The integumentary system consists of the skin and its derivatives--hair, nails, and cutaneous glands. Even though this body system is our most visible one, we are accustomed to pretty much ignoring it in our day-to-day life unless something hurts or we get preoccupied with the visible signs that it's aging. We're about to give it the attention it deserves. The study and treatment of the integumentary system is called dermatology.

Something handy to know that is not in your book is how to estimate skin area. You often hear about someone who's been in an accident or has burns that he's lost some percentage of his skin. You may have wondered how this is measured. Well, generally these figures are estimates based on something called the Rule of 9s. The Rule of 9s says that the percentage of your skin area in given body locations is always a multiple of nine. Let's take a look:

The skin on the head and neck is about 9% of your total skin area. That on each upper extremity (arm and hand) is also about 9%--18% for both together. The trunk has about 18% of your skin on its front and about 18% of your skin on its back. The lower extremities (legs and feet) have about 18% each--36% together. If you add all of this skin area up--9% for the head, 18% for upper extremities, 36% for the trunk, and 36% for the lower extremities--you'll get 99%. The last 1% is found on the perineum, the diamond-shaped area between the thighs.

The skin is the body's largest organ. You've probably never really thought of the skin as an organ at all, but it fits the definition--a collection of related tissues functioning together. The average adult has about twenty or thirty square feet of skin; and it comprises about 15% of our weight--figure twenty pounds or so.

Skin has two layers, the epidermis, which consists of epithelial tissue, and the dermis, which is connective tissue. Below these is a connective tissue layer called hypodermis, or subcutaneous tissue. This hypodermis isn't really skin at all, but we'll study it with the skin because it is so closely related.

Skin is amazingly intricate. Each square inch of skin contains twenty feet of blood vessels, about 95 sebaceous (oil) glands, 20 million or so cells, and about 1200 nerve endings for pain alone. That's a lot of stuff when you think about it!

The thickness of skin varies from person to person and from one place to another on your body. Your thinnest skin is found on your eyelids, your thickest across your shoulders. This variation is mostly due to variation in the thickness of your dermis. When we classify skin as thick or thin though, we usually consider only the epidermal layer. When we look at it this way, the thickest skin is found on your palms and soles, fingers and toes.

The epidermal layer of skin consists of keratinized stratified squamous epithelium--this means the tissue is quite sturdy and occurs in layers. The entire surface layer is dead cells packed with keratin, a tough protein which resists water and chemicals. It's your waterproofing--the reason you don't gain weight in the bathtub or dry out in the sun; it is also your protection against many organisms and substances which might be harmful--more on that later.

The epidermis lacks blood vessels, so it depends on the underlying dermis for nutrition. It also has very few nerve endings for touch and pain, so it also relies on receptors found in the dermis for sensation. Several kinds of cells are found in epidermis. Keep track of keratinocytes, the cells which contain keratin, the protein that makes epidermis so tough, and melanocytes, cells which make the skin pigment melanin. You may safely ignore most of the other cell types and the specifics of the 5 epidermal zones.

You will want to remember that the deepest layers of epidermis are the reproductive layers, the ones in which keratinocytes and melanocytes reproduce to replace surface cells as they exfoliate or desquamate (flake off). These deep layers reproduce and push their cells toward the surface. As cells are pushed upward, they stop dividing and flatten out; they also produce more and more keratin. Cells in about the middle of epidermis produce a lipid layer which helps to waterproof the skin. A problem is that this lipid also cuts off the supply of nutrients from blood vessels far below in the dermis; this means that cells above this lipid layer die. This brings us back to that surface zone which is composed of layers and layers of dead, densely packed, flattened, scaly, keratinized cells.

From birth to death, keratinocytes take from 30 to 40 days to push to the surface and exfoliate. This process slows as an individual ages. It speeds up in cases of injury or stress; injured epidermis regenerates faster than any other tissue in the body. Overall, the rate of exfoliation matches the rate of mitosis. The process of mitosis also accelerates in areas of skin that suffer mechanical stress, such as repeated rubbing; this is why manual laborers have thick calluses on their hands and people with ill-fitting shoes have thick calluses on their feet. (Or check out the feet of someone who goes barefoot a lot; the skin thickness here is sometimes amazing!)

Dermis is mostly collagen along with some fibrous proteins. It contains blood vessels, sweat glands, oil glands, hair follicles, nail roots, nerve endings, and muscle tissue. The boundary between epidermis and dermis is irregular, which allows these two layers to interlock, resisting slippage, and also produces fingerprint ridges, which help with grip. The most sensitive areas of skin have dermis near the surface.

There are two zones in the dermis. The deeper and thicker one is called the reticular zone. It is composed of strong, dense connective tissue that has limited elasticity. When stretched beyond its limits, the collagen tears, producing striae or stretch marks. The much thinner papillary zone is nearer the surface and acts as a support layer. It is more vascular than necessary to meet its nutritional needs; this increased vascularity is essential to the body's temperature control mechanism. The nervous system monitors blood flow in this zone, enabling response to temperature and emotional changes. This looser connective tissue also allows for mobility of white blood cells and other defensive constituents in protecting the body from infection.

The hypodermis or subcutaneous tissue is composed of superficial fascia. There is adipose connective tissue in this layer, which allows skin movement. Hypodermis functions to bind skin to underlying tissue, provide padding to the body, store energy as fat and glycogen, and provide thermal insulation. Those with thinner layers of subcutaneous fat are more susceptible to cold; a condition of having excess subcutaneous fat is called obesity. Women have a thicker layer of subcutaneous fat than men, and this fat is distributed differently in the sexes. Because this fat layer is highly vascular, it makes a good site for injection of medications (hypodermic injection) because drugs are absorbed rapidly from the adipose tissue.

The characteristic color of skin seen in individuals comes from three pigments.

The first of these is hemoglobin, the red pigment that makes red blood cells red. Hemoglobin in dermal blood vessels imparts a pink color to the skin; combined with white collagen fibers, it is responsible for the flesh tones seen in Caucasians. Areas where skin in darker red, for example, the lips, are areas where the blood flow is very near the surface.

The second pigment that contributes to skin color is melanin. This pigment imparts a black, brown, tan, or yellow color to skin. Everyone has about the same number of melanocytes (melanin-producing cells), but differences in skin color result from different degrees of clumping exhibited by the melanin that is present. Fair-skinned people have more clumped melanin and more melanin near the nucleus of skin cells; dark-skinned people have melanin more dispersed throughout their cells.

The amount of melanin results from a combination of heredity and sun (ultraviolet light) exposure. Melanin production is affected by levels of two anterior pituitary hormones, melanocyte-stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH). Ultraviolet light stimulates MSH to trigger melanin synthesis; then keratinocytes take in fragments of melanocytes by phagocytosis. This ingested melanin localizes on the light side of the cells' nuclei, protecting their fragile DNA from the damaging effects of ultraviolet light. The melanin also darkens the skin in a suntan. The tan fades as melanized keratinocytes exfoliate naturally over time.

