoid mater (the middle layer of the brain cover, the meninges ) and the pia mater (the layer of the meninges closest to the brain). It constitutes the content of all intracerebral ventricles, cisterns, and sulci (singular sulcus), as well as the central canal of the spinal cord. It acts as a cushion or buffer for the cortex, providing a basic mechanical and immunological protection for the brain inside the skull and serving a vital function in cerebral autoregulation of cerebral blood flow .

Figure 15.8 Image description: The dorsal root ganglia are collections of the cell bodies of sensory neurons located lateral to the spinal cord emerging at each spinal level. They are responsible for conveying various sensory stimuli, including pain, touch, vibration, proprioception, and temperature, from the peripheral nervous system to the central nervous system.

Figure 15.9 Image description: Knee-jerk reflex, sudden kicking movement of the lower leg in response to a sharp tap on the patellar tendon, which lies just below the kneecap. One of the several positions that a subject may take for the test is to sit with knees bent and with one leg crossed over the other so that the upper foot hangs clear of the floor. The sharp tap on the tendon slightly stretches the quadriceps, the complex of muscles at the front of the upper leg. In reaction these muscles contract, and the contraction tends to straighten the leg in a kicking motion. Exaggeration or absence of the reaction suggests that there may be damage to the central nervous system. The knee jerk can also be helpful in recognizing thyroid disease.

Figure 15.10 Image description: To respond to a threat—to fight or to run away—the sympathetic system causes diverse effects as many different effector organs are activated together for a common purpose. More oxygen needs to be inhaled and delivered to skeletal muscle. The respiratory, cardiovascular, and musculoskeletal systems are all activated together. Additionally, sweating keeps the excess heat that comes from muscle contraction from causing the body to overheat. The digestive system shuts down so that blood is not absorbing nutrients when it should be delivering oxygen to skeletal muscles. To coordinate all these responses, the connections in the sympathetic system diverge from a limited region of the central nervous system (CNS) to a wide array of ganglia that project to the many effector organs simultaneously. The complex set of structures that compose the output of the sympathetic system make it possible for these disparate effectors to come together in a coordinated, systemic change. The sympathetic division of the autonomic nervous system influences various organ systems of the body through connections emerging from the first thoracic (T1) and second lumbar (L2) spinal segments . It is referred to as the thoracolumbar system to reflect this anatomical basis. Sympathetic preganglionic neurons are located in the lateral horns of any of these spinal regions and project to ganglia adjacent to the vertebral column through the ventral roots of the spinal cord.

Figure 15.11 Image description: The parasympathetic nervous system (PSNS) is a division of the autonomic nervous system (ANS) that controls the activity of the smooth and cardiac muscles and glands.  It works in synergy with the  sympathetic nervous system (SNS) , which complements the PSNS activity. The parasympathetic nervous system is also called the  craniosacral division of the ANS, as its central nervous system components are located within the  brain  and the sacral portion of the  spinal cord . The  functions  of the PNS are commonly described as the  “rest and digest”  response, since it is involved in slowing down the heart rate, relaxing the sphincter muscles in the gastrointestinal and urinary tracts and increasing intestinal and gland activity. The final result is conserving energy and regulating basic bodily functions such as digestion and urination. It is contrasted to the sympathetic nervous system, which is described as the “fight and flight” response that occurs in stressful situations and has mainly opposite functions.

Figure 15.15 Image description: Proteins that extend all the way across the membrane are called transmembrane proteins. The portions of an integral membrane protein found inside the membrane are hydrophobi c , while those that are exposed to the cytoplasm or extracellular fluid tend to be hydrophilic.

Figure 15.16 Image description: The prototypic ligand-gated ion channel is the nicotinic acetylcholine receptor. It consists of a pentamer of protein subunits (typically ααβγδ), with two binding sites for acetylcholine (one at the interface of each alpha subunit). Allow sodium, potassium, and calcium to cross.

Figure 15.17 Image description: Mechanically-gated ion channels are proteins found in eukaryotic and prokaryotic cell membranes that open in response to mechanical stress. Tension, compression, swelling, and shear stress can alter the conformation of the protein, opening a transmembrane channel that allows the passage of ions for signal transmission. In eukaryotes, mechanically-gated channels are distributed in several regions like the neurons, lungs, skin, bladder, and heart, where they play critical roles in numerous physiological and pathophysiological processes.

Figure 15.18 Image description: Voltage-gated ion channel s  are a class of transmembrane proteins that form ion channels that are activated by changes in the electrical membrane potential near the channel. The membrane potential alters the conformation of the channel proteins, regulating their opening and closing. Cell membranes are generally impermeable to ions, thus they must diffuse through the membrane through transmembrane protein channels. They have a crucial role in excitable cells such as neuronal and muscle tissues, allowing a rapid and coordinated depolarization in response to triggering voltage change. Found along the axon and at the synapse, voltage-gated ion channels directionally propagate electrical signals. Voltage-gated ion-channels are usually ion-specific, and channels specific to sodium (Na + ), potassium (K + ), calcium (Ca 2+ ), and chloride (Cl − ) ions have been identified. The opening and closing of the channels are triggered by changing ion concentration, and hence charge gradient, between the sides of the cell membrane.

Figure 15.19 Image description: Leakage channels are the simplest type of ion channel, in that their permeability is more or less constant. The types of leakage channels with the greatest significance in neurons are potassium and chloride channels. Although they are the simplest in theory, most conduct better in one direction than the other (they are rectifiers) and some are capable of being shut off by ligands even though they do not require ligands in order to operate.

Figure 15.20 Image description: Measuring Charge across a Membrane with a Voltmeter   A recording electrode is inserted into the cell and a reference electrode is outside the cell. By comparing the charge measured by these two electrodes, the transmembrane voltage is determined. It is conventional to express that value for the cytosol relative to the outside.

Figure 15.21 Image description: What has been described here is the action potential, which is presented as a graph of voltage over time . It is the electrical signal that nervous tissue generates for communication. The change in the membrane voltage from −70 mV at rest to +30 mV at the end of depolarization is a 100-mV change. That can also be written as a 0.1-V change. To put that value in perspective, think about a battery. An AA battery that you might find in a television remote has a voltage of 1.5 V, or a 9-V battery (the rectangular battery with two posts on one end) is, obviously, 9 V. The change seen in the action potential is one or two orders of magnitude less than the charge in these batteries. In fact, the membrane potential can be described as a battery. A charge is stored across the membrane that can be released under the correct conditions. A battery in your remote has stored a charge that is “released” when you push a button.

