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The Muscular System

The muscular system is the body's network of tissues that controls movement both of the body and within it. Walking, running, jumping: all these actions propelling the body through space are possible only because of the contraction (shortening) and relaxation of muscles. These major movements, however, are not the only ones directed by muscular activity. Muscles make it possible to stand, sit, speak, and blink. Even more, were it not for muscles, blood would not rush through blood vessels, air would not fill lungs, and food would not move through the digestive system. In short, muscles are the machines of the body, allowing it to work.

DESIGN: PARTS OF THE MUSCULAR SYSTEM

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The muscles of the body are divided into three main types: skeletal, smooth, and cardiac. As their name implies, skeletal muscles are attached to the skeleton and move various parts of the body. They are composed of tissue fibers that are striated or striped. The alternating bands of light and dark result from the pattern of the filaments (threadlike proteins) within each muscle cell. Skeletal muscles are called voluntary muscles because a person controls their use, such as in the flexing of an arm or the raising of a foot.

There are just over 650 skeletal muscles in the whole human body. Some authorities state there are as many as 850 muscles in the body. No exact figure is available because scientists disagree about which ones are separate muscles and which ones are part of larger muscles. There is also some variability in muscular structure between individuals.

Smooth muscles are found in the stomach and intestinal walls, in artery and vein walls, and in various hollow organs. They are called involuntary muscles because a person generally cannot consciously control them. They are regulated by the autonomic nervous system (a division of the nervous system that affects internal organs such as the heart, lungs, stomach, and liver). Unlike skeletal muscles, smooth muscles have no striations or stripes.

In a vessel or organ, smooth muscles are arranged in sheets or layers. Often, there are two layers, one running circularly (around) and the other longitudinally (up and down). As the two layers alternately contract and relax, the shape of the vessel or organ changes and fluid or food is propelled along. Smooth muscles contract slowly and can remain contracted for a long period of time without tiring.

The Muscular System: Words to Know

<dl><dt>Acetylcholine (ah-see-til-KOE-leen):</dt><dd>Neurotransmitter chemical released at the neuromuscular junction by motor neurons that translates messages from the brain to muscle fibers.</dd><dt>Adenosine triphosphate (ah-DEN-o-seen try-FOS-fate):</dt><dd>High-energy molecule found in every cell in the body.</dd><dt>Aerobic metabolism (air-ROH-bic muh-TAB-uhlizm):</dt><dd>Chemical reactions that require oxygen in order to create adenosine triphosphate.</dd><dt>Antagonist (an-TAG-o-nist):</dt><dd>Muscle that acts in opposition to a prime mover.</dd><dt>Cramp:</dt><dd>Prolonged muscle spasm.</dd><dt>Fascicle (FA-si-kul):</dt><dd>Bundle of myofibrils wrapped together by connective tissue.</dd><dt>Lactic acid (LAK-tik ASS-id):</dt><dd>Chemical waste product created when muscle fibers break down glucose without the proper amount of oxygen.</dd><dt>Muscle tone:</dt><dd>Sustained partial contraction of certain muscle fibers in all muscles.</dd><dt>Myofibrils (my-o-FIE-brilz):</dt><dd>cylindrical structures lying within skeletal muscle fibers that are composed of repeating structural units called sarcomeres.</dd><dt>Myofilament (my-o-FILL-ah-ment):</dt><dd>Protein filament composing the myofibrils; can be either thick (composed of myosin) or thin (composed of actin).</dd><dt>Neuromuscular junction (nu-row-MUSS-ku-lar-JUNK-shun):</dt><dd>Region where a motor neuron comes into close contact with a muscle fiber.</dd><dt>Prime mover (or agonist):</dt><dd>Muscle whose contractions are chiefly responsible for producing a particular movement.</dd><dt>Rigor mortis (RIG-er MOR-tis):</dt><dd>Rigid state of the body after death due to irreversible muscle contractions.</dd><dt>Sarcomere (SAR-koh-meer):</dt><dd>Unit of contraction in a skeletal muscle fiber containing a precise arrangement of thick and thin myofilaments.</dd><dt>Spasm:</dt><dd>Sudden, involuntary muscle contraction.</dd><dt>Strain:</dt><dd>Slight tear in a muscle; also called a pulled muscle.</dd><dt>Synergist (SIN-er-jist):</dt><dd>Muscle that cooperates with another to produce a particular movement.</dd><dt>Tendon (TEN-den):</dt><dd>Tough, white, cordlike tissue that attaches muscle to bone.</dd></dl>

Cardiac muscle, called the myocardium, is found in only one place in the body: the heart. It is a unique type of muscle. Like skeletal muscle, it is

striated. But like smooth muscle, it is involuntary, controlled by the autonomic nervous system. The myocardium is composed of thick bundles of muscle that are twisted and whorled into ringlike arrangements. Forming the walls of the chambers of the heart, the myocardium contracts to pump blood throughout the body (for a further discussion of its actions, see chapter 1).