The amount of melanin varies widely from place to place on the body. It is concentrated in freckles (also hereditary) and moles; the backs of the hands and tops of the feet; the nipple and areola of the breast; around the anus, scrotum, penis, and labia majora.

The third pigment that contributes to skin color is the yellow pigment, carotene. This is a substance with a dietary source in egg yolks and yellow and orange vegetables; depending on the diet, it concentrates in the uppermost layer of epidermis and in subcutaneous fat. Carotene is most conspicuous in the heels and in calluses, which often look yellowish due to their thick epidermal layers.

Abnormal skin colors can result from a variety of conditions and take a variety of appearances.

Cyanosis, or blueness of skin, is due to an oxygen deficiency in the circulating blood. It can be caused by any condition that prevents the blood from picking up adequate oxygen or which causes blood to flow so slowly through skin that most of its oxygen load is extracted before freshly-oxygenated blood arrives. These conditions may include airway obstruction, cold environmental temperatures, and cardiac arrest.

Erythema, abnormal redness of skin, occurs with exercise, hot environmental temperatures, sunburn, anger, and embarrassment. It results from increased blood flow in dilated cutaneous vessels or from pooling of blood in skin after it has leaked from blood vessels.

Yellowing of skin, jaundice, results from increased levels of bilirubin, a yellow pigment, in skin. Bilirubin is a breakdown product of hemoglobin and is normally disposed of by the liver; jaundice means either an increased rate of red cell destruction or a decreased ability of the liver to keep up, either due to damage or to immaturity (as in premature babies).

Bronzing, a golden-brown color, is associated with Addison disease. This is a deficiency of hormone secretion by the adrenal glands.

Pale skin, called pallor, is the result of deficient blood flow through the skin; white collagen then provides the predominant color in the skin. This can occur in times of emotional stress, when blood pressure is abnormally low, in circulatory shock, cold environmental temperatures, and severe anemia.

Albinism is a genetic lack of melanin in skin, hair, and eyes. The result is white hair, pale skin, and pink (from blood vessels) eyes.

A hematoma is a bruise, a mass of clotted blood showing through the skin. These are generally from trauma.

Hemangiomas are patches of discolored skin caused by abnormal dermal blood capillaries. They are frequently called birthmarks and may be flat or slightly swollen, bright red to dull purple, present only in childhood or for life.

Freckles are flat melanized patches of skin which depend on heredity and sun exposure. Those with a genetic predisposition to freckling will note that their freckles darken with sun exposure.

Moles are elevated patches of melanized skin which are generally harmless. They should be watched for signs of malignancy, since some have a tendency to become malignant.

Fingerprint ridges or friction ridges are what leave oily fingerprints on items you touch. These form during fetal development and remain throughout life. No two people have identical fingerprints, even identical twins.

Flexion ridges are the creases caused by years of closing the hand and bending the wrist and elbow.

Barrier. The skin resists trauma better than other organs; it also heals more quickly. Most physical injuries affect the skin.

The skin is an inhospitable place for microorganisms; it is dry, its pH is too low, and sebum is antibacterial. There are generally large numbers of bacteria on the skin; these organisms must find ways to survive the unfavorable conditions. Microorganisms cannot easily enter through intact skin; most of them must rely on injuries or insect vectors to get them into deeper tissues. Even then, phagocytic cells living in deeper layers of skin usually make short work of such invaders. Skin makes a great barrier against infection.

We've already mentioned the waterproofing offered by intact skin. Between the tightly packed cells, the lipids produced in epidermis, and the oil produced by sebaceous glands, the skin offers excellent protection against water absorption and water loss.

Skin is also a barrier to ultraviolet light. The melanin caps over the nuclei of skin cells protect them from mutations caused by ultraviolet light; and the melanin presented in keratinocytes protects deeper tissues from ultraviolet damage.

Vitamin D Synthesis. Vitamin D is required in order to regulate levels of calcium and phosphate in the blood stream. While vitamin D can be acquired in the diet, it can also be made in the skin from a derivative of cholesterol. This process requires ultraviolet light (which was a bad guy just a few minutes ago). Vitamin D deficiency used to be common in children, especially dark-skinned ones, who lived in industrialized areas with heavy air pollution and cold winters. Because their skin didn't absorb ultraviolet light as well as fair skin and because they had less exposure to sunlight, these children were at risk for the bone deformity associated with rickets. Now that most milk is fortified with vitamin D, this deficiency disease is mostly a thing of the past. And, for the record, this does NOT give you a good excuse for tanning; just 15 minutes of sunlight on face and arms every few days is plenty to get all the vitamin D you need.

Cutaneous Absorption. Most chemicals can't penetrate skin; however a small amount of oxygen diffuses through skin and into the blood, and fat-soluble vitamins (A, D, E, and K) can also be absorbed. Some drugs and poisons can also be absorbed through the skin. The skin also gives off some carbon dioxide, a metabolic waste, and some other organic substances. Among these are volatile chemicals that attract mosquitoes; many mosquito repellents contain chemicals intended to mask the their odor.

Sensory Functions. Generally, when we think of sense organs, we think first of eyes, ears, nose, and taste buds. In fact, skin is your largest sense organ. You have nerve endings in your dermis which respond to heat, cold, pressure, touch, texture, vibration, and pain. These receptors detect not only the presence or absence of one of these stimuli, but also their intensity; and they identify the location where the stimulus is occurring. Sense receptors are especially numerous on the palms and fingers, the soles of the feet, the face, and the genitals--all the body areas we are accustomed to thinking of as sensitive.

Thermoregulation. This function is carried out by a sort of partnership among the skin, the nervous system, the muscles, and the endocrine system. It is one of the important homeostatic functions in the body, maintaining normal temperatures within a fairly narrow range. The deepest tissues are warmest; surface temperatures normally run around 37^oC (98^oF). Newborns run higher normal temperatures than adults.

While normal muscular activity won't increase body temperature, increased body activity will. When you sleep, your temperature falls slightly. But when your environmental temperature changes, your body temperature remains remarkably constant; this is due to the body's thermoregulation mechanisms.

Thermoreceptors in the skin are found in dermis; some respond when skin temperature rises, others when temperature falls. These transmit signals to the hypothalamus in the brain, which also monitors spinal fluid temperature and input from other receptors and then signals the body to dilate or constrict dermal blood vessels, increase or decrease sweating, initiate or stop shivering, increase or decrease activity, and regulate heat production in cells.

Social Functions. This is a function of skin to which we pay lots of attention in the rest of our lives, but generally ignore when studying physiology. The skin, especially on the face, provides an important means of nonverbal communication. We often--perhaps too often--choose friends and mates based on the appearance of the skin; we use it to identify individuals and to tell the sexes apart.