Figure 15.22 Image description: An action potential has three phases: depolarization, overshoot, repolarization. There are two more states of the membrane potential related to the action potential. The first one is hypopolarization which precedes the depolarization, while the second one is hyperpolarization, which follows the repolarization.

Figure 15.23 Image description: Action potentials in neurons that lack myelin sheaths travel much more slowly  than action potentials in equivalent neurons sheathed in myelin. The speed of action potentials is also dependent on the diameter of the axon. Wider axons have lower resistance than narrow axons and signals can travel faster in large axons.

Figure 15.24 Image description: When myelination is present, the action potential propagates differently. Sodium ions that enter the cell at the initial segment start to spread along the length of the axon segment, but there are no voltage-gated Na +  channels until the first node of Ranvier. Because there is not constant opening of these channels along the axon segment, the depolarization spreads at an optimal speed. The distance between nodes is the optimal distance to keep the membrane still depolarized above threshold at the next node. As Na +  spreads along the inside of the membrane of the axon segment, the charge starts to dissipate. If the node were any farther down the axon, that depolarization would have fallen off too much for voltage-gated Na +  channels to be activated at the next node of Ranvier. If the nodes were any closer together, the speed of propagation would be slower.

Figure 15.25 image description:   Communication at a chemical synapse: Communication at chemical synapses requires release of neurotransmitters. When the presynaptic membrane is depolarized, voltage-gated Ca2+ channels open and allow Ca2+ to enter the cell. The calcium entry causes synaptic vesicles to fuse with the membrane and release neurotransmitter molecules into the synaptic cleft. The neurotransmitter diffuses across the synaptic cleft and binds to ligand-gated ion channels in the postsynaptic membrane, resulting in a localized depolarization or hyperpolarization of the postsynaptic neuron. [Return to Figure 15.37.]

Layer of dense irregular connective tissue surrounding cartilage.

cartilage cells

Region of bone development in the epiphyses.

Region of the epiphyseal plate that anchors the plate to the osseous tissue of the epiphysis.

Region of the epiphyseal plate that makes new chondrocytes to replace those that die at the diaphyseal end of the plate and contributes to longitudinal growth of the epiphyseal plate.

Region of the epiphyseal plate where chondrocytes from the proliferative zone grow and mature and contribute to the longitudinal growth of the epiphyseal plate.

Region of the epiphyseal plate closest to the diaphyseal end; functions to connect the epiphyseal plate to the diaphysis.

(In connective tissue) extracellular material which is produced by the cells embedded in it, containing ground substance and fibres.

Central, vertical axis of the body, including the skull, vertebral column, and thoracic cage.

Entire sequence of bones that extend from the skull to the tailbone.

Consists of 12 pairs of ribs and sternum.

Small, U-shaped bone located in upper neck that does not contact any other bone.

Three small bones in the middle ear.

Individual bone in the neck and back regions of the vertebral column.

Single bone located near the inferior end of the adult vertebral column that is formed by the fusion of five sacral vertebrae; forms the posterior portion of the pelvis.

Lowest part of the vertebral column; 'tailbone'

Describes the front or direction toward the front of the body; also referred to as ventral.

Fourteen bones that support the facial structures and form the upper and lower jaws and the hard palate.

Unpaired bone that forms the lower jaw bone; the only moveable bone of the skull.

Expanded area of fibrous connective tissue that separates the brain case bones of the skull prior to birth and during the first year after birth.

Fibrous joint that connects the bones of the skull (except the mandible); an immobile joint (synarthrosis).

Division of the posterior (dorsal) cavity that houses the brain.

Paired bones that form the upper, lateral sides of the skull.

Paired bones that form the lateral, inferior portions of the skull, with squamous, mastoid, and petrous portions.

Unpaired bone that forms forehead, roof of orbit, and floor of anterior cranial fossa.

Unpaired bone that forms the posterior portions of the brain case and base of the skull.

Unpaired bone that forms the central base of skull.

Unpaired bone that forms the roof and upper, lateral walls of the nasal cavity, portions of the floor of the anterior cranial fossa and medial wall of orbit, and the upper portion of the nasal septum.

(Also, maxilla) paired bones that form the upper jaw and anterior portion of the hard palate.

anterior portion of each vertebra that supports the body weight.

Bony arch formed by the posterior portion of each vertebra that surrounds and protects the spinal cord.

Opening associated with each vertebra defined by the vertebral arch that provides passage for the spinal cord.

Bony passageway within the vertebral column for the spinal cord that is formed by the series of individual vertebral foramina.

Paired bony processes that extends laterally from the vertebral arch of a vertebra.

Unpaired bony process that extends posteriorly from the vertebral arch of a vertebra.

Bony process that extends upward from the vertebral arch of a vertebra that articulates with the inferior articular process of the next higher vertebra.

Bony process that extends downward from the vertebral arch of a vertebra that articulates with the superior articular process of the next lower vertebra.

First cervical (C1) vertebra.

Second cervical (C2) vertebra.

Bony projection (odontoid process) that extends upward from the body of the C2 (axis) vertebra.

Expanded, superior portion of the sternum.

Small process that forms the inferior tip of the sternum.

Hyaline cartilage structure attached to the anterior end of each rib that provides for either direct or indirect attachment of most ribs to the sternum.

Vertebrosternal ribs 1–7 that attach via their costal cartilage directly to the sternum.

Vertebrochondral ribs 8–12 whose costal cartilage either attaches indirectly to the sternum via the costal cartilage of the next higher rib or does not attach to the sternum at all.

Vertebral ribs 11–12 that do not attach to the sternum or to the costal cartilage of another rib.

All bones of the upper and lower limbs, plus the girdle bones that attach each limb to the axial skeleton.

An encircling or confining structure; in anatomy, the pectoral or pelvic girdle.

Shoulder girdle; the set of bones, consisting of the scapula and clavicle, which attaches each upper limb to the axial skeleton.

Hip girdle; consists of a single hip bone, which attaches a lower limb to the sacrum of the axial skeleton.

Describes the middle or direction toward the middle of the body.

Describes the back or direction toward the back of the body; also referred to as dorsal.

Describes the side or direction toward the side of the body.

(Also, glenoid fossa) shallow depression located on the lateral scapula, between the superior and lateral borders.