Of the three types of muscle, skeletal are probably the most familiar. They stabilize joints, help maintain posture, and give the body its general shape. In men, they make up about 40 percent of the body's mass or weight; in women, about 23 percent. Since the term muscular system refers specifically to skeletal muscles, the remainder of this chapter will focus on them.

Structure of muscle cells

Each muscle is made of hundreds to thousands of individual muscle cells. Unlike most other cells in the body, these cells are unusually shaped: they are elongated like a cylinder or a long rod. Because of their shape, muscle cells are normally referred to as muscle fibers. Whereas most cells have a single nucleus (the part of the cell that controls its activities), muscle fibers have as many as 100 or more nuclei. The nuclei are located on the surface of the fiber, just under its thin membrane. Another difference between muscle fibers and other body cells is their size. They can extend the entire length of a muscle. For example, a muscle fiber in a thigh muscle could measure 0.0004 inch (0.001 centimeter) in diameter and 12 to 16 inches (30 to 40 centimeters) in length.

RIGOR MORTIS

When a person dies, blood stops circulating through the body. The skeletal muscles (along with all other parts of the body) are deprived of oxygen and nutrients, including ATP. Calcium ions leak out of their storage area in the membranes of muscle fibers, causing thick myofilaments to attach to and pull thin myofilaments. While the muscle fibers still have a stored supply of ATP, the heads of thick myofilaments are able to detach from the thin myofilaments. When the supply of ATP runs out, however, the heads cannot detach and the muscle fibers stay in a contracted position.

The rigid state of muscle contraction that results is called rigor mortis. Depending on the person's physical condition at death, the onset of rigor mortis may vary from ten minutes to several hours after death. Facial muscles are usually affected first, followed by other parts of the body. Rigor mortis lasts until the muscle fibers begin to decompose fifteen to twenty-five hours after death.

Each muscle fiber is composed of hundreds of smaller filaments or threads called myofibrils (the prefix myocomes from the Latin word myos, meaning "muscle"). Each myofibril contains bundles of threadlike proteins or filaments called myofilaments, which can be either thick or thin. The larger thick myofilaments are made mostly of bundled molecules of the protein myosin. The thin myofilaments are composed of the protein actin.

In each myofibril, the thick and thin myofilaments are combined into thousands of units or segments that repeat over and over. These units are called sarcomeres. Thick myofilaments lie in the center of a sarcomere. Thin myofilaments are attached at either end of a sarcomere and extend toward the center, passing among the thick myofilaments. This regular arrangement of the varying myofilaments within each sarcomere produces the striated or striped appearance of each myofibril and, by extension, of muscle fibers.

As are most living cells, muscle fibers are soft and fragile. Even so, they can exert tremendous power without being ripped apart. The reason is that muscles are composed of different types of tissue (like all other organs in the body). In addition, those tissues are bundled together, providing strength and support. Each myofibril is enclosed in a delicate sheath or covering made of connective tissue (tissue found everywhere in the body that connects body parts, providing support, storage, and protection). Numerous sheathed myofibrils are then bundled together and wrapped with thicker connective tissue to form what is called a fascicle (from the Latin word fasciculus, meaning "a bundle"). Many fascicles are then bundled together by an even tougher coat of connective tissue to form the muscle.

Tendons

The layers of connective tissue that bundle the various parts of a muscle usually converge or come together at the end of the muscle to form a tough, white, cord-like tissue called a tendon. Tendons attach muscles to bone. Because they contain fibers of the tough protein collagen, tendons are much stronger than muscle tissue. The collagen fibers are arranged in a tendon in a wavy way so that it can stretch and provide additional length at the muscle-bone junction. As muscles are used, the tendons are able to withstand the constant pulling and tugging.

Muscles are always attached at both of their ends. The end that is attached to a bone that moves when the muscle contracts is called the insertion. The other end, attached to a bone that does not move when the muscle contracts, is called the origin. It is important to note that not all muscles are attached to bones at both ends. The ends of some muscles are attached to other muscles; some are attached to the skin.

Major muscles of the body

Skeletal muscles that support the skull, backbone, and rib cage are called axial skeletal muscles. These include the muscles of the head and neck and those of the trunk. Roughly 60 percent of all skeletal muscles in the body are axial muscles. The skeletal muscles of the limbs (arms and legs) are called distal or appendicular skeletal muscles. These include the muscles of the shoulders and arms and those of the hip and legs.

Muscle names are descriptive. Some muscles are named according to their location in the body. For example, the frontalis muscle overlies the frontal bone of the skull. Other muscles are named for their relative size. Terms such as maximus (largest), minimus (smallest), and longus (long) are often used as part of a muscle's name. Still other muscles are named for their shape. The deltoid muscle is so named because it has the shape of the Greek letter delta, which is triangular-shaped. And some muscles are named for their actions. Terms such as flexor (to flex or bend in), extensor (to extend or straighten out), adductor (to draw toward a line that runs down the middle of the body), and abductor (to draw away from a line that runs down the middle of the body) are often added as part of a muscle's name.