The hair and nails are known as accessory organs or appendages of skin. These, like the surface layers of skin, are composed of dead keratinized cells. The keratin in skin and nails is called hard keratin, which is diffeent from the soft keratin in skin. It is much more compact and tougher than soft keratin. (Funny, isn't it, that everything you see when you look at a friend is dead? There are no living tissues at all that are visible on humans. Seems odd that we place so much importance on the appearance of a lot of dead stuff.)

Structure of the Hair and Follicle. The hair filament itself is called a pilus. It grows from a slanted tube in the skin called a follicle.

Around the base of a follicle is a bundle of muscle fibers which respond to cold, fear, and other stimuli by pulling the hair upright. These muscles, the arrector pili, serve a function in animals, whose coats fluff up to trap insulating air in cold weather and to give a larger more threatening appearance when frightened. They result in goose bumps in humans, which appear to have no useful purpose, but do increase the sensitivity of the skin to touch.

There are three zones along the length of a hair. The bulb is at the base where the hair originates in the dermis; this is an area of high cell regeneration where keratin is produced. The bulb is the only living portion of a hair; it is nourished by blood vessels in the connective tissue around the follicle. The root is inside the follicle; and the shaft is the portion of the hair that projects above the surface of the skin.

In cross-section, a hair has three layers. The innermost layer is the medulla; it consists of loosely arranged cells with air spaces between them. The cortex, the middle layer consists of densely packed keratinized cells; and the cuticle on the outside of the shaft has a single layer of overlapping scaly cells. Since the free edges of cuticle cells face the tip of the hair and the cells lining the follicle face the opposite direction, the hair resists pulling. When you pull a hair out, these follicle cells come with it. The texture of hair is related to its cross-sectional shape; straight hair is round, wavy hair is oval, and kinky hair is flat.

The color of hair is due to pigments. Melanin produced by melanocytes in the bulb is transferred to the cortex cells to produce black or brown hair. Blonde hair has very little melanin, which allows a hay-colored pigment called phaeomelanin to show through. Red hair has an iron-containing pigment called trichosiderin. Hair which is gray is actually a mixture of pigmented hairs and white hairs; white hair's melanin has been replaced by air.

Growth. Hair grows much the same way as skin does. Mitosis occurs in cells of the root sheath, pushing older cells out and away toward the surface. The cells become progressively keratinized and die as they are pushed upward away from their blood supply. Hair grows at varying rates, from 10 to 20 cm per year. A hair normally grows for a few years, then stops. A new hair begins to grow beneath it, pushing the old hair out ahead of it until the old hair falls out. A person normally loses from 10 to 100 hairs a day.

Eyebrows and lashes don't get as long as scalp hair because they grow for only three or four months before they go dormant and are pushed out. Hair appears to thin with age largely because it is replaced with finer hair as individuals age. Some thinning is normal in both sexes and is worsened by disease, poor nutrition, fever, emotional trauma, radiation, and chemotherapy. Baldness, or alopecia, is much more common in men than in women. That's because the most common form of baldness, pattern baldness, is a sex-influenced trait, expressed mostly in men. There is a combination of genetic and hormonal influences in play; the gene is expressed only in the presence of high levels of testosterone. That means the trait is seen only in men and in women who have unusually high levels of testosterone (probably from a tumor of the adrenal gland).

Hirsutism, the opposite problem, is excessive hairiness, especially in women and children. In younger people it is usually due to a tumor which causes secretion of high levels of testosterone; it is also associated with menopause in normal women.

Functions of Hair. Hair has little function in humans, mostly because we have so little hair. It keeps animals warm, but our body hair is too sparse to serve in this way. It does insulate the scalp, an area with very little subcutaneous fat, and protect it from sunburn. Guard hairs in the nostrils and ears and eyelashes do serve a protective function, trapping debris and liquids.

Nails are hard clear modifications of the top layer of epidermis. The cells in nails are very thin, scaly, and densely packed. They contain hard keratin and are composed of dead cells with very little melanin. They protect the tips of the fingers and toes. Fingernails also serve to stiffen up the soft tissue at the ends of the fingers, providing a better grip; they help us to manipulate objects too. Fingernails grow very slowly, about 1 mm a week; toenails grow more slowly. New cells are added by mitosis in the nail matrix at the base of the nail under the skin. The flat visible part of the nail is the nail plate.

Sweat Glands. Sweat glands are also called sudoriferous glands. There are two kinds, merocrine or eccrine and apocrine. Eccrine sweat glands are the most numerous and produce watery perspiration; we have three or four million of these, pretty much all over the body. The nervous system controls these glands; they respond to temperature changes and emotions. They function in thermoregulation; evaporating perspiration from the skin uses energy, cooling the skin.

Sweat consists of mostly water, with a small concentration of electrolytes. As the water evaporates, it leaves the other substances behind on the skin. One of these is salt, the reason sweat tastes salty.

Even when we don't feel wet, we are producing perspiration. This is called insensible perspiration. We produce about half a quart of perspiration a day in moderate temperatures, most of it without being aware of it. If we exercise or experience high environmental temperatures, we sweat enough to produce visible skin wetness. This is called diaphoresis. Water losses due to perspiration can reach a quart per hour in very hot weather if we are active.

Perspiration also has a protective function; its pH is between 4 and 6. This is too acid for many bacteria to survive.

The apocrine sweat glands are found only in the groin, anal region, axilla (armpits), areola, and beard area. These glands empty into hair follicles, not directly on to the skin as do eccrine glands. The sweat from apocrine glands is thicker and milker because it contains fatty acids along with water and electrolytes. When trapped in clothing, these fatty acids are degraded by skin bacteria, producing body odor. These glands correspond to the scent glands found in many animals; in animals they are often associated with specialized tufts of hair. They occur in humans on skin usually covered with hair as well. The hair retains the secretion from the eccrine glands, contributing to the characteristic odor.

Sebaceous Glands. Sebaceous glands produce an oily secretion called sebum. These glands empty into hair follicles and directly on the skin surface. Sebum serves to keep the skin and hair from becoming dry and brittle. Brushing your hair makes it shine because it distributes sebum from the roots to the ends. Sebum enhances the water barrier effect of skin.

Mammary Glands. Mammary glands are modified apocrine sweat glands that produce a secretion richer than apocrine glands. Milk is produced in mammary glands and channeled through ducts to the nipple. The glands are present in males and females, but only develop during pregnancy, so never develop in men.

Since skin is the organ most exposed to the environment, it is also the one most vulnerable to injury and disease.

Skin Cancer. Skin cancer is nearly always induced by ultraviolet radiation from the sun. It is most common on areas of skin frequently exposed to sunlight, the head and neck. The most common and least dangerous form is known as basal cell carcinoma. It seldom spreads to other tissues and is usually treated by surgical removal and radiation therapy. Squamous cell carcinoma arises from keratinocytes in one of the middle zones of epidermis. It can spread to lymph nodes if left untreated. A less frequent and potentially deadly form of skin cancer is malignant melanoma. It arises from the melanocytes in an existing mole and metastasizes quickly. It requires immediate treatment.