Shoulder joint; formed by the articulation between the glenoid cavity of the scapula and the head of the humerus.

Describes a position in a limb that is nearer to the point of attachment or the trunk of the body.

Wrist joint, located between the forearm and hand regions of the upper limb; articulation formed proximally by the distal end of the radius and the fibrocartilaginous pad that unites the distal radius and ulna bone, and distally by the scaphoid, lunate, and triquetrum carpal bones.

Describes a position in a limb that is farther from the point of attachment or the trunk of the body.

Articulation between adjacent phalanx bones of the hand or foot digits.

Superior portion of the hip bone.

Anterior portion of the hip bone.

Joint formed by the articulation between the auricular surfaces of the sacrum and ilium.

Joint formed by the articulation between the pubic bodies of the right and left hip bones.

Large, roughened protuberance that forms the posteroinferior portion of the hip bone; weight-bearing region of the pelvis when sitting.

Large, U-shaped indentation located on the posterior margin of the ilium, superior to the ischial spine.

A bone embedded in tendon; the only major sesamoid bone is the patella.

Muscle deep to the gluteus maximus on the lateral surface of the thigh that laterally rotates the femur at the hip.

Tarsal bone that articulates superiorly with the tibia and fibula at the ankle joint; also articulates inferiorly with the calcaneus bone and anteriorly with the navicular bone.

Heel bone; posterior, inferior tarsal bone that forms the heel of the foot.

Human Anatomy and Physiology I Copyright © 2024 by Priscilla Stewart is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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6.1 The Functions of the Skeletal System

Learning objectives.

By the end of this section, you will be able to:

List and describe the functions of the skeletal system

• Attribute specific functions of the skeletal system to specific components or structures

The skeletal system is the body system composed of bones, cartilages, ligaments and other tissues that perform essential functions for the human body. Bone tissue, or osseous tissue , is a hard, dense connective tissue that forms most of the adult skeleton, the internal support structure of the body. In the areas of the skeleton where whole bones move against each other (for example, joints like the shoulder or between the bones of the spine), cartilages, a semi-rigid form of connective tissue, provide flexibility and smooth surfaces for movement. Additionally, ligaments composed of dense connective tissue surround these joints, tying skeletal elements together (a ligament is the dense connective tissue that connect bones to other bones). Together, they perform the following functions:

Support, Movement, and Protection

Some functions of the skeletal system are more readily observable than others. When you move you can feel how your bones support you, facilitate your movement, and protect the soft organs of your body. Just as the steel beams of a building provide a scaffold to support its weight, the bones and cartilages of your skeletal system compose the scaffold that supports the rest of your body. Without the skeletal system, you would be a limp mass of organs, muscle, and skin. Bones facilitate movement by serving as points of attachment for your muscles. Bones also protect internal organs from injury by covering or surrounding them. For example, your ribs protect your lungs and heart, the bones of your vertebral column (spine) protect your spinal cord, and the bones of your cranium (skull) protect your brain (see Figure 6.1.1 ).

Mineral and Fat Storage, Blood Cell Formation

On a metabolic level, bone tissue performs several critical functions. For one, the bone tissue acts as a reservoir for a number of minerals important to the functioning of the body, especially calcium, and phosphorus. These minerals, incorporated into bone tissue, can be released back into the bloodstream to maintain levels needed to support physiological processes. Calcium ions, for example, are essential for muscle contractions and are involved in the transmission of nerve impulses.

Bones also serve as a site for fat storage and blood cell production. The unique connective tissue that fills the interior of most bones is referred to as bone marrow . There are two types of bone marrow: yellow bone marrow and red bone marrow. Yellow bone marrow contains adipose tissue, and the triglycerides stored in the adipocytes of this tissue can be released to serve as a source of energy for other tissues of the body. Red bone marrow is where the production of blood cells (named hematopoiesis, hemato- = “blood”, -poiesis = “to make”) takes place. Red blood cells, white blood cells, and platelets are all produced in the red bone marrow. As we age, the distribution of red and yellow bone marrow changes as seen in the figure ( Figure 6.1.2 ).

Career Connection – Orthopedist

An orthopedist is a doctor who specializes in diagnosing and treating disorders and injuries related to the musculoskeletal system. Some orthopedic problems can be treated with medications, exercises, braces, and other devices, but others may be best treated with surgery ( Figure 6.1.3 ).

While the origin of the word “orthopedics” (ortho- = “straight”; paed- = “child”), literally means “straightening of the child,” orthopedists can have patients who range from pediatric to geriatric. In recent years, orthopedists have even performed prenatal surgery to correct spina bifida, a congenital defect in which the neural canal in the spine of the fetus fails to close completely during embryologic development.

Orthopedists commonly treat bone and joint injuries but they also treat other bone conditions including curvature of the spine. Lateral curvatures (scoliosis) can be severe enough to slip under the shoulder blade (scapula) forcing it up as a hump. Spinal curvatures can also be excessive dorsoventrally (kyphosis) causing a hunch back and thoracic compression. These curvatures often appear in preteens as the result of poor posture, abnormal growth, or indeterminate causes. Mostly, they are readily treated by orthopedists. As people age, accumulated spinal column injuries and diseases like osteoporosis can also lead to curvatures of the spine, hence the stooping you sometimes see in the elderly.

Some orthopedists sub-specialize in sports medicine, which addresses both simple injuries, such as a sprained ankle, and complex injuries, such as a torn rotator cuff in the shoulder. Treatment can range from exercise to surgery.

Section Review

The major functions of the skeletal system are body support, facilitation of movement, protection of internal organs, storage of minerals and fat, and blood cell formation.

Review Questions

Critical thinking questions.

• Suppose your red bone marrow could not be formed. What functions would your body not be able to perform?
• Suppose your osseous tissue could not store calcium. What functions would your body not be able to perform?

Answers for Critical Thinking Questions

• Without red bone marrow, you would not be able to produce blood cells. The red bone marrow is responsible for forming red and white blood cells as well as platelets. Red blood cells transport oxygen to tissues, and remove carbon dioxide. Without red blood cells, your tissues would not be able to produce ATP using oxygen. White blood cells play a role in the immune system fighting off foreign invaders in our body – without white blood cells you would not be able to recover from infection. Platelets are responsible for clotting your blood when a vessel ruptures. Without platelets you would bleed to death and die.
•  The calcium in osseous tissue provides mineral support to bones. Without this calcium, the bones are not rigid and cannot be supportive. The calcium in osseous tissue is also an important storage site, that can release calcium when needed. Other organ systems rely on this calcium for action (specifically, muscle contraction and neural signaling). Without calcium storage, blood calcium levels change dramatically and affect muscle contraction and neural signaling.

