Muscle

Muscle is the tough, elastic tissue that makes body parts move. All animals except the simplest kinds have some type of muscle.

People use muscles to make various movements, such as walking, jumping, or throwing. Muscles also help in performing activities necessary for growth and for maintaining a strong, healthy body. For example, people use muscles in the jaw to chew food. Other muscles help move food through the stomach and intestines, and aid in digestion. Muscles in the heart and blood vessels force the blood to circulate. Muscles in the chest make breathing possible.

Muscles are found throughout the body. As a person grows, the muscles also get bigger. Muscle makes up nearly half the body weight of an adult.

This article primarily discusses the muscles of human beings. The last section describes muscles in other animals.

Kinds of muscles

The human body has more than 600 major muscles. About 240 of them have specific names. There are two main types of muscles: (1) skeletal muscles and (2) smooth muscles. A third kind of muscle, called cardiac muscle, has characteristics of both skeletal and smooth muscles. It is found only in the heart.

Skeletal muscles

help hold the bones of the skeleton together and give the body shape. They also make the body move. Skeletal muscles make up a large part of the legs, arms, abdomen, chest, neck, and face. These muscles vary greatly in size, depending on the type of job they do. For example, eye muscles are small and fairly weak, but the muscles of the thigh are large and strong.

All muscles are made up of cells called muscle fibers. Each skeletal muscle is composed of thousands of long, cylindrical muscle fibers. When viewed under a microscope, these fibers show alternating light and dark bands called striations. For this reason, skeletal muscles are also called striated muscles. The striations occur because thick and thin filaments (strands) repeatedly overlap each other. The thick filaments consist of a protein called myosin. The thin filaments are made up chiefly of the protein actin.

Muscle fibers have a variety of other specialized parts. Each skeletal muscle fiber has many elements called nuclei. These nuclei contain growth-producing substances that repair or remake various parts of the muscle fiber as they wear out. Each muscle fiber also has thousands of tiny mitochondria. These structures produce the energy that the fiber needs in order to live and do its work.

Muscle fibers are held together by connective tissue. The ends of most skeletal muscles are joined to bones by a tough, flexible connective tissue called tendon. One end of the muscle is attached to a bone that does not move when the muscle contracts (draws together). This end of the muscle is called the origin. The other end, called the insertion, is attached to a bone that moves when the muscle contracts.

When a person stands erect, many skeletal muscles contract to make the body rigid. The skeletal muscles also can make one part of the body move while another part stays stiff. Skeletal muscles act both ways because they work in pairs. One muscle of each pair is called the flexor. It bends a joint and brings a limb closer to the body. The other muscle, the extensor, does the opposite. For example, the biceps muscle in the front of the upper arm is a flexor. When this muscle contracts, the elbow bends and the forearm and hand move toward the shoulder. The triceps muscle in the back of the upper arm is an extensor. When it contracts, the elbow straightens and the forearm and hand move away from the shoulder. At the same time, the biceps relaxes so the triceps can pull it back to its original length.

Skeletal muscles contract and pull on the bones they attach to when a nerve stimulates them. They usually move voluntarily (under conscious control) and are sometimes called voluntary muscles. But skeletal muscles also may move involuntarily (without conscious control). For example, involuntary movement occurs when a person jerks his or her hand away from a hot object before thinking about doing it.

Skeletal muscles adapt to exercise in special ways, depending on how they are required to work. For example, muscles grow larger and stronger if a person lifts heavy weights for a short period of time each day. Such exercise causes muscle nuclei to increase production of thick and thin filaments in each exercised muscle fiber. In addition, bone and tendon grow stronger. Muscles adapt differently if a person regularly performs lighter exercise for long periods—such as 30 minutes of bicycling, running, or swimming. In this case, muscle fibers increase their ability to produce energy needed to keep up such muscular work.

Smooth muscles

are found in various organs of the body. For example, smooth muscles are found in the walls of the stomach, intestines, blood vessels, and bladder. The fibers of smooth muscles are not striated like those of skeletal muscles. They also are smaller than skeletal muscle fibers and have only one nucleus.

Smooth muscles operate slowly and automatically in a natural, rhythmic pattern of contraction followed by relaxation. In this way, they control various body processes. For example, the steady action of smooth muscles in the stomach and intestines moves food along for digestion. Because they are not under conscious control by the brain, smooth muscles are also known as involuntary muscles.

Smooth muscles are stimulated by a special set of nerves that belong to the autonomic nervous system, and by body chemicals (see Nervous system (The autonomic nervous system) ). In certain circumstances, the autonomic nerves and hormones act to change the speed and strength of smooth muscle contractions. For example, they slow the pace of contractions in the intestines if a person feels fear or anxiety. They can even stop contractions in the intestines if these feelings become severe. For this reason, people under emotional stress often find it difficult to digest food.