Please note: in the naming of the major muscles of the body on the following pages, pronunciations are provided in parenthesis when necessary.

MUSCLES OF THE HEAD AND NECK. The muscles of the face are unique: they are attached to the skull on one end and to the skin or other muscles on the other end. Muscles that are attached to the skin of the face allow people to express emotions through actions such as smiling, frowning, pouting, and kissing.

As mentioned, the frontalis (frun-TA-lis) covers the frontal bone or forehead. The temporalis (tem-po-RAL-is) is a fan-shaped muscle overlying the temporal bone on each side of the head above the ear. The orbicularis oculi (or-bik-u-LAR-is OK-u-lie) encircles each eye and helps close the eyelid. The orbicularis oris (or-bik-u-LAR-is OR-is) is the circular muscle around the lips. It closes and extends the lips.

The masseter (mas-SE-ter), located over the rear of the lower jaw on each side of the face, opens and closes the jaw, allowing chewing. The buccinator (BUK-si-na-tor), running horizontally across each cheek, flattens the cheek and pulls back the corners of the mouth. The sternocleidomastoid (ster-nokli-do-MAS-toyd), located on either side of the neck and extending from the

clavicle or collarbone to the temporal bone on the side of the head, allows the head to rotate and the neck to flex.

MUSCLES OF THE TRUNK. On the front part of the trunk or torso, the pectoralis major (pek-to-RA-lis MA-jor) are the large, fan-shaped muscles that cover the upper part of the chest. They flex the shoulders and pull the arms into the body. The rectus abdominis (REK-tus ab-DOM-i-nis) are the strap-like muscles of the abdomen, extending from the ribs to the pelvis. Better known as the stomach muscles, they flex the vertebral column or backbone and provide support for the abdomen and its many organs. The muscles making up the side walls of the abdomen are the external oblique (ex-TER-nal o-BLEEK). In addition to helping compress the abdomen, they rotate the trunk and allow it to bend sideways.

On the rear part of the trunk, the trapezius (trah-PEE-zee-us) are the kite-shaped muscles that run from the back of the neck and upper back down to the middle of the back. They raise, lower, and adduct the shoulders. The large, flat muscles that cover the lower back are the latissimus dorsi (lah-TIS-i-mus DOR-see). They adduct and rotate the arms and help extend the shoulders.

MACHINES WITH MUSCLES

Robots and machines that move and pick up objects like humans may no longer be found only in science fiction novels and movies. Scientists have created various artificial muscles that contract and expand just like human muscles. Unlike human muscles, however, artificial muscles have no limit to their strength.

One such artificial muscle is made out of artificial silk, which is cooked and then boiled to make a rubbery, semiliquid substance. The substance is similar in structure to human muscle, composed of smaller and smaller fibers. These fibers are naturally negatively charged with electricity.

When an acid (which has a positive electrical charge) is applied to this substance, the negative and positive ions attract each other and the substance contracts. When a base material (which has a negative charge) is applied, the ions repel each other and the material expands.

The National Aeronautics and Space Administration (NASA) has plans for artificial muscles. A small NASA rover destined to explore an asteroid in 2002 will be equipped with artificial muscles. Scientists hope tests like this one will eventually lead to the creation of space robots with humanlike flexibility and movement. Beyond that, they hope artificial muscles may someday be used to replace defective muscles in humans.

MUSCLES OF THE SHOULDERS AND ARMS. The fleshy, triangular-shaped muscles that form the rounded shape of the shoulders are the deltoid (DEL-toyd). They help abduct the arm, or move it away from the middle of the body. The most familiar muscle of the upper arm is the biceps brachii (BI-seps BRAY-key-eye.) Located on the front of the upper arm, the bicep makes a prominent bulge as it flexes the elbow. On the rear portion of the upper arms is the triceps brachii (TRY-seps BRAY-key-eye). Its action is just the opposite of the biceps: it extends or straightens the forearm.

The muscles of the forearm, which move the bones of the hands, are thin and long. Of these many muscles, the flexor carpi (FLEX-or CAR-pee) bend the wrist and the flexor digitorum (FLEX-or di-ji-TOR-um) bend the fingers. The muscles that have the opposite effect, extending the wrist and fingers, are the extensor carpi and the extensor digitorum.

MUSCLES OF THE HIPS AND LEGS. Muscles of the lower limbs cause movement at the hip, knee, and foot joints. These muscles are among the largest and strongest muscles in the body. Muscles on the thigh (upper portion of the leg) are especially massive and powerful since they hold the body upright against the force of gravity.

The gluteus maximus (GLOO-tee-us MAX-i-mus) are the large muscles that form most of the flesh of the buttocks. These powerful muscles help extend the hip in activities such as climbing stairs and jumping. The adductor (ah-DUC-ter) muscles are a group of muscles that form a mass on the inside of the thighs. As their name indicates, they adduct or press the thighs together.