This work, Anatomy & Physiology, is adapted from Anatomy & Physiology by OpenStax , licensed under CC BY . This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

Images, from Anatomy & Physiology by OpenStax , are licensed under CC BY except where otherwise noted.

Access the original for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction .

Anatomy & Physiology Copyright © 2019 by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU, and OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License , except where otherwise noted.

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The Anatomy of the Skeletal System

The skeletal system comprises 206 bones and has two main parts—the axial skeleton and the appendicular skeleton. The skeletal system includes your bones, ligaments that attach bone to bone, and cartilage that provides padding between your bones.

This article discusses the anatomy of the skeletal system—what it's made of, how it's organized, conditions that affect it, and tests that assess it.

SDI Productions / Getty Imgaes

Skeletal System: Labeled Diagram of Major Organs

In addition to the bones, organs of the skeletal system include ligaments that attach bones to other bones and cartilage that provides padding between bones that form joints throughout your body.

The bones are divided into two main categories—the axial skeleton , which contains the bones that support the middle of your body, and the a ppendicular skeleton, which includes bones that make up your appendages—arms and legs—and bones that attach your limbs to your axial skeleton.

Axial Skeleton

The axial skeleton forms the "axis" that runs down the center of the body. There are 80 bones that make up the axial skeleton.

Skull (Cranium)

Your skull ( cranium ) is made up of cranial and facial bones. Cranial bones protect your brain, while facial bones make up your facial structure. Skull bones include the following:

Cranial bones include:

Facial bones include:

• Maxillae (upper jaw)
• Mandible (lower jaw)
• Zygomatic (cheekbones)
• Inferior nasal conchae

Auditory Ossicles

The auditory ossicles consist of a total of six tiny bones, with three in each ear. They are located in the inner ear and are structures that help create sound in your body. The auditory ossicles include the following:

The hyoid bone is a horseshoe-shaped bone located in the throat. It is part of bodily functions like speaking, swallowing, and airway maintenance.

Vertebral column

The vertebral column (spine) protects your spinal cord, supports your head, and allows bodily movement. It contains the sacrum (made up of four bones) and coccyx (the tailbone, which is made up of five bones), and 24 vertebrae, including:

• Cervical vertebrae : Seven bones in the neck region
• Thoracic vertebrae : Twelve bones attached to the ribs
• Lumbar vertebrae : Five bones in the low back region

The thorax contains the sternum (breastbone) and the thoracic (rib) cage . The thoracic cage comprises 12 pairs of ribs connecting to the thoracic vertebrae and the sternum. Your rib cage protects your heart.

Appendicular Skeleton

The appendicular skeleton includes 126 bones that comprise your appendages—your arms and legs—and the bones that attach your limbs to your axial skeleton.

Upper Extremities

Your upper extremities refer to your shoulders and arms. Bones in the upper extremities include:

• Scapula (shoulder blade)
• Clavicle (collarbone)
• Humerus (upper arm)
• Radius and ulna (forearm bones)
• Carpals (eight tiny bones in the wrist)
• Metacarpals (in the palm)
• Phalanges (bones of the fingers)

Lower Extremities

Bones in the lower extremities make up your hips and legs and include:

• Femur (thigh bone)
• Patella (kneecap)
• Tibia and fibula (lower leg bones)
• Tarsals (eight tiny bones in the ankle)
• Metatarsals (in the middle of the foot)
• Phalanges (bones of the toes)

Which Bones Are Most Commonly Broken?

The most commonly fractured bones include the distal radius (on the thumb side of your wrist), the ankle, the femur (thigh bone), the humerus (upper arm bone), and the metacarpals (bones of the palms).

With osteoporosis, the most commonly fractured bones are the vertebrae (in the spine).

What Is the Purpose of the Skeletal System?

The primary purpose of the skeletal system is to give the body its shape and to provide attachment points for the muscles that move the body.

Other purposes of the skeletal system include:

• Storing minerals (such as calcium) and fats
• Producing red blood cells
• Protecting internal organs

Calcium in Your Bones

Most of the body's calcium is stored in your bones.

Skeletal System Associated Conditions

Various conditions and injuries can affect the skeletal system. Examples include:

• Fractures (broken bones)
• Ligament sprains
• Osteoarthritis
• Osteoporosis
• Osteogenesis imperfecta
• Osteomalacia
• Paget's disease of bone
• Osteomyelitis
• Avascular necrosis (osteonecrosis)
• Marfan syndrome
• Ehlers-Danlos syndrome
• Osteopetrosis
• Ankylosing spondylitis
• Bone marrow diseases

Skeletal System Tests

Many different tests can help diagnose conditions that affect the skeletal system.

Imaging Tests

Healthcare providers use various imaging tools to get detailed pictures of your bones. Depending on the reason for imaging, a healthcare provider will conduct one or more of the following tests:

• X-rays : This common test can help diagnose conditions that affect the bones and joints, such as fractures or arthritis.
• Computed tomography (CT scan), computerized axial tomography (CAT scan) : This test provides three-dimensional pictures that help diagnose fractures that aren't clear on X-rays, or other bone conditions, such as cancer.
• Magnetic resonance imaging (MRI) : This type of imaging often helps diagnose conditions that affect soft tissues of the skeletal system (ligaments, cartilage).
• Bone scintigraphy (bone scan) : These scans can provide detailed information about a bone injury or condition, such as the staging of bone cancer.
• Positron emission tomography (PET scan) : This test, which uses an injected radioactive tracer, can help stage bone (and other types of) cancer.
• Bone density test : These tests are primarily for determining how dense bones are—the key factor in diagnosing osteoporosis.

Other Skeletal System Tests

Healthcare providers may perform additional tests if they need further information about your bones or skeletal system. These tests may include:

• Joint aspiration : This test involves removing a sample of fluid from a joint to help diagnose infection.
• Biopsy : For the skeletal system, this procedure can involve removing a small sample of bone or bone marrow so that it can be tested for conditions such as cancer.
• Blood tests : These tests help diagnose infections that can affect the skeletal system.