Burns. Burns are a leading cause of accidental death and can be caused by fire, hot substances or objects, sunlight, radiation, chemicals, and electric shock. The cause of death in a burned person is generally related to the loss of skin with its barrier functions. Fluid loss and infection, along with the toxic effects of the burned tissue itself, are the primary problems facing a burn victim.

Burns are classified according to the depth of the involved tissue. First-degree burns are those that involve only epidermis. The patient experiences redness, slight swelling, and pain in the affected area. These burns generally heal fairly quickly as the dermal layer beneath the damage quickly regenerates the epidermal layers. Second-degree burns involve epidermis and a portion of the dermis, but leave some dermis undamaged in the affected area. The skin will look red, tan, or white. These burns take longer to heal as the epidermis is regenerated from undamaged dermis underneath and adjacent to the burned area. There may be some scarring as healing occurs. Third-degree burns destroy the entire thickness of skin, epidermis and dermis as well. There may be involvement and damage to deeper tissues as well. Since there is no dermis left in the burned area, tissue regeneration must occur from the sides of the burned area; this is a slow process, which may take months. Scarring is common as these burns heal.

In a burned person, the most urgent treatment considerations are related to the loss of the skin barrier. Because so much fluid can be lost through destroyed skin, an immediate need is to replace fluids and electrolytes, along with protein which seeps out from the damaged area. Losses can mount to several quarts per day. Without fluid replacement, the patient can lose most of her blood volume in the first few hours, go into circulatory shock (low blood volume) and kidney failure, causing cardiac arrest. Rapid replacement of large volumes of fluid and provision of several thousand Calories per day constitute an immediate need. The other issue for burned patients is the risk of infection from organisms that take advantage of the absence of the skin's protective barrier to enter deeper tissues. Burned patients are kept in aseptic (germ-free) environments and given antibiotics to prevent infection. The eschar (charred tissue) must also be removed in a process called debridement to prevent it becoming infected and releasing toxins into the tissues.

That's all there is in Chapter 7. Now that you've read the chapter and worked your way through these notes, it is time to work on the chapter review sheets. Then use the chapter objectives to build a study guide for this portion of the test. When you've accomplished this and feel confident that you understand the concepts presented here, you're ready to move on to Chapter 8.

Osteology is the study of bones.

The skeletal system is composed of bones, cartilage, and ligaments which form a strong framework for the body. Without your skeleton, you'd be a soft mass of tissue in a sort of puddle on the floor, incapable of movement. Cartilage is the forerunner of bone in early development. It persists at joint surfaces in many joints of the adult skeleton. Ligaments join bone to bone, holding bones together in a strong, yet flexible framework. Tendons attach muscles to bones to provide for movement.

The skeleton provides support for the body. It holds our organs and tissues in place.

It also acts as protection for many organs and structures in the body. Examples are the brain, spinal cord, lungs, heart, pelvic organs, and bone marrow.

The skeleton is responsible for the body's movement. It provides a rigid attachment and leverage for skeletal muscles, which move the various parts of the body and the entire body.

Blood cell formation occurs in the bone marrow; many immune system cells are produced in marrow as well.

The bones form a significant reservoir of minerals which are used to maintain proper electrolyte balance in the bloodstream and tissues. They store calcium and phosphate, releasing them to meet physiologic needs.

Because bones can absorb or release alkaline salts, they function in maintaining the acid-base balance of the body. These salts buffer the blood against pH changes.

And bones operate in detoxification of blood. They remove heavy metals and other substances from blood, then release them slowly over time so they can be safely excreted.

The bones are composed of connective tissue, a matrix hardened by deposits of calcium phosphate and other minerals. The mineralization or hardening process is called ossification. Bones contain osseous (bone) tissue, blood, bone marrow, cartilage, adipose tissue, nervous tissue, and fibrous connective tissue. A bone is an organ made of all of these substances.

Bones are classified into four groups based on shape.

Long bones are longer than they are wide. Examples are the long bones in the arms and legs. They act as levers moved by muscles to produce movement.

Short bones are nearly equal in length and width. These include bones in the wrist and ankle. Short bones have more limited motion.

Flat bones like the shoulder blades and breast bone are mostly useful to enclose and protect important organs. Their broad surfaces provide a wide area for muscle attachment.

Irregular bones like the vertebrae are bones that don't fit the other shape categories.

Many bones consist of an outer layer of dense white tissue called compact bone. It is heavy and strong and makes bones good supports. At the ends of some bones and in the middles of others is found spongy bone. Spongy bone is much looser and lighter, but still quite strong. Many bones also contain a central space called the medullary cavity. Here is where bone marrow is found. The hollow nature of bones also lightens them significantly. Bones composed entirely of compact bone would be too heavy for our muscles to move.

Long bones have certain features in common. These include the shaft of the bone, the diaphysis. This is composed of compact bone with a medullary cavity. The expanded ends of the long bone are the epiphyses. These larger ends, composed of spongy bone with a compact bone covering, provide additional support where pressure is great and provide adequate surface for the attachment of the ligaments and tendons. In children the epiphyseal plate, a zone of cartilage between the diaphysis and epiphysis, is the site of growth as the bone elongates. In the adult this cartilage has all ossified to bone.

At many joints the bone ends are covered with a smooth glassy layer of cartilage called the articular cartilage (joints are called articulations). It covers joint surfaces and secretes a lubricating fluid that makes the joint more friction-free.

Bones have tiny holes through which blood vessels travel to the inner layers of the bone. These are nutrient foramina.

Bones are covered with a tough fibrous membrane composed of collagen. This is the periosteum. It is continuous with tendons as they anchor to the bone. The inner layer of the periosteum is composed of osteogenic (bone-producing) cells which can add to the thickness of the bone or participate in repairs when the bone is damaged.

The inner surface of the bone is also covered with a membranous layer of connective tissue called endosteum.

Osseous tissue consists of various kinds of cells embedded in a ground substance or matrix.

Cells. The basic cell in bone tissue is the osteogenic cell. These cells multiply continuously; some of them develop into osteoblasts, the bone-forming cells that synthesize the organic matter of the bone matrix and help to mineralize the bone. Osteoblasts are unable to reproduce; new ones must constantly develop from osteogenic cells. As matrix mineralizes during bone formation, osteoblasts become trapped in the matrix and turn into osteocytes. They're found in tiny cavities surrounded by channels through which they send their cytoplasm to communicate with other trapped cells. They detect mechanical stress on the bone and communicate it to osteoblasts on the surface so that bones under stress will grow thicker and stronger.