On the front of the thigh is a group of four muscles known collectively as the quadriceps (KWOD-ri-seps). Together, the quadriceps help powerfully extend or straighten the knee, such as when an individual kicks a soccer ball. On the back of the thigh, a group of three muscles performs the

opposite effect. Known as hamstrings (HAM-strings), these muscles flex or bend the knee.

The sartorius (sar-TOR-ee-us) is long, straplike muscle that crosses the front of the thigh diagonally from the outside of the hip to the inside of the knee. Although it is not that powerful, it does lie on upper surface of the thigh and is easily seen. The sartorius helps rotate the leg so an individual can sit in a cross-legged position with the knees wide apart.

On the back part of the lower leg is the calf muscle, properly known as the gastrocnemius (gas-trok-NEE-me-us). This diamond-shaped muscle, formed in two sections, helps extend or lower the foot, such as when an individual walks on his or her toes. The strong tendon that attaches the gastrocnemius to the heel of the foot is the well-known Achilles tendon (ah-KI-leez; in Greek mythology, a hero of the Trojan War who is killed by an arrow shot into his heel). The main muscle on the front part of the lower leg, the tibialis anterior (tib-ee-A-lis), opposes the action of the gastrocnemius. It flexes and inverts or elevates the foot. When runners and other athletes experience tenderness and pain in the front part of the lower leg, a condition commonly known as shin splints, the tibialis anterior has been strained or pulled.

WORKINGS: HOW THE MUSCULAR SYSTEM FUNCTIONS

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Muscles have three important functions: to produce movement, maintain posture, and generate heat. Almost all movements by the human body result from muscle contraction. Muscles lend support to the body and help it maintain posture against the force of gravity. Even when the body is at rest (or asleep), muscle fibers are contracting to maintain muscle tone. Finally, any activity by muscles generates heat as a byproduct, which is vital in maintaining normal body temperature.

MUSCLE FACTS

Smallest muscle in the body?

Stapedius: the muscle that activates the stirrup, the small bone that sends vibrations from the eardrum to the inner ear. It measures just 0.05 inch (0.13 centimeter) in length.

Largest muscle in the body?

Latissimus dorsi: the large, flat muscle pair that covers the middle and lower back.

Longest muscle in the body?

Sartorius: the straplike muscle that runs diagonally from the waist down across the front of the thigh to the knee.

Strongest muscle in the body?

Gluteus maximus: the muscle pair of the hip that form most of the flesh of the buttocks.

Fastest-reacting muscle in the body?

Orbicularis oculi: the muscle that encircles the eye and closes the eyelid. It contracts in less than 0.01 second.

Number of muscles used to make a smile?

Seventeen.

Number of muscles used to make a frown?

Forty-three.

The link between nerve cells and muscle fibers

In order to contract or shorten, muscle fibers must be stimulated by nerve impulses sent through motor neurons or nerves. These impulses originate in the brain, then run down the spine. From there, they branch out to all parts of the body.

A single motor neuron may stimulate a few muscle fibers or hundreds of them. A motor neuron along with all the fibers it stimulates is called a motor unit. When a motor neuron reaches a muscle fiber, it does not touch the fiber, but fits into a hollow on the surface of the muscle fiber. This region where the end of the motor neuron and the membrane of the muscle fiber come close together is called the neuromuscular junction.

When a nerve impulse reaches the end of the motor neuron at the neuromuscular junction, acetylcholine (a neurotransmitter chemical) is released. Acetylcholine then travels across the small gap between the motor neuron and the muscle fiber and attaches to receptors on the membrane of the muscle fiber. This triggers an electrical charge that quickly travels from one end of the muscle fiber to the other, causing it to contract.

DISCOVERING THE LINK BETWEEN NERVES AND MUSCLES

Swiss biologist Victor Albrecht von Haller (1708–1777) was the first scientist to discover the relationship between nerves and muscles. Prior to his research, scientists knew little about the structure and function of nerves or about their interaction with muscles. A popular theory at the time even held that nerves were hollow tubes through which a spirit or fluid flowed.

Haller rejected this theory, especially since no one had ever been able to locate or identify such a spirit or fluid. Instead, he sought to prove that a muscle contracts when a stimulus is applied to it. Haller labeled this action irritability.

In his research, Haller soon found that irritability increased when a stimulus was applied to a nerve connected to a muscle. He then rightly concluded that in order for a muscle to contract, a stimulus had to come from its connecting nerve.

The sliding filament theory

In 1950, while working to explain exactly how muscles contract, two teams of scientists developed the same theory at the same time: the sliding filament theory. Today, medical researchers accept this theory as a good description of what happens to make a muscle contact.

According to the sliding filament theory, thick myofilaments have branches or arms that extend out from their main body. At the end of the branches are thickened heads (the appearance of a thick myofilament can be likened to a racing shell or a long narrow boat with many oars attached on either side). Normally, when a muscle is relaxed, the thick and thin myofilaments do not interact. When the muscle is stimulated to contract, they do.