Cardiac muscle

makes up the walls of the heart. When cardiac muscle cells contract, they push blood out of the heart and into the arteries. The blood then circulates throughout the body, bringing nourishment to all body cells. Cardiac muscle has characteristics of both skeletal and smooth muscles. Like skeletal muscle fibers, cardiac muscle cells have striations. Like smooth muscle fibers, each cardiac muscle cell has only one nucleus and contracts automatically.

The heart also contains a group of specialized cells called the sinoatrial node, or S-A node. The S-A node starts up each contraction of the cardiac muscle by giving off rhythmic signals to neighboring muscle cells. As these cells contract, they cause others to contract as well. By this process, all the cardiac muscle cells contract together. The autonomic nerves that stimulate the S-A node control how often the cardiac muscle contracts. The S-A node thus acts as the heart’s “pacemaker,” because it determines how often the heart beats to pump blood through the body. For further information about the heart, its parts, and how they work, see Heart.

How muscles work

All muscles contract when they are stimulated. Scientists have done much research to determine how muscles contract and how they are stimulated.

How muscles contract.

During the mid-1900’s, the English scientist H. E. Huxley developed what has become the most widely accepted theory to explain how muscles contract. This theory, called the sliding filament theory, proposes that the muscle fiber’s thick myosin filaments have numerous small projections. These projections, called myosin cross-bridges, spring away from the myosin filaments when the muscle fibers are stimulated. These cross-bridges attach themselves to the thin actin filaments that run parallel to the myosin filaments. The cross-bridges pull on the actin filaments, causing them to slide between the myosin filaments. As the actin filaments slide, they pull the ends of the muscle toward its middle, making the muscle fibers shorten.

In order for the myosin cross-bridges to work, the substance adenosine triphosphate (ATP) must be produced by muscle cells. ATP provides the energy to slide actin filaments. It is produced when oxygen in muscle fibers combines with chemicals from food. Each muscle fiber contains only a small amount of ATP. When muscles work hard, the body’s ability to turn food and oxygen into energy increases to make the needed ATP.

How muscles are stimulated.

Muscle cells are excitable because the membrane of each cell is electrically charged. Thus, a muscle cell is said to have electric potential. This electric potential results from the presence of sodium and potassium ions (electrically charged particles) on each side of the membrane. Potassium ions easily move through the membrane and accumulate in the cell. Sodium ions do not enter the cell as easily. In addition, the membrane has a special mechanism that pumps potassium into the cell and pumps sodium out. Consequently, the cell normally contains much potassium but little sodium.

Muscle cells are stimulated by nerves or by hormones, depending on the muscle involved. When the muscle cell is stimulated, its electric potential changes rapidly. The excited membrane allows sodium to rush into the cell and potassium to flow out. The sodium-potassium pumping mechanism quickly reverses this change, returning the cell to its normal condition.

The change in electric potential in the muscle cell triggers the release of calcium from storage areas inside the cell. The calcium then builds up in the cell and eventually causes the actin filaments to attract myosin cross-bridges and produce contraction. The cell relaxes when the level of calcium drops back to normal.

Disorders of the muscles

Muscles function through an amazing coordination of many elements. Occasionally, however, the normal operation of muscles is disturbed. For example, a person may experience painful cramps of certain skeletal muscles if he or she exercises too hard or for a long time. Skeletal muscle cramps involve spastic (sudden and violent) muscle contractions. No one knows exactly why such cramps occur. They probably result from having too much or too little salt in the fluids surrounding muscle fibers. With proper rest and nutrition, the body can correct the problem, and cramping stops. Cramps also may develop in smooth muscle organs, such as the stomach and intestine. Doctors use heat, massage, and medicines in treating cramps. See Cramp.

Hard muscular work also may cause skeletal muscles to become sore. In severe cases, the soreness may last up to four days. The cause of muscle soreness is not completely understood, but it probably involves damage to muscle and connective tissue. With proper exercise, the muscles and body can adapt to strenuous muscle work and greatly reduce the risk of tissue damage.

Numerous diseases affect skeletal muscles. Two major classes of muscle diseases are muscular atrophy diseases and myopathies. Atrophy diseases attack and damage the nervous system, including nerves that stimulate muscles. As a result, muscles progressively shrink and become weak. Amyotrophic lateral sclerosis—also called ALS or Lou Gehrig’s disease—is an example of an atrophy disease (see Amyotrophic lateral sclerosis ). Muscular weakness also occurs in myopathies. In these diseases, weakness results because the muscle itself does not function properly. Certain myopathies, such as various muscular dystrophies, are characterized by gradual wasting away of skeletal muscles.

Muscles in other animals

All of the more advanced animals have some kind of muscle tissue. The muscles of human beings and of other animals have many similarities. For example, human beings and other vertebrates (animals with backbones) have three kinds of muscles—skeletal, smooth, and cardiac. Some invertebrates (animals without backbones), such as insects, have striated muscles. Other invertebrates, such as scallops and squids, have both striated and smooth muscles. In most cases, the muscles of other animals operate in much the same way as human muscles. However, the speed at which striated muscles contract varies widely among species. In general, contractions occur faster in a small animal, such as the rat, than in a large animal like the cow.