The same precursor cells in bone marrow that develop into monocytes for the blood stream can begin to fuse and form osteoclasts that remain in bone. These large cells hang around on the inner and outer surfaces of a bone and function in dissolving bone when there is a need for a change in shape.

Matrix. Bone's ground substance is about two-thirds inorganic, unusual in tissue. The organic component of bone matrix is primarily collagen; it gives the bone flexibility, which helps the bone to resist tension. The inorganic component is mostly hydroxyapatite, a crystallized salt of calcium phosphate, with smaller amounts of calcium carbonate and a few other minerals. This gives the bone rigidity and strength to resist compression.

The development of bone is called ossification or osteogenesis, and it occurs by two different processes. Some bone starts out as membrane, which then calcifies in a process called intramembranous ossification. This happens a lot as embryonic tissue calcifies for the first time. Osteogenic cells develop in sheets of connective tissue and deposit organic matrix; osteoblasts deposit calcium phosphate and become trapped, developing into osteocytes. Other bone starts out as cartilage, which is broken down and replaced with calcified tissue in a process called endochondral ossification.

Throughout life, osteoblasts continue to deposit and calcify bone tissue while osteoclasts dissolve it in an ongoing process called remodeling. This permits bones to change in size, strength, and shape to meet the body's changing needs, as well as to provide ready mechanisms for damage repair.

Mineralization. The process of crystallization requires depositing calcium and phosphate ions from serum into bone; this can happen only if the concentration of ions in the tissue reaches a threshold level. Now it's handy that most tissues (the soft ones) have chemical inhibitors to prevent ion concentration from getting high enough. This keeps soft tissues soft. In developing bone, osteoblasts neutralize these inhibitors, which explains how mineralization gets started in bone tissue; calcium and phosphate concentrations build up until crystallization begins. The first few crystals ct as seed crystals, then more ions crystallize around them in a positive feedback process that adds more and more mineral to the bone.

It is possible for what should be soft tissues to form mineral deposits; this is an indication that something has gone wrong in the control mechanism which was supposed to keep the ion concentration below threshold levels. When it happens, it is called ectopic (out of place) ossification. This may take the form of generalized hardening of a tissue or the formation of a stone or calculus in tissue.

Mineral Resorption. Mineral resorption is carried out by osteoclasts, which dissolve bone using hydrochloric acid and enzymes. In doing so, they release minerals to the blood, so these are then available for other needs in the body.

Non-bone Functions of Minerals. While the largest deposits of calcium and phosphate in the body are found in bone, each has other important functions in the body. Phosphate is a critical component of DNA, RNA, ATP, phospholipids, and many other compounds in cells. In addition, the phosphate buffering system is one of the primary means of maintaining blood pH. Calcium functions in neuron communication, muscle contraction, blood clotting, exocytosis, as a second messenger in cell signaling, and as a cofactor for some enzymes.

These other functions of phosphate and calcium are important enough that the skeleton serves as a reservoir for these minerals. There are two calcium reserves in bone (which hoards up to 99% of the body's calcium), the stable hydroxyapatite which is not easily released to the bloodstream, and the exchangeable calcium which is easily released to tissue fluid. This is a tiny portion of the total calcium in the bones, but sufficient as a reserve most of the time. Throughout life, due to remodeling, the adult skeleton exchanges nearly one-fifth of its calcium every year. The calcium concentration of plasma is maintained within a very narrow range so that it is available to cells when needed.

Serum Calcium Levels. Phosphate is also drawn from bones as needed, but changes in serum phosphate levels are not at all as serious as changes in calcium levels. Deficient levels of serum calcium, hypocalcemia, cause excessive excitability of the nervous system, accompanied by tremors, spasms, and tetany (constant uncontrolled contraction). The consequences can include suffocation due to spasm of the larynx. This happens because calcium ions normally contribute to the membrane potential maintained in nerve and muscle cells; lack of calcium causes excessive inflow of sodium to the cells, which stimulates them, causing excessive excitability.

Hypercalcemia has the opposite effect, increasing the membrane potential and making it difficult to open the sodium channels to begin a stimulus. So the nervous system exhibits depression resulting in emotional disturbances, muscle weakness, sluggish reflexes, and perhaps cardiac arrest.

While hypercalcemia is fairly rare, hypocalcemia is not. It can accompany or result from vitamin D deficiency (because vitamin D helps the body absorb calcium from the diet), diarrhea, thyroid tumors, hypoactive parathyroid glands, pregnancy and lactation.

Calcium-Phosphate Homeostasis. Maintaining homeostasis of calcium and phosphate levels is dependent on maintaining a balance among dietary intake, losses, and exchange with bones. The whole process is regulated by hormones.

One of these regulatory hormones is calcitriol, which is the active form of vitamin D. Calcitriol stimulates the intestine to absorb calcium and phosphate while reducing urinary excretion of these substances and promotes osteoclast activity. This means the overall effect of calcitriol is to increase serum levels of calcium and phosphate. A deficiency of vitamin D, caused by a combination of dietary deficiency and lack of sunlight, leads to rickets in children and osteomalacia in adults. These diseases cause softened bones.

Another hormone important in regulating calcium levels is calcitonin, which is poroduced in the thyroid gland when blood calcium levels rise. Calcitonin reduces osteoclast activity while increasing osteoblast activity. While the effect of calcitonin is greater in children than in adults, it may be used to protect the bones of pregnant and lactating women and people with osteoporosis. Its overall effect is to lower serum calcium levels.

The third homone involved in calcium-phosphate homeostasis is parathyroid hormone (PTH), produced by the parathyroid glands when blood calcium levels fall. PTH inhibits osteoblasts while stimulating osteoclasts; decreases calcium excretion while increasing phosphate excretion, which prevents deposition of calcium phosphate in bones; and stimulates calcitriol synthesis. Its overall effect is to increase serum calcium levels.

Other Factors Affecting Bone. In adolescence, growth hormone, estrogen, and testosterone production promote ossification; this leads to the well-recognized growth spurt. Because estrogen has a stronger effect than testosterone, girls tend to grow faster at this time, but because girls grow for fewer years, they do not grow as tall as boys do. Once the cartilage in the epiphyseal plates is depleted, then no further growth in length can occur in the long bones, and the final height has been reached.

Fractures. Fractures are breaks in bones. These can be stress fractures caused by trauma or pathologic fractures seen in bones weakened by disease. Healing of fractures is a slow process, taking a period of months at best, longer in older people or if the fracture is complicated. Fractures are treated by reducing the fracture, that is, lining up the bone pieces in their original position, and immobilizing the fractured bone until the pieces have time to heal together.