The electrical charge triggered by acetylcholine stimulates the release of calcium ions (an ion is an atom or group or atoms that has an electrical charge) stored within the muscle fiber. The ions attach to the thin myofilaments and remove their protective coverings. The arms of the thick myofilaments then reach out, and the heads on the arms attach to open sites on the thin myofilaments. The arms pivot (an action called a power stroke), pulling the thin myofilaments toward the center of the sarcomere. This shortens the sarcomere. As this event occurs simultaneously throughout all sarcomeres in a muscle fiber, the muscle fiber shortens or contracts.

A single nerve impulse produces only one contraction, which lasts between 0.01 and 0.04 second. For a muscle fiber to remain contracted, the brain must send additional nerve impulses. When nerve impulses cease, so do the electrical charges, the release of calcium ions, and the connection between thin myofilaments and thick myofilaments.

MUSCLES IN SPACE

In the zero gravity of space, astronauts face many challenges. Chief among these is the effect of weightlessness on muscles. Even after spending as little as four or five days in space, astronauts have experienced significant muscle and bone changes.

The reason is that more than half the muscles in the human body are designed primarily to fight gravity. In a weightless environment, those muscles are not used. As a result, they quickly weaken and atrophy or waste away. Without the stress of pumping blood through the body against the force of gravity, the muscles of the heart also begin to weaken considerably.

Exercising during space flights is one way astronauts have tried to counter the effects of zero gravity. Unfortunately, they have had to exercise two to three hours a day just to maintain muscle and cardiovascular strength. The National Aeronautics and Space Administration (NASA) and research centers are currently working to develop exercising devices that recreate the forces on Earth so astronauts can spend more time exploring instead of exercising.

When a muscle fiber contracts, it does so completely and always produces the same amount of pull (tension). The muscle fiber is either "on" or "off." This is known as the all-or-nothing principle of muscle contraction. While this principle applies to individual muscle fibers, it does not apply to entire muscles. A muscle would be useless if it could only contract completely or not at all. The amount of tension or pull in a muscle can vary depending on how many muscle fibers in that muscle are stimulated to contract.

Muscle fiber energy

In order to contract, muscles need energy. That energy comes from adenosine triphosphate (ATP), a high-energy molecule found in every cell in the body. ATP is the only energy source that muscles can use to power their activity. Thick myofilaments need ATP in order to detach their heads from thin myofilaments. They then use the energy from the ATP to complete their next power stroke.

Yet, muscle fibers store only a limited supply of ATP—about 4 to 6 seconds' worth. For muscles to continue working, ATP must be supplied continuously. The most abundant energy source for ATP is glycogen—a starch form of the simple sugar glucose made up of thousands of glucose units. In the human body, the liver stores glucose by converting it to glycogen. When the body needs energy, the liver is stimulated to change glycogen back into glucose and secrete it into the bloodstream for use by the cells.

In the cells, glucose combines with oxygen to yield or produce carbon dioxide, water, heat, and ATP. This process of energy production that uses oxygen in the reaction is called aerobic ("with air") metabolism. Carbon dioxide, water, and heat are all waste products of this chemical reaction. Carbon dioxide moves from the cells into the blood to be carried to the lungs, where it is exhaled. The water becomes a necessary part of a cell's internal fluid. The heat contributes to normal body temperature. If too much heat is generated, such as during vigorous physical activities, the excess heat is carried away and removed from the body through the process of sweating.

EXERCISE AND MUSCLE FATIGUE. Even though muscle fibers store some oxygen, that oxygen is quickly used up, especially during strenuous exercise. In order to convert glucose into ATP so they can continue working, muscles must receive more oxygen via the blood. That is why respiration or breathing rate increases during physical exertion. In times where work or play activities are exhausting, muscle fibers may literally run out of oxygen. If not enough oxygen is present in muscle fibers, the fibers convert glucose into lactic acid, a chemical waste product.

WHY DOES THAT HAPPEN?

Q: Why do I shiver when I become cold?

A: When muscles need to create ATP, their only energy source, they combine glucose with oxygen. This reaction also creates heat as a by-product. The body uses this heat to maintain normal body temperature.

When the temperature of the body drops below normal, the brain signals the muscles to contract rapidly—what we perceive as shivering. The heat generated by these rapid muscle contractions helps to raise or at least stabilize body temperature.

When lactic acid builds up in muscle fibers, it increases the acidity in the fibers. Key enzymes in the fibers are then deactivated, and the fibers can no longer function properly. As a result, muscles are not as effective, contracting less and less. This condition is known as muscle fatigue.

In a state of fatigue, muscle contractions may be painful. Finally, muscles may simply stop working.

Lactic acid is normally carried away from muscles by the blood. It is then transported to the liver, where it is changed back into glucose. In order to do this, however, the liver needs ATP. To produce ATP in the liver, oxygen is once again needed. This is why breathing rate remains high even after vigorous physical activity is stopped. Only after the liver produces the necessary ATP does breathing gradually return to normal.

