What are muscles?

Structure and Functions

More than half of the body weight of humans is made up of muscle. Three types of muscles are found in the body: skeletal muscle, cardiac muscle, and smooth muscle. These muscles are composed of different types of muscle cells and perform different functions within the body. The characteristics and functions of each of these three muscle types will be discussed separately, starting with skeletal muscle.

Skeletal muscles attach to and cover bones. This type of muscle is often referred to as voluntary muscle because it is the only muscle type that can be controlled or made to move by consciously thinking about it. Skeletal muscles perform four important functions: bringing about body movement, helping to maintain posture, helping to stabilize joints such as the knee, and generating body heat.

Nearly all body movement is dependent upon skeletal muscle. Skeletal muscle is needed not only to be able to run and jump but also to speak, to write, and to move and blink the eyes. These movements are brought about by the contraction or shortening of skeletal muscles. These muscles are attached to two bones or other structures by tough thin strips or cords of tissue known as tendons. When a muscle contracts or shortens, it pulls the tendons, which then pull on the bones or other structures to which they are connected. In this way, the desired movement is brought about.

Skeletal muscles also aid in the maintenance of posture. Posture is defined as the ability to maintain a position of the body or body parts: for example, the ability to stand or to sit erect. The constant force of gravity must be overcome in order to maintain a standing or seated posture. Small adjustments to the force of gravity are constantly being made through slight contractions of skeletal muscle.

Skeletal muscles—or, more appropriately, their tendons—help to maintain joint stability. Many of the tendons that connect muscles to bones cross movable joints such as the knee and the shoulder. These tendons are kept taut by the constant contraction of the muscles to which they are attached. As a result, they act as walls to prevent the joints from dislocating or shifting out