Diseases. We've already discussed rickets and osteomalacia, caused by vitamin D deficiency. Osteoporosis is a loss of bone mass that results in brittle bones which are very susceptible to fracture. It occurs most frequently in people who don't get enough exercise and/or are deficient in estrogen after menopause. While the vast majority of osteoporosis patients are women, in fact men may suffer from osteoporosis as well. Osteomyelitis is inflammation of bone and its marrow, usually due to bacterial infection. This is prevalent following fractures that break the skin, especially in children, and is very difficult to treat successfully. There are also a variety of tumors, some benign and some malignant, which may affect bone, among them osteoma (benign), osteochondroma, osteosarcoma, and chondrosarcoma.

That's it for Chapter 8. Now that you've read the chapter and worked your way through these notes, it is time to work on the chapter review sheets. Then use the chapter objectives to build a study guide for this portion of the test. When you've accomplished this and feel confident that you understand the concepts presented here, you're ready to move on to Chapter 9.

Now that you have a little background in the stuff skeletons are made of, it's time to look at the bones as a system, the skeletal system. We will not be learning the names and features of lots of bones in this chapter. What we'll focus on is mostly the general regions and structure of the skeleton, the relationships among these parts, and their overall functions. This means that there are large parts of this chapter we'll be skipping right over. Don't spend a lot of time studying things that aren't in your reading assignment or objectives.

The skeleton is usually divided for the purpose of study into two general parts, the axial skeleton and the appendicular skeleton. Since axis means central support, the axial skeleton is just that, the central support for the rest of the body. It consists of the skull, vertebral column (the spine), and the thoracic cage (rib cage--ribs and sternum or breastbone). The word appendicular comes from the word appendage, so the appendicular skeleton consists of the limbs (or appendages) and their attachments. This includes the bones of the upper limbs (arms) and pectoral girdle (shoulder assembly) and the bones of the lower limbs (legs) and pelvic girdle (hip assembly).

At birth you had about 270 bones; then in childhood, you grew even more. But now you have a lot fewer bones, probably around 206. What happened? Well, as you develop and mature, many formerly individual bones fuse together. Not everyone has the same number of bones because some of us grow some additional bones called sesamoid bones. We develop these within tendons in response to stress; they're intended to protect vulnerable areas. The biggest sesamoid bone most of us have (and everyone develops this one) is the patella, or kneecap. Some of us have a few extra skull bones as well. The skeleton is a body feature that varies quite a bit from person to person.

Bones have all sorts of bumps and grooves on them. There are smooth and rough areas, little pores, and ridges. Some of these surface features are sites for attachment of tendons and ligaments; some provide joint surfaces; others have other purposes. A bone can be identified by its general size and shape, along with its characteristic surface features.

The skull, even though it seems simple and smooth, is the most complex part of the skeleton. There are more than 22 individual bones in the skull, many joined with immovable joints called sutures. There are several cavities in the skull; the largest, called the cranial cavity, is for the brain. There are also orbits for the eyes and nasal, buccal, middle-ear, and inner-ear cavities. The paranasal sinuses are hollow places in the bones which are filled with air. These lighten the skull and add resonance to the voice. The skull also has foramina, tiny holes for nerves and blood vessels, and the large foramen magnum (which means large hole) where the spinal cord enters the skull.

The brain doesn't directly contact the skull bones. There is a three-layered membrane, collectively called meninges, between them. The meninges have fluid between them to provide lubrication and shock absorption.

One of the problems with the inflexibility of skull joints is that, if the brain is injured and swells, there is very little "give" in the skull. This means that a great deal of pressure can build up inside the skull; this pressure can cause damage to the brain tissue.

The skull sutures are not fused yet at birth. This is a good thing. One of the gains mammals made in evolutionary history is having a very large brain, which allows us to do many tasks a smaller brain wouldn't permit. This causes a problem in birthing babies, however, because that great big brain means a fairly large head, which is tougher to fit through the mother's pelvic bones at birth. The problem is compounded in humans because we are adapted to walk upright, which means a smaller pelvic outlet (so our abdominal organs don't fall out as we walk along). Now we have the huge head complicated by a small pelvic outlet. One of the reasons human babies are born so helpless compared to babies of other mammals is that we have to get them out of the mother while the head has any chance of fitting through her pelvis; this means less time to finish developing before birth. It also means that birth needs to occur while the skull joints are still soft so the skull bones slide over one another, allowing temporary compression of the head, which eases the tight fit at birth. Many babies are born with pretty misshapen heads because of this compression; they straighten out after a few days.

You can feel the soft places between these unfused skull bones on the head of a baby. These soft spots are called fontanels and will ossify later, most of them by two years of age. The jaw also starts out in two pieces which fuse later, usually by the time a child begins school.

In order to achieve the learning required in the first few years of life, the brain must grow very rapidly, much faster than the rest of the skeleton. It has reached three-quarters of adult size by the age of two and full adult size by second or third grade. This causes a very out-of-proportion appearance in small children; their heads are very large compared to their body size. This is true to some degree in all baby mammals, something that causes the cute appearance that awakens the parenting instincts in adults of many species.

The functions of the vertebral column are to support the skull and trunk; to allow for movement; to protect the spinal cord; to absorb jolts and stresses produced by walking, running, lifting, and other movements; and to provide attachment points for the limbs, the thoracic cage, and muscles which maintain posture.

You have a chain of about 33 vertebrae, although the exact number varies from person to person. There are intervertebral discs made of fibrous cartilage between the individual bones. The vertebrae are usually divided into five regions: cervical (neck), thoracic (chest), lumbar (lower back), sacral (base of spine), and coccygeal (tailbone).

Each vertebra has its own specific shape, but all of them have a body, which is the largest solid part, and a vertebral foramen which, together with the foramen in each of the other vertebrae, provides a canal for passage of the spinal cord. The vertebrae also have various surface features, some of which are points for the attachment of muscles.

These discs are pads composed of an inner gelatinous substance and a surrounding ring of fibrocartilage. These function to bind the vertebrae together, to support body weight, and to absorb shocks. The discs bulge out to the sides to offset the effects of jolts and bumps; however excessive jolting can crack the disc, allowing the gelatinous insides to leak out. This is what we call a herniated or slipped disc; and it can put pressure on the spinal cord or on spinal nerves, resulting in significant pain and disability.

This includes the thoracic vertebrae, the sternum, and the ribs. It functions as a protective container for the lungs and heart, provides attachment points for the pectoral girdle and upper limbs, protects some abdominal organs (spleen, parts of the liver and kidneys), and participates in breathing.

This set of structures supports the arm. It consists of only two bones on each side, the clavicle or collarbone and the scapula or shoulder blade. The collarbone is a long thin bone that braces the shoulder joint and is the most-fractured bone in the body. It is very close to the surface and because people tend to reach out with their arms when they are falling, tends to take a lot of trauma. If you work hard with your shoulders and arms, your clavicle will be much thicker than other people's. The scapula is a fairly flat triangular plate that lies over the ribs on the back. It is broad, so has many attachment points for muscles, which attach on all sides to hold the bone in place during the stresses it undergoes.