Movement and muscle arrangement

Muscles cannot push; they can only pull. In order to create movement, muscles must act in pairs. Muscles are arranged on the skeleton in such a way that the flexing or contracting of one muscle or group of muscles is usually balanced by the lengthening or relaxation of another muscle or group of muscles. In other words, when a muscle performs an action, another can undo or reverse that action.

For example, when the biceps (muscle on the front of the upper arm) contracts, the forearm moves in at the elbow toward the biceps; at the same time, the triceps (muscle on the rear of the upper arms) lengthens. When the forearm is moved out in a straight-arm position, the opposite occurs: the triceps contracts and the biceps lengthens.

A muscle whose contraction is responsible for producing a particular movement is called a prime mover (or an agonist). A muscle that opposes or reverses the movement of a prime mover is called an antagonist. Generally, antagonistic muscles are located on the opposite side of a limb or portion of the body from prime mover or agonist muscles.

In the previous example, the biceps is the prime mover behind the flexing of the elbow. In this movement, the triceps is the antagonist of the biceps. When the forearm is straightened out (and the elbow is extended), the triceps becomes the prime mover and the biceps is the antagonist.

Most muscles do not act by themselves to produce a particular movement. Muscles that help prime movers by producing the same movement or by reducing unnecessary movement are called synergists. When the biceps flexes the elbow, smaller muscles in the upper arm also come into play. If the elbow is flexed with the palm of the hand up, the biceps is the prime mover. However, if the elbow is flexed with the palm down or the thumb up (palm in), the other muscles become the prime movers. These particular synergistic muscles allow for greater mobility or movement of the hand when the elbow is flexed.

Although prime movers are mainly responsible for producing certain body movements, the actions of antagonists and synergists are equally important. Without the combined efforts of all three types of muscles, body movements would not be smooth, coordinated, and precise.

Muscle tone

Even when the body is at rest, certain muscle fibers in all muscles are contracting. This activity is directed by the brain and cannot be controlled consciously. This state of continuous partial muscle contractions is known as muscle tone. These contractions are not strong enough to produce movement, but do tense and firm the muscles. In doing so, they keep the muscles firm, healthy, and ready for action. Muscles with moderate muscle tone are firm and solid, whereas ones with little muscle tone are limp and soft.

Muscle tone is the result of different motor units throughout a muscle being stimulated by the nervous system in an orderly way. First one group of motor units is stimulated, then another. Alternate fibers contract so the muscle as a whole does not become fatigued.

Muscle tone is important because it helps human beings maintain an upright posture. Without muscle tone, an individual would not be able to sit up straight in a chair or hold his or her head up. Muscle tone is also important because it generates heat to help maintain body temperature. Normal muscle tone accounts for about 25 percent of the heat in a body at rest.

AILMENTS: WHAT CAN GO WRONG WITH THE MUSCULAR SYSTEM

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With their rich supply of blood, skeletal muscles are fairly resistant to infection. When following a healthy lifestyle, few people will experience a life-threatening muscular ailment. Though rare, serious disorders can target the muscles. A few disorders can affect the muscles indirectly by attacking the nerves that stimulate muscles. Among these ailments are botulism and tetanus.

The following are a few of the disorders that can affect the muscular system, from common injuries caused by misuse to indirectly caused serious disorders.

Botulism

Botulism, or severe food poisoning, is caused by a toxin (poison) produced by a certain bacteria that is sometimes present in foods not properly canned or preserved. Once released by the bacteria in the body, the toxin prevents motor neurons from releasing acetylcholine at neuromuscular junctions. Muscle fibers are then not stimulated to contract and paralysis (partial or complete loss of the ability to move) results. As botulism progresses, the muscles controlling breathing fail and the affected individual suffocates.

Botulism is a serious disease that requires prompt medical attention. Antibiotics are not effective in preventing or treating the disease. Medical researchers have developed an antitoxin (antibody capable of acting against a toxin) for treating botulism. However, since it only works on the toxin when it is not attached to nerve endings, the antitoxin must be given to an infected individual as soon as possible. Motor neuron endings that have already been affected by the toxin cannot be saved. If an individual survives a severe case of botulism, it may weeks to months to years for the body to recover fully, if at all.

Muscular dystrophy

The most common type of genetic (inherited) muscular disorder is muscular dystrophy. This disease causes skeletal muscles to waste away slowly and progressively. Medical researchers generally recognize nine types of muscular dystrophy. The causes behind some of these types are not well understood. In others, researchers believe that proteins used by muscle fibers to protect their membranes are defective, leading to deterioration of the membranes and the muscle fibers.

The most frequent and most dreaded type of muscular dystrophy appears in boys aged three to seven. (Boys are usually affected because it is a sex-linked condition; girls are carriers of the disease and are usually not affected.) In the United States, this type of the disease occurs in about 1 in 3,500 births and affects approximately 8,000 boys and young men.