The skeletal system is made up of your bones, ligaments, and cartilage. Though its main function is to provide structural support for the body, it also stores important minerals—such as calcium—forms red blood cells, and protects your internal organs. The skeletal system can break down into two main categories—the axial skeleton, which forms the "long axis" of the body, and the appendicular skeleton, which forms your arms and legs.

Many different injuries and diseases can affect the skeletal system. Imaging procedures, such as X-rays, MRI, and bone density tests, and other tests, such as blood work or tissue biopsy, can help to diagnose these conditions.

National Cancer Institute. Cranium .

National Cancer Institute SEER Training Modules. Axial skeleton (80 bones) .

Fisher E, Austin D, Werner HM, et al. Hyoid bone fusion and bone density across the lifespan: prediction of age and sex .  Forensic Sci Med Pathol . 2016;12(2):146-157. doi:10.1007/s12024-016-9769-x

Osmosis from Elsevier. Bones of the vertebral column .

National Cancer Institute SEER Training Modules. Appendicular skeleton (126 bones) .

American Academy of Orthopaedic Surgeons. Osteoporosis and spinal fractures .

National Cancer Institute SEER Training Modules. Introduction to the skeletal system .

MedlinePlus. Calcium and bones .

National Institute of Arthritis and Musculoskeletal and Skin Diseases. Muscle and bone diseases .

MedlinePlus. Diagnostic imaging .

InformedHealth.org. Understanding tests used to detect bone problems .

Johns Hopkins Medicine. Joint aspiration .

By Aubrey Bailey, PT, DPT, CHT Aubrey Bailey is a physical therapist and professor of anatomy and physiology with over a decade of experience providing in-person and online education for medical personnel and the general public.

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6.1: The Functions of the Skeletal System

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Learning Objectives

• Define bone, cartilage, and the skeletal system
• List and describe the functions of the skeletal system

Bone , or osseous tissue , is a hard, dense connective tissue that forms most of the adult skeleton, the support structure of the body. In the areas of the skeleton where bones move (for example, the ribcage and joints), cartilage , a semi-rigid form of connective tissue, provides flexibility and smooth surfaces for movement. The skeletal system is the body system composed of bones and cartilage and performs the following critical functions for the human body:

• supports the body
• facilitates movement
• protects internal organs
• produces blood cells
• stores and releases minerals and fat

Support, Movement, and Protection

The most apparent functions of the skeletal system are the gross functions—those visible by observation. Simply by looking at a person, you can see how the bones support, facilitate movement, and protect the human body.

Just as the steel beams of a building provide a scaffold to support its weight, the bones and cartilage of your skeletal system compose the scaffold that supports the rest of your body. Without the skeletal system, you would be a limp mass of organs, muscle, and skin.

Bones also facilitate movement by serving as points of attachment for your muscles. While some bones only serve as a support for the muscles, others also transmit the forces produced when your muscles contract. From a mechanical point of view, bones act as levers and joints serve as fulcrums (Figure \(\PageIndex{1}\)). Unless a muscle spans a joint and contracts, a bone is not going to move. For information on the interaction of the skeletal and muscular systems, that is, the musculoskeletal system, seek additional content.

Bones also protect internal organs from injury by covering or surrounding them. For example, your ribs protect your lungs and heart, the bones of your vertebral column (spine) protect your spinal cord, and the bones of your cranium (skull) protect your brain (Figure \(\PageIndex{2}\)).

CAREER CONNECTION: Orthopedist

An orthopedist is a doctor who specializes in diagnosing and treating disorders and injuries related to the musculoskeletal system. Some orthopedic problems can be treated with medications, exercises, braces, and other devices, but others may be best treated with surgery (Figure \(\PageIndex{3}\)).

While the origin of the word “orthopedics” (ortho- = “straight”; paed- = “child”), literally means “straightening of the child,” orthopedists can have patients who range from pediatric to geriatric. In recent years, orthopedists have even performed prenatal surgery to correct spina bifida, a congenital defect in which the neural canal in the spine of the fetus fails to close completely during embryologic development.

Orthopedists commonly treat bone and joint injuries but they also treat other bone conditions including curvature of the spine. Lateral curvatures (scoliosis) can be severe enough to slip under the shoulder blade (scapula) forcing it up as a hump. Spinal curvatures can also be excessive dorsoventrally (kyphosis) causing a hunch back and thoracic compression. These curvatures often appear in preteens as the result of poor posture, abnormal growth, or indeterminate causes. Mostly, they are readily treated by orthopedists. As people age, accumulated spinal column injuries and diseases like osteoporosis can also lead to curvatures of the spine, hence the stooping you sometimes see in the elderly.

Some orthopedists sub-specialize in sports medicine, which addresses both simple injuries, such as a sprained ankle, and complex injuries, such as a torn rotator cuff in the shoulder. Treatment can range from exercise to surgery.

Mineral Storage, Energy Storage, and Hematopoiesis

On a metabolic level, bone tissue performs several critical functions. For one, the bone matrix acts as a reservoir for a number of minerals important to the functioning of the body, especially calcium, and potassium. These minerals, incorporated into bone tissue, can be released back into the bloodstream to maintain levels needed to support physiological processes. Calcium ions, for example, are essential for muscle contractions and controlling the flow of other ions involved in the transmission of nerve impulses.

Bone also serves as a site for fat storage and blood cell production. The softer connective tissue that fills the interior of most bone is referred to as bone marrow (Figure \(\PageIndex{4}\)). There are two types of bone marrow: yellow marrow and red marrow. Yellow marrow contains adipose tissue; the triglycerides stored in the adipocytes of the tissue can serve as a source of energy. Red marrow is where hematopoiesis —the production of blood cells—takes place. Red blood cells, white blood cells, and platelets are all produced in the red marrow.

Chapter Review

The major functions of the bones are body support, facilitation of movement, protection of internal organs, storage of minerals and fat, and hematopoiesis. Together, the muscular system and skeletal system are known as the musculoskeletal system.

Review Questions

Q. Which function of the skeletal system would be especially important if you were in a car accident?

A. storage of minerals

B. protection of internal organs

C. facilitation of movement

D. fat storage

Q. Bone tissue can be described as ________.

A. dead calcified tissue

B. cartilage

C. the skeletal system

D. dense, hard connective tissue

Q. Without red marrow, bones would not be able to ________.

A. store phosphate

B. store calcium

C. make blood cells

D. move like levers

Q. Yellow marrow has been identified as ________.

A. an area of fat storage

B. a point of attachment for muscles

C. the hard portion of bone

D. the cause of kyphosis

Q. Which of the following can be found in areas of movement?

A. hematopoiesis

C. yellow marrow

D. red marrow

Q. The skeletal system is made of ________.

A. muscles and tendons

B. bones and cartilage

C. vitreous humor

D. minerals and fat

Critical Thinking Questions

The skeletal system is composed of bone and cartilage and has many functions. Choose three of these functions and discuss what features of the skeletal system allow it to accomplish these functions.