The attachment points to the upper limb in the shoulder are fairly loose. This provides for a great deal of flexibility, but also makes the joint fairly easy to dislocate.

The arms have about thirty bones each. They are generally divided into four regions:

1) The brachium is the actual arm. It reaches from the shoulder to the elbow and contains just one bone, a long bone called the humerus.

2) The forearm, or antebrachium, reaches from the elbow to the wrist. It has just two bones, both long bones, the lateral radius and the medial ulna.

3) The carpus, or wrist, has eight small carpal bones, arranged in two rows. These have somewhat more limited movement than the arm and forearm.

4) The manus, hand, has 19 bones, 5 metacarpals in the palm and 14 phalanges in the fingers.

The pelvic girdle has just four bones in the adult. These are the right and left os coxae (hipbones), the sacrum, and the coccyx. The top flares of the hipbones can be felt at your hips; they're the bones you feel just below your waist on either side. The sacrum and coccyx each consist of several fused vertebrae. The pelvic girdle has the important function of supporting the entire trunk on the legs, as well as protecting the pelvic organs, lower intestine, bladder, and reproductive organs.

The legs are similar to the arms, but they are adapted for weight bearing and locomotion. Once again, there are four regions which roughly correspond to the four regions of the upper limb:

1) The femoral region is the thigh and contains one bone, the femur. This is the body's largest bone. Right at the junction of this region with the next is found the patella, a sesamoid bone also known as the kneecap.

2) The crural region is the leg proper and reaches from knee to ankle. It contains two bones, the medial tibia and the lateral fibula.

3) The tarsal region is in the ankle; its bones are treated as part of the foot.

4) The foot is the pedal region and contains 26 bones, 7 tarsal bones, 5 metatarsals, and 14 phalanges in the toes.

This is enough about specific bones until we get to the skeleton in lab.

So we finish Chapter 9. Now that you've read the chapter and worked your way through these notes, it is time to work on the chapter review sheets. Then use the chapter objectives to build a study guide for this portion of the test. When you've accomplished this and feel confident that you understand the concepts presented here, you're ready to move on to Chapter 10.

Joints are known as articulations, the point at which two bones meet. Specific joints are frequently named for the bones involved. The study of joints is arthrology. The design of every joint is the result of a trade-off between flexibility and the need for support. Highly flexible joints allow for smooth movement, but offer less support; inflexible joints are very strong, but don't permit much movement. Different joints trade off these two factors in various ways. They can be classified as to the freedom of movement possible at the joint:

1) Diarthroses are joints that are freely movable. These are the joints we usually think of as joints, like the knee and hip, shoulder and elbow.

2) Amphiarthroses are joints with only limited movement. We often don't even recognize the presence of a joint in these places, like the spine, the palm of the hand and wrist.

3) Synarthroses are immovable joints, like the sutures of the skull.

A common type of joint is synovial, where the bones are separated by a space filled with a slippery lubricant substance called synovial fluid. These tend to be the most freely movable joints. The bone surfaces at the joint are covered with thick smooth cartilage called articular cartilage. Some joints that take a great deal of weight and stress have an additional pad of cartilage called a meniscus for shock absorption and to guide bones across one another to reduce the chance of dislocation.

In addition, at a synovial joint there will be tendons, strips or sheets of tough connective tissue that attach muscle to bone. These play an important role in keeping the joint stable. There will also be ligaments, similar to tendons, but with the job of holding the bones themselves together at the joint. In addition, where tendon passes over bone and between muscles, there is often one or more bursae, fibrous synovial-fluid-filled sacs. These provide ease of movement and enhance the mechanical effects of muscles.

There are a great many kinds of movements available at various freely movable joints. A few that it is good to know include the following:

Flexion and Extension. Flexion is bending at a joint, for example, bending the elbow, bending the knee, bending at the waist. It decreases the angle between the two bones involved in the joint. Extension is the opposite motion; it occurs when you un-flex the same joint. So straightening the elbow or knee or straightening up at the waist are extensions. Straightening returns the body part to anatomical position. Hyperextension is extension of the joint beyond anatomical position and can be a normal movement permitted by the skeletal and muscular systems or an injury caused by abnormal pulling or pressure on the joint.

Abduction and Adduction. Abduction is movement of a body part away from the midline of the body, for example, raising the arm away from the body. It is easy to remember if you recall that abducting a child is taking it away from the parent; ab- means away, and abduct means to "draw away." Adduction is the opposite action and involves returning the body part to anatomical position.

Circumduction. Circumduction is making a circular motion with one end of an appendage while the other end remains relatively motionless. For example, if you make circles in the air with your hand, your arm is circumducting; the shoulder remains fairly stable while the hand makes a circle. Circumduction actually involves a sequence of other kinds of movements, flexion, extension, etc.

Rotation. Rotation is turning a bone on its longitudinal axis. The best example of this is turning the head. It can also occur at the waist, leg, and arm.

Supination and Pronation. This set of actions is seen only in the forearm and hand. Supination is rotating the hand so the palm faces forward or upward. The name comes from the word supine, which means lying on the back, facing the body upward. Pronation is the opposite action, rotating the hand so the palm faces backward or downward. It comes from the word prone, which means lying on the belly, face down.

Dorsiflexion and Plantar Flexion. These actions are done only with the foot. Dorsiflexion is raising the toes. If you are standing barefoot on the floor and then raise your toes off the floor (leaving the rest of your foot in contact with the floor), that is dorsiflexion. This is because you are flexing the dorsum (top) of the foot. Plantar flexion is again the opposite action, extending the foot so that the toes point downward. Think of a dancer pointing the toes; this is plantar flexion, called that because the plantar (sole of the foot) surface is flexed.

That's it for Chapter 10. Now that you've read the chapter and worked your way through these notes, it is time to work on the chapter review sheets. Then use the chapter objectives to build a study guide for this portion of the test. When you've accomplished this and feel confident that you understand the concepts presented here, you're ready to move on to Chapter 11.

Muscles are organs specialized to produce movement of a body part. Their ability to contract achieves many purposes.

Movement. Muscles enable the body to move from place to place, allow the movement of individual body parts, and allow body contents to be moved during breathing, circulation, defecation, urination, and childbirth.

Stability. The muscles help to maintain posture and prevent unwanted movements. They also hold some bones in place by maintaining tension on the ligaments and tendons that anchor these bones. So muscles not only permit desired movement, but inhibit unwanted movement.

Communication. Muscles enable communication by speech and writing and permit nonverbal communication by means of facial expression and body movements.

Control of Body Openings and Passages. The sphincters, ring-shaped muscles around passageways, control admission and elimination of substances from the body. Other circular muscles move substances through passageways in the body.

Heat Production. Skeletal muscles produce much of the heat used to maintain body temperature.

Endomysium is a layer of connective tissue which surrounds each muscle cell. It allows room for capillaries and nerve fibers to reach each muscle fiber.