MUSCULAR SYSTEM DISORDERS

Botulism (BOCH-a-liz-em): Form of food poisoning in which a bacterial toxin prevents the release of acetylcholine at neuromuscular junctions, resulting in paralysis.

Muscular dystrophy (MUS-kyu-lar DIS-tro-fee): One of several inherited muscular diseases in which a person's muscles gradually and irreversibly deteriorate, causing weakness and eventually complete disability.

Myasthenia gravis (my-ass-THEH-nee-ah GRA-vis): Autoimmune disease in which antibodies attack acetylcholine, blocking the transmission of nerve impulses to muscle fibers.

Tetanus (TET-n-es): Bacterial disease in which a bacterial toxin causes the repetitive stimulation of muscle fibers, resulting in convulsive muscle spasms and rigidity.

The first symptom of this disease type is clumsiness in walking and a tendency to fall due to muscle weakness in the legs and pelvis. The disease then spreads to other areas in the body. Sometimes, muscle tissue is replaced by fatty tissue, giving the false impression that the muscles have become enlarged. By the age of ten, a boy is usually confined to a wheelchair or a bed. Death usually occurs before adulthood because of a respiratory infection brought on by the weakness of respiratory or breathing muscles.

Another type of muscular dystrophy appears later in life and affects both sexes equally. The first signs appear in adolescence. The muscles affected are those in the face, shoulders, and upper arms. The hips and legs may also be affected. This type of muscular dystrophy occurs in about 1 out of every 20,000 people. Individuals afflicted with this disease may survive until middle age.

Currently, there is no known cure for any type of muscular dystrophy. Certain drugs have been developed that slow the progression of some types. Physical therapy involving regular, nonstrenuous exercise is often prescribed to help maintain general good health.

Myasthenia gravis

Myasthenia gravis is an autoimmune disease that causes muscle weakness. An autoimmune disease is one in which antibodies (proteins normally produced by the body to fight infection) attack and damage the body's own normal cells, causing tissue destruction. In myasthenia gravis, antibodies attack receptors on the membranes of muscle fibers that receive acetylcholine from motor neurons. Unable to receive acetylcholine, the muscle fibers cannot be stimulated to contract and weakness develops.

About 30,000 people in the United States are affected by myasthenia gravis. The disease can occur at any age, but it is most common in women between the ages of twenty and forty. The muscles of the neck, throat, lips, tongue, face, and eyes are primarily affected. Muscles of the arms, legs, and trunk may also be involved. Depending on the severity of the disease, a person may have difficulty moving their eyes, seeing clearly, walking, speaking clearly, chewing and swallowing, and even breathing. Physical exertion, heat from the Sun, hot showers, hot drinks, and stress may all increase symptoms.

There is no cure for myasthenia gravis, but drugs have been developed that effectively control the symptoms in most people. The disease only causes early death if the respiratory muscles are affected and stop functioning properly.

Spasms and cramps

Muscle spasms and cramps are spontaneous, often painful muscle contractions. Cramps are usually defined as spasms that last over a period of time. Any muscle in the body may be affected, but spasms and cramps are most common in the calves, feet, and hands. While painful, spasms and cramps are harmless and are not related to any disorder, in most cases.

Spasms or cramps may be caused by abnormal activity at any stage in the muscle contraction process, from the brain sending an electrical signal to the muscle fiber relaxing. Prolonged exercise, where sensations of pain and fatigue are often ignored, can lead to such severe energy shortages that a muscle cannot relax, causing a spasm or cramp. Dehydration—the loss of fluids and salts through sweating, vomiting, or diarrhea—can disrupt ion balances in both muscles and nerves. This can prevent them from responding and recovering normally, which can lead to spasms and cramps.

Most simple spasms and cramps require no treatment other than patience and stretching. Gentle stretching and massaging of the affected muscle may ease the pain and hasten recovery.

Strains

Strains are tears in a muscle. Sometimes called pulled muscles, they usually occur because of overexertion (too much tension placed on a muscle) or improper lifting techniques. Strains are common and can affect anyone. Symptoms of strains range from mild muscle stiffness to great soreness.

Mild strains can be treated at home. Basic first aid consists of RICE: R est, I ce for forty-eight hours, C ompression (wrapping in an elastic bandage), and E levation. Strains can be prevented by stretching and warming up before exercising and using proper lifting techniques.

Tetanus

Like botulism, tetanus is also caused by a toxin released by a bacteria. This bacteria invades the body most often through deep puncture wounds exposed to contaminated soil. Many people associate tetanus with wounds from rusty nails or other dirty objects, but any wound can be a source. In the body, the tetanus bacteria releases its toxin, which affects motor neurons at neuromuscular junctions. Its effect, however, is opposite that of the botulism toxin. This toxin causes the repetitive stimulation of muscle fibers, resulting in convulsive muscle spasms and rigidity.