It supports the body. The rigid, yet flexible skeleton acts as a framework to support the other organs of the body.

It facilitates movement. The movable joints allow the skeleton to change shape and positions; that is, move.

It protects internal organs. Parts of the skeleton enclose or partly enclose various organs of the body including our brain, ears, heart, and lungs. Any trauma to these organs has to be mediated through the skeletal system.

It produces blood cells. The central cavity of long bones is filled with marrow. The red marrow is responsible for forming red and white blood cells.

It stores and releases minerals and fat. The mineral component of bone, in addition to providing hardness to bone, provides a mineral reservoir that can be tapped as needed. Additionally, the yellow marrow, which is found in the central cavity of long bones along with red marrow, serves as a storage site for fat.

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38.1: Types of Skeletal Systems

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Skills to Develop

• Discuss the different types of skeletal systems
• Explain the role of the human skeletal system
• Compare and contrast different skeletal systems

A skeletal system is necessary to support the body, protect internal organs, and allow for the movement of an organism. There are three different skeleton designs that fulfill these functions: hydrostatic skeleton, exoskeleton, and endoskeleton.

Hydrostatic Skeleton

A hydrostatic skeleton is a skeleton formed by a fluid-filled compartment within the body, called the coelom. The organs of the coelom are supported by the aqueous fluid, which also resists external compression. This compartment is under hydrostatic pressure because of the fluid and supports the other organs of the organism. This type of skeletal system is found in soft-bodied animals such as sea anemones, earthworms, Cnidaria, and other invertebrates (Figure \(\PageIndex{1}\)).

Movement in a hydrostatic skeleton is provided by muscles that surround the coelom. The muscles in a hydrostatic skeleton contract to change the shape of the coelom; the pressure of the fluid in the coelom produces movement. For example, earthworms move by waves of muscular contractions of the skeletal muscle of the body wall hydrostatic skeleton, called peristalsis, which alternately shorten and lengthen the body. Lengthening the body extends the anterior end of the organism. Most organisms have a mechanism to fix themselves in the substrate. Shortening the muscles then draws the posterior portion of the body forward. Although a hydrostatic skeleton is well-suited to invertebrate organisms such as earthworms and some aquatic organisms, it is not an efficient skeleton for terrestrial animals.

Exoskeleton

An exoskeleton is an external skeleton that consists of a hard encasement on the surface of an organism. For example, the shells of crabs and insects are exoskeletons (Figure \(\PageIndex{2}\)). This skeleton type provides defence against predators, supports the body, and allows for movement through the contraction of attached muscles. As with vertebrates, muscles must cross a joint inside the exoskeleton. Shortening of the muscle changes the relationship of the two segments of the exoskeleton. Arthropods such as crabs and lobsters have exoskeletons that consist of 30–50 percent chitin, a polysaccharide derivative of glucose that is a strong but flexible material. Chitin is secreted by the epidermal cells. The exoskeleton is further strengthened by the addition of calcium carbonate in organisms such as the lobster. Because the exoskeleton is acellular, arthropods must periodically shed their exoskeletons because the exoskeleton does not grow as the organism grows.

Endoskeleton

An endoskeleton is a skeleton that consists of hard, mineralized structures located within the soft tissue of organisms. An example of a primitive endoskeletal structure is the spicules of sponges. The bones of vertebrates are composed of tissues, whereas sponges have no true tissues (Figure \(\PageIndex{1}\)). Endoskeletons provide support for the body, protect internal organs, and allow for movement through contraction of muscles attached to the skeleton.

The human skeleton is an endoskeleton that consists of 206 bones in the adult. It has five main functions: providing support to the body, storing minerals and lipids, producing blood cells, protecting internal organs, and allowing for movement. The skeletal system in vertebrates is divided into the axial skeleton (which consists of the skull, vertebral column, and rib cage), and the appendicular skeleton (which consists of the shoulders, limb bones, the pectoral girdle, and the pelvic girdle).

Human Axial Skeleton

The axial skeleton forms the central axis of the body and includes the bones of the skull, ossicles of the middle ear, hyoid bone of the throat, vertebral column, and the thoracic cage (ribcage) (Figure \(\PageIndex{1}\)). The function of the axial skeleton is to provide support and protection for the brain, the spinal cord, and the organs in the ventral body cavity. It provides a surface for the attachment of muscles that move the head, neck, and trunk, performs respiratory movements, and stabilizes parts of the appendicular skeleton.

The bones of the skull support the structures of the face and protect the brain. The skull consists of 22 bones, which are divided into two categories: cranial bones and facial bones. The cranial bones are eight bones that form the cranial cavity, which encloses the brain and serves as an attachment site for the muscles of the head and neck. The eight cranial bones are the frontal bone, two parietal bones, two temporal bones, occipital bone, sphenoid bone, and the ethmoid bone. Although the bones developed separately in the embryo and fetus, in the adult, they are tightly fused with connective tissue and adjoining bones do not move (Figure \(\PageIndex{5}\)).

The auditory ossicles of the middle ear transmit sounds from the air as vibrations to the fluid-filled cochlea. The auditory ossicles consist of six bones: two malleus bones, two incus bones, and two stapes on each side. These are the smallest bones in the body and are unique to mammals.

Fourteen facial bones form the face, provide cavities for the sense organs (eyes, mouth, and nose), protect the entrances to the digestive and respiratory tracts, and serve as attachment points for facial muscles. The 14 facial bones are the nasal bones, the maxillary bones, zygomatic bones, palatine, vomer, lacrimal bones, the inferior nasal conchae, and the mandible. All of these bones occur in pairs except for the mandible and the vomer (Figure \(\PageIndex{6}\)).

Although it is not found in the skull, the hyoid bone is considered a component of the axial skeleton. The hyoid bone lies below the mandible in the front of the neck. It acts as a movable base for the tongue and is connected to muscles of the jaw, larynx, and tongue. The mandible articulates with the base of the skull. The mandible controls the opening to the airway and gut. In animals with teeth, the mandible brings the surfaces of the teeth in contact with the maxillary teeth.