Perimysium is a connective tissue sheath on each bundle of muscle fibers called a fascicle. This holds the fibers in parallel strands. The strength of a muscle contraction depends on the number of fascicles of the muscle involved in the contraction.

Epimysium is a connective tissue layer which surrounds the entire muscle. This layer gradually turns into the connective tissue sheaths called fascia. Deep fascia occur between adjacent muscles; and superficial fascia occur between muscles and skin.

Muscles connect to bone in two different ways. Direct attachment occurs when the fibers of epimysium are continuous with the periosteum of the bone itself. These muscle fibers appear to grow right out of the bone. Indirect attachment occurs when collagen fibers of the epimysium taper into a strong tendon that is continuous with the periosteum of the bone. Some fibers of the periosteum continue right into the matrix of the bone, giving a very strong structure. Tendons are more likely to tear than to detach from either their muscle or the bone.

Some muscles connect to a broad sheet-like tendon called an aponeurosis. And some groups of tendons from separate muscles pass together under a band of connective tissue called a retinaculum, which holds the tendons in place while they can slide easily under it.

Most skeletal muscles are attached to different bones at their two ends and span a joint. Contracting the muscle moves the bone at one end; the other acts as an anchor for the movement to provide leverage. The origin of a muscle is the end that attaches to the stable bone; and the insertion of the muscle is the end that attaches to the bone which moves. The thicker middle portion of the muscle is called its belly.

Individual muscles produce movements called their action, but muscles seldom act independently. They generally function in groups whose coordinated actions produce a particular movement at a joint. Sometimes the same muscle acts in different ways at the same joint, depending on what other muscles are acting at the same time.

In a muscle group, the agonist or prime mover is the muscle mostly responsible for the force at a joint. A muscle that aids the prime mover in producing its action is called a synergist. The synergist may add power to a movement or may simply stabilize the joint during its movement, restrict undesirable movements, or modify the direction of movement. Antagonists are muscles that oppose the prime movers. These may have a variety of effects. Sometimes they simply relax to permit the prime mover control over an action. Other times they operate in opposition to a movement to provide some moderation to the movement, preventing excessive movement and joint injury. Antagonists are also the muscles that return the body part to its original position. Since muscles can't push—only pull—on a bone, another muscle with opposite action is required to return a part to its starting position. Fixators are muscles that hold bones steady so that other muscles attached to them can contract to produce some other movement.

That's it for Chapter 11. You'll note that once again we're ignoring the specific muscles in various body locations; these are not important in this course. Now that you've read the chapter and worked your way through these notes, it is time to go to work on the chapter review sheets. Than use the chapter objectives to build a study guide for this portion of the test. When you've accomplished this and feel confident that you understand the concepts presented here, you're ready to move on to Chapter 12.

All muscle tissue exhibits responsiveness or excitability. This ability is highly developed. Stimuli must reach a threshold in order to stimulate muscle contraction.

Muscles also exhibit contractility, that is, they contract or shorten. This is their response to a stimulus. This contraction pulls on bones or other tissue to create movements.

Extensibility is another characteristic of muscles; they can stretch out to their full length between contractions.

And they have elasticity, the ability to return to their original shape after tension on them has been released.

Muscles are generally classified according to two characteristics, whether they're voluntary, that is, under conscious control, and whether they have striations, which look like light and dark bands.

Muscle which is voluntary and striated is skeletal muscle. Skeletal muscles are usually attached to one or more bones to produce movements. Most of the rest of this chapter is about skeletal muscles.

Muscle which is involuntary and striated is cardiac muscle. This is found only in the heart. These muscle fibers contract rhythmically without receiving a stimulus from the nervous system; they are called autorhythmic. The heart has a built-in pacemaker that stimulates waves of contraction so that the heart's muscle fibers contract in a coordinated pattern. The autonomic nervous system does exert some control over the rate and strength of contractions in heart muscle, but the contractions themselves are independent of nervous system control. These muscle fibers operate almost exclusively aerobically and are very resistant to fatigue, but are vulnerable to an interruption in their oxygen supply.

Involuntary, unstriated muscle is called smooth muscle. Some smooth muscles contract independently without nervous stimulation. These may respond to hormonal stimulation or changes in their chemical environments (like changing pH, oxygen levels, and carbon dioxide levels). Others have their own pacemaker cells and contract autorhythmically like heart muscle. Others respond to nervous stimulation. Still others respond to stretch, which by itself can stimulate contraction; this is the mechanism by which muscles move the contents down a tubular organ. This wavelike sequential contraction of circular muscles around a tubular structure is called peristalsis, and it is often triggered by stretch as the structure fills.

A nerve signal reaches the end of a neuron (nerve cell) and spreads out into the terminal branches of the cell. These branches stimulate all the muscle fibers supplied by that particular neuron. One nerve fiber and all the muscle fibers it innervates are known as a motor unit. The muscle fibers of a single motor unit tend to be dispersed throughout a muscle to produce a weak contraction over a wide area. Smaller motor units (only a few muscle fibers per nerve fiber) give very fine control of small movements; large motor units (up to 1000 muscle fibers per nerve fiber) provide strong movements with less fine control. The large units are much harder to stimulate than the small ones. With multiple motor units in a single muscle, the individual muscle fibers work in shifts and are less prone to fatigue because they take turns resting.

The contraction cycle in a skeletal muscle fiber has four phases:

1) Excitation, when a stimulus is applied to the muscle fiber

2) Latent period in which the muscle fiber prepares to contract

3) Contraction in which the muscle fiber develops tension and may shorten

4) Relaxation in which the fiber returns to its resting length

A muscle fiber exhibits an all-or-none response to stimulation, meaning that fibers can't contract to varying degrees; they simply contract when the stimulus reaches the threshold level and don't contract when it doesn't reach the threshold. Different contraction strengths are caused by the number of fibers involved in a muscle contraction.

Conditions that influence the strength of a muscle contraction in the entire muscle include the amount of oxygen available, the amount of available glucose, the amount of waste products in the muscle, and the overall health of the cells.

Not all contraction leads to shortening of the muscle. Some contraction simply produces tension against an external resistance so that the muscle stays the same length. This sort of contraction is called isometric contraction. This is what produces muscle tone. It doesn't involve movements, but maintains posture and prevents atrophy, which is a loss of muscle tone.

Isotonic contraction is contraction that does involve a change in length of the muscle, but no change in tension on the muscle. An isotonic contraction begins when internal tension builds to the point where tension overcomes resistance to movement, so the muscle shortens.

That's it for Chapter 12. Once again, we've left out a great deal; you can't learn everything, so we're concentrating on the information I think will be most helpful to you in your health careers. You've finished the final chapter in Lesson Two! Now it's time to pull out the last chapter review sheet, then use the objectives to build a study guide for this last chapter. When you're confident you've learned the information in these six chapters and understand the concepts presented here, request a test via e-mail.