Tetanus is often called "lockjaw" because one of the most common symptoms is a stiff jaw, unable to be opened. The disease sometimes affects the body only at the site of infection. More often, it spreads to the entire body. The uncontrollable muscle spasms produced are sometimes severe enough to cause broken bones. Tetanus results in death when the muscles controlling breathing become "locked" and cannot function.

Up to 30 percent of tetanus victims in the United States die. Prompt medical attention is crucial in handling the disease. Treatment, which can take several weeks, includes antibiotics to kill the bacteria and shots of antitoxin to neutralize the toxin. Recovery can then take six or more weeks. Tetanus, however, is easily preventable through vaccination, which helps the body develop antibodies against the bacteria.

TAKING CARE: KEEPING THE MUSCULAR SYSTEM HEALTHY

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As humans age, all muscle tissues decrease in size and power. Muscle fibers die and are replaced by fibrous connective tissue or by fatty tissue. The connective tissue makes the muscles less flexible. Movement is limited. Even muscles with normal tone will atrophy or waste away.

The effects of this eventual decline in the muscular system can be offset by regular exercise throughout an individual's life. Exercise helps control body weight, strengthens bones, tones and builds muscles, and generally improves the quality of life for people of all ages.

Some types of exercise help to strengthen the heart and lungs. These activities are called aerobic exercises. The American College of Sports Medicine and the Centers for Disease Control and Prevention recommend that people engage in moderate to intense aerobic activity four or more times per week for at least thirty minutes at a time. Walking, jogging, cycling, swimming, and climbing stairs are just a few examples of aerobic activity. These exercises also force the large muscles of the body to use oxygen more efficiently, as well as store greater amounts of ATP.

Exercises that increase the size and strength of muscles are called anaerobic exercises. These types of exercises require quick bursts of energy. Weight lifting (also known as strength training) and sprinting are just two examples anaerobic activity. As muscles grow larger, they require more energy to work, even when the body is at rest. To meet this increased need, the body is forced to use its stored nutrients more efficiently.

When combined with exercise, the following help keep the muscular system operating at peak efficiency: proper nutrition, healthy amounts of good-quality drinking water, adequate rest, and stress reduction.

The "Food Guide" Pyramid developed by the U.S. Departments of Agriculture and Health and Human Services provides easy-to-follow guidelines for a healthy diet. In general, foods that are low in fat (especially saturated fat), low in cholesterol, and high in fiber should be eaten. Fat should make up no more than 30 percent of a person's total daily calorie intake. Breads, cereals, pastas, fruits, and vegetables should form the bulk of a person's diet; meat, fish, nuts, and cheese and other dairy products should make up a lesser portion.

Stress taxes all body systems. Any condition that threatens the body's homeostasis or steady state is a form of stress. Conditions that cause stress may be physical, emotional, or environmental. When stress lasts longer than a few hours, higher energy demands are placed on the body. Combining exercise with proper amounts of sleep, relaxation techniques, and positive thinking will help reduce stress and keep the body in balance.

FOR MORE INFORMATION

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Books

Avila, Victoria. How Our Muscles Work. New York: Chelsea House, 1995.

Ballard, Carol. The Skeleton and Muscular System. Austin, TX: Raintree/Steck-Vaughn, 1997.

Feinberg, Brian. The Musculoskeletal System. New York: Chelsea House, 1993.

Llamas, Andreu. Muscles and Bones. Milwaukee, WI: Gareth Stevens, 1998.

Parker, Steve. Muscles. Brookfield, CT: Copper Beech Books, 1997.

Silverstein, Alvin, Virginia Silverstein, and Robert Silverstein. The Muscular System. New York: Twenty-First Century Books, 1994.

WWW Sites

Cyber Anatomy: Muscular System
http://tqd.advanced.org/11965/html/cyber-anatomy_musboth.html
Site provides detailed front and rear views of the human body with major muscle groups labeled. Also includes brief paragraph descriptions of the three types of muscle tissue.

Hosford Muscle Tables: Skeletal Muscles of the Human Body
http://www.ptcentral.com/muscles/
Web site is an index containing detailed information about the skeletal muscles of the human body. Included is each muscle's origin, insertion, action, blood supply, and innervation.

Muscular Dystrophy Association
http://www.mdausa.org
Homepage of the Muscular Dystrophy Association.

Muscular System
http://hyperion.advanced.org/2935/Natures_Best/Nat_Best_Low_Level/Muscular_page.L.html
Site offers a description of cardiac, smooth, and skeletal muscle tissues and an investigation on how groups of muscles function in different areas of the body.

Muscular System
http://www.innerbody.com/image/musfov.html
Site includes large images of the human muscular system (front and rear views) with each major muscle linked to a paragraph explanation of its structure and function. Other links on the site feature a paragraph overview of the muscular system, an explanation of the muscle cell types, and nerve/muscle connections.

http://www.encyclopedia.com/medicine/news-wires-white-papers-and-books/muscular-system

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