The Vertebral Column

The vertebral column , or spinal column, surrounds and protects the spinal cord, supports the head, and acts as an attachment point for the ribs and muscles of the back and neck. The adult vertebral column comprises 26 bones: the 24 vertebrae, the sacrum, and the coccyx bones. In the adult, the sacrum is typically composed of five vertebrae that fuse into one. The coccyx is typically 3–4 vertebrae that fuse into one. Around the age of 70, the sacrum and the coccyx may fuse together. We begin life with approximately 33 vertebrae, but as we grow, several vertebrae fuse together. The adult vertebrae are further divided into the 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae (Figure \(\PageIndex{7}\)).

Each vertebral body has a large hole in the center through which the nerves of the spinal cord pass. There is also a notch on each side through which the spinal nerves, which serve the body at that level, can exit from the spinal cord. The vertebral column is approximately 71 cm (28 inches) in adult male humans and is curved, which can be seen from a side view. The names of the spinal curves correspond to the region of the spine in which they occur. The thoracic and sacral curves are concave (curve inwards relative to the front of the body) and the cervical and lumbar curves are convex (curve outwards relative to the front of the body). The arched curvature of the vertebral column increases its strength and flexibility, allowing it to absorb shocks like a spring (Figure \(\PageIndex{7}\)).

Intervertebral discs composed of fibrous cartilage lie between adjacent vertebral bodies from the second cervical vertebra to the sacrum. Each disc is part of a joint that allows for some movement of the spine and acts as a cushion to absorb shocks from movements such as walking and running. Intervertebral discs also act as ligaments to bind vertebrae together. The inner part of discs, the nucleus pulposus, hardens as people age and becomes less elastic. This loss of elasticity diminishes its ability to absorb shocks.

The Thoracic Cage

The thoracic cage , also known as the ribcage, is the skeleton of the chest, and consists of the ribs, sternum, thoracic vertebrae, and costal cartilages (Figure \(\PageIndex{8}\).8). The thoracic cage encloses and protects the organs of the thoracic cavity, including the heart and lungs. It also provides support for the shoulder girdles and upper limbs, and serves as the attachment point for the diaphragm, muscles of the back, chest, neck, and shoulders. Changes in the volume of the thorax enable breathing.

The sternum , or breastbone, is a long, flat bone located at the anterior of the chest. It is formed from three bones that fuse in the adult. The ribs are 12 pairs of long, curved bones that attach to the thoracic vertebrae and curve toward the front of the body, forming the ribcage. Costal cartilages connect the anterior ends of the ribs to the sternum, with the exception of rib pairs 11 and 12, which are free-floating ribs.

Human Appendicular Skeleton

The appendicular skeleton is composed of the bones of the upper limbs (which function to grasp and manipulate objects) and the lower limbs (which permit locomotion). It also includes the pectoral girdle, or shoulder girdle, that attaches the upper limbs to the body, and the pelvic girdle that attaches the lower limbs to the body (Figure \(\PageIndex{9}\)).

The Pectoral Girdle

The pectoral girdle bones provide the points of attachment of the upper limbs to the axial skeleton. The human pectoral girdle consists of the clavicle (or collarbone) in the anterior, and the scapula (or shoulder blades) in the posterior (Figure \(\PageIndex{10}\)).

The clavicles are S-shaped bones that position the arms on the body. The clavicles lie horizontally across the front of the thorax (chest) just above the first rib. These bones are fairly fragile and are susceptible to fractures. For example, a fall with the arms outstretched causes the force to be transmitted to the clavicles, which can break if the force is excessive. The clavicle articulates with the sternum and the scapula.

The scapulae are flat, triangular bones that are located at the back of the pectoral girdle. They support the muscles crossing the shoulder joint. A ridge, called the spine, runs across the back of the scapula and can easily be felt through the skin (Figure \(\PageIndex{10}\)). The spine of the scapula is a good example of a bony protrusion that facilitates a broad area of attachment for muscles to bone.

The Upper Limb

The upper limb contains 30 bones in three regions: the arm (shoulder to elbow), the forearm (ulna and radius), and the wrist and hand (Figure \(\PageIndex{11}\)).

An articulation is any place at which two bones are joined. The humerus is the largest and longest bone of the upper limb and the only bone of the arm. It articulates with the scapula at the shoulder and with the forearm at the elbow. The forearm extends from the elbow to the wrist and consists of two bones: the ulna and the radius. The radius is located along the lateral (thumb) side of the forearm and articulates with the humerus at the elbow. The ulna is located on the medial aspect (pinky-finger side) of the forearm. It is longer than the radius. The ulna articulates with the humerus at the elbow. The radius and ulna also articulate with the carpal bones and with each other, which in vertebrates enables a variable degree of rotation of the carpus with respect to the long axis of the limb. The hand includes the eight bones of the carpus (wrist), the five bones of the metacarpus (palm), and the 14 bones of the phalanges (digits). Each digit consists of three phalanges, except for the thumb, when present, which has only two.

The Pelvic Girdle

The pelvic girdle attaches to the lower limbs of the axial skeleton. Because it is responsible for bearing the weight of the body and for locomotion, the pelvic girdle is securely attached to the axial skeleton by strong ligaments. It also has deep sockets with robust ligaments to securely attach the femur to the body. The pelvic girdle is further strengthened by two large hip bones. In adults, the hip bones, or coxal bones , are formed by the fusion of three pairs of bones: the ilium, ischium, and pubis. The pelvis joins together in the anterior of the body at a joint called the pubic symphysis and with the bones of the sacrum at the posterior of the body.

The female pelvis is slightly different from the male pelvis. Over generations of evolution, females with a wider pubic angle and larger diameter pelvic canal reproduced more successfully. Therefore, their offspring also had pelvic anatomy that enabled successful childbirth (Figure \(\PageIndex{12}\)).

The Lower Limb

The lower limb consists of the thigh, the leg, and the foot. The bones of the lower limb are the femur (thigh bone), patella (kneecap), tibia and fibula (bones of the leg), tarsals (bones of the ankle), and metatarsals and phalanges (bones of the foot) (Figure \(\PageIndex{13}\)). The bones of the lower limbs are thicker and stronger than the bones of the upper limbs because of the need to support the entire weight of the body and the resulting forces from locomotion. In addition to evolutionary fitness, the bones of an individual will respond to forces exerted upon them.

