CM Q

Welcome to the audio review of Chapter 7: Skeletal System.

The skeletal system has many functions!

It supports and gives shape to the body.

It protects internal organs and helps make movements possible when bones at moveable joints are pulled by muscles.

The skeletal system also stores vital substances, including calcium and fat. Hormones regulate calcium storage: calcitonin increases storage and parathyroid hormone reduces stores of calcium.

Fat is stored in the cavities of some bones.

Another vital function of bone is hematopoiesis, blood cell formation in red bone marrow.

To review the structure of bones, you need to recall the four major types, which are based on the overall shape of the bone:

Long bones, for example, the humerus (or upper arm),

Short bones, such as the carpals (or wrist) bones,

Flat bones, such as the frontal or skull bone, and

Irregular bones, for example, the vertebrae (they make up the spinal column).

Some also recognize a sesamoid (or round) bone category for bones such as the patella or kneecap.

The structure of long bones contains several terms with which you should be familiar.

The diaphysis or shaft is the hollow tube of hard compact bone.

The medullary cavity is the hollow area inside the diaphysis that contains yellow marrow.

The epiphyses, or ends of the bone, are spongy bone that contain red bone marrow.

Articular cartilage covers the epiphyses and functions as a cushion.

The periosteum is the strong membrane covering bone everywhere except at joint surfaces.

The endosteum is the thin membrane lining the medullary cavity.

The structure of flat bones differs from that of long bones. Flat bones have a spongy bone layer sandwiched between two compact bone layers. Diploe is the term for the spongy bone layer of a flat bone.

It is also important for you to understand the microscopic structure of bones.

There are two types of bone: cancellous or spongy bone, and compact bone.

The texture of cancellous (or spongy) bone results from needlelike threads of bone called trabeculae surrounded by a network of open spaces. Cancellous bone is found in the epiphyses of bones. The spaces of spongy bone contain red bone marrow.

The structural unit of compact bone is an osteon, a calcified matrix arranged in multiple layers or rings, each called a concentric lamella.

Bone cells are called osteocytes and are found inside spaces called lacunae, which are connected by tiny tubes called canaliculi.

Cartilage both resembles and differs from bone.

Cartilage cells are called chondrocytes.

The matrix of cartilage is gellike and lacks blood vessels.

Listen on to find out cartilage’s functions in bone development.

Bone development involves making and remodeling bone tissue.

Before birth, bone structure consists of cartilage and fibrous structures. Three types of bone cells are involved: osteoblasts, osteocytes, and osteoclasts.

Osteoblasts build new bone matrix by encrusting collagen fibers with calcium crystals. (think “b-b: blasts build”) Osteocytes are inactive osteoblasts.

Osteoclasts dissolve bone, releasing calcium ions for reabsorption into the bloodstream.

Remodeling is a combined action of making and dissolving bone matrix that eventually sculpts bone into the adult shape.

There are two types of ossification: endochondral and intramembranous.

In endochondral ossification, cartilage models are gradually replaced by calcified bone.

In intramembranous ossification, fibrous membranes are ossified into hard bone plates.

For example, fontanels of newborns are soft, not-yet-ossified regions.

Next, we’ll review the major bones of the skeleton.

The skeleton can be divided into central axial and peripheral appendicular regions. The axial skeleton includes 80 bones: bones of the skull, vertebral column, and thorax, as well as the hyoid bone of the neck, the only bone in the body that does not directly connect with another bone.

The skull has 8 bones of the cranium, 14 in the face, and 6 in the middle ear.

Within the skull are spaces called paranasal sinuses.

The vertebral column or spine has 24 total vertebrae: 7 cervical, 12 thoracic, and 5 lumbar, as well as the sacrum and coccyx.

The thorax contains 24 ribs and the sternum. Those are all the bones of the axial skeleton.

The appendicular skeleton contains 126 bones of the upper and lower extremities.

The 64 bones of the upper extremity include bones of the pectoral or shoulder girdle, the arm and forearm, and the wrist and hand.

The pectoral girdle has 2 scapulae and 2 clavicles.

Each arm has a humerus and each forearm has a radius and an ulna, for a total of 6 bones.

The wrists and hands contain 16 carpal bones, 10 metacarpal bones, and 28 phalanges.

The 62 bones of the lower extremity include the pelvic or hip girdle, the thigh and leg, and the ankle and foot.

The pelvic girdle has 2 coxal bones.

The thighs and legs contain a total of 2 femurs, 2 patellas, 2 tibias, and 2 fibulas.

The ankles and feet have a total of 14 tarsal bones, 10 metatarsal bones, and—just like the hands—28 phalanges.

Note that the arched structure of the foot provides dynamic support for the entire skeleton.

Skeletal variations include male and female differences, age differences, and environmental factors.

A comparison of male and female skeletons reveals that the male skeleton is generally larger. The shape of the pelvis is different; the male pelvis is deep and narrow, whereas the female pelvis is shallow and broad.

The size of the pelvic inlet also differs: the female pelvic inlet is generally wider, normally large enough for a baby’s head to pass through it.

Similarly, the pubic angle, the angle between the pubic bones, is generally wider in females.

Age differences apply to both men and women.

Bones enlarge and become more ossified until maturity at age 25. Bones actively remodel (dissolve and rebuild) in middle adulthood, then bones become less dense during the elderly years.

Environmental factors influencing the skeleton include nutrition and mechanical stress.

Nutrition affects growth and maintenance of bone tissue. Mechanical stress, including exercise, affects bone remodeling.

Next, we’ll review joints. An articulation is a joint between two or more bones. Every bone except the hyoid (which anchors the tongue) connects to at least one other bone.

There are three kinds of joints: synarthroses, amphiarthroses, and diarthroses.

Synarthroses permit no movement; fibrous connective tissue grows between articulating bones. For example, sutures of the skull are synarthroses.

Amphiarthroses allow slight movement. In amphiarthroses, cartilage connects articulating bones; for example, in the symphysis pubis.

Diarthroses are freely moveable joints. Most joints belong to this class.

There is a common structure to freely moveable joints; the joint capsule and ligaments hold adjoining bones together, but permit movement at the joint.

Articular cartilage covers the ends of bones where they form joints with other bones.

A synovial membrane lines the joint capsule and secretes lubricating fluid. The space between joint ends of bones is termed the joint cavity.

A bursa is a fluid-filled pouch that absorbs shock; inflammation of a bursa is called bursitis.

Some different types of joints are the ball-and-socket, hinge, pivot, saddle, gliding, and condyloid types. Different kinds of movements are determined by the structure of each joint.

This concludes the audio review of Chapter 7.