The femur , or thighbone, is the longest, heaviest, and strongest bone in the body. The femur and pelvis form the hip joint at the proximal end. At the distal end, the femur, tibia, and patella form the knee joint. The patella , or kneecap, is a triangular bone that lies anterior to the knee joint. The patella is embedded in the tendon of the femoral extensors (quadriceps). It improves knee extension by reducing friction. The tibia , or shinbone, is a large bone of the leg that is located directly below the knee. The tibia articulates with the femur at its proximal end, with the fibula and the tarsal bones at its distal end. It is the second largest bone in the human body and is responsible for transmitting the weight of the body from the femur to the foot. The fibula , or calf bone, parallels and articulates with the tibia. It does not articulate with the femur and does not bear weight. The fibula acts as a site for muscle attachment and forms the lateral part of the ankle joint.

The tarsals are the seven bones of the ankle. The ankle transmits the weight of the body from the tibia and the fibula to the foot. The metatarsals are the five bones of the foot. The phalanges are the 14 bones of the toes. Each toe consists of three phalanges, except for the big toe that has only two (Figure \(\PageIndex{14}\)). Variations exist in other species; for example, the horse’s metacarpals and metatarsals are oriented vertically and do not make contact with the substrate.

Evolution Connection: Evolution of Body Design for Locomotion on Land

The transition of vertebrates onto land required a number of changes in body design, as movement on land presents a number of challenges for animals that are adapted to movement in water. The buoyancy of water provides a certain amount of lift, and a common form of movement by fish is lateral undulations of the entire body. This back and forth movement pushes the body against the water, creating forward movement. In most fish, the muscles of paired fins attach to girdles within the body, allowing for some control of locomotion. As certain fish began moving onto land, they retained their lateral undulation form of locomotion (anguilliform). However, instead of pushing against water, their fins or flippers became points of contact with the ground, around which they rotated their bodies.

The effect of gravity and the lack of buoyancy on land meant that body weight was suspended on the limbs, leading to increased strengthening and ossification of the limbs. The effect of gravity also required changes to the axial skeleton. Lateral undulations of land animal vertebral columns cause torsional strain. A firmer, more ossified vertebral column became common in terrestrial tetrapods because it reduces strain while providing the strength needed to support the body’s weight. In later tetrapods, the vertebrae began allowing for vertical motion rather than lateral flexion. Another change in the axial skeleton was the loss of a direct attachment between the pectoral girdle and the head. This reduced the jarring to the head caused by the impact of the limbs on the ground. The vertebrae of the neck also evolved to allow movement of the head independently of the body.

The appendicular skeleton of land animals is also different from aquatic animals. The shoulders attach to the pectoral girdle through muscles and connective tissue, thus reducing the jarring of the skull. Because of a lateral undulating vertebral column, in early tetrapods, the limbs were splayed out to the side and movement occurred by performing “push-ups.” The vertebrae of these animals had to move side-to-side in a similar manner to fish and reptiles. This type of motion requires large muscles to move the limbs toward the midline; it was almost like walking while doing push-ups, and it is not an efficient use of energy. Later tetrapods have their limbs placed under their bodies, so that each stride requires less force to move forward. This resulted in decreased adductor muscle size and an increased range of motion of the scapulae. This also restricts movement primarily to one plane, creating forward motion rather than moving the limbs upward as well as forward. The femur and humerus were also rotated, so that the ends of the limbs and digits were pointed forward, in the direction of motion, rather than out to the side. By placement underneath the body, limbs can swing forward like a pendulum to produce a stride that is more efficient for moving over land.

The three types of skeleton designs are hydrostatic skeletons, exoskeletons, and endoskeletons. A hydrostatic skeleton is formed by a fluid-filled compartment held under hydrostatic pressure; movement is created by the muscles producing pressure on the fluid. An exoskeleton is a hard external skeleton that protects the outer surface of an organism and enables movement through muscles attached on the inside. An endoskeleton is an internal skeleton composed of hard, mineralized tissue that also enables movement by attachment to muscles. The human skeleton is an endoskeleton that is composed of the axial and appendicular skeleton. The axial skeleton is composed of the bones of the skull, ossicles of the ear, hyoid bone, vertebral column, and ribcage. The skull consists of eight cranial bones and 14 facial bones. Six bones make up the ossicles of the middle ear, while the hyoid bone is located in the neck under the mandible. The vertebral column contains 26 bones, and it surrounds and protects the spinal cord. The thoracic cage consists of the sternum, ribs, thoracic vertebrae, and costal cartilages. The appendicular skeleton is made up of the limbs of the upper and lower limbs. The pectoral girdle is composed of the clavicles and the scapulae. The upper limb contains 30 bones in the arm, the forearm, and the hand. The pelvic girdle attaches the lower limbs to the axial skeleton. The lower limb includes the bones of the thigh, the leg, and the foot.

Critical Thinking Questions

The skeletal system is composed of bone and cartilage and has many functions. Choose three of these functions and discuss what features of the skeletal system allow it to accomplish these functions.

What are the structural and functional differences between a tarsal and a metatarsal?

What are the structural and functional differences between the femur and the patella?

If the articular cartilage at the end of one of your long bones were to degenerate, what symptoms do you think you would experience? Why?

In what ways is the structural makeup of compact and spongy bone well suited to their respective functions?

In what ways do intramembranous and endochondral ossification differ?

Considering how a long bone develops, what are the similarities and differences between a primary and a secondary ossification center?

What is the difference between closed reduction and open reduction? In what type of fracture would closed reduction most likely occur? In what type of fracture would open reduction most likely occur?

In terms of origin and composition, what are the differences between an internal callus and an external callus?

If you were a dietician who had a young female patient with a family history of osteoporosis, what foods would you suggest she include in her diet? Why?

During the early years of space exploration our astronauts, who had been floating in space, would return to earth showing significant bone loss dependent on how long they were in space. Discuss how this might happen and what could be done to alleviate this condition.

An individual with very low levels of vitamin D presents themselves to you complaining of seemingly fragile bones. Explain how these might be connected.

Describe the effects caused when the parathyroid gland fails to respond to calcium bound to its receptors.

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• Authors: J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix
• Publisher/website: OpenStax
• Book title: Anatomy and Physiology
• Publication date: Apr 25, 2013
• Location: Houston, Texas
• Book URL: https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
• Section URL: https://openstax.org/books/anatomy-and-physiology/pages/6-critical-thinking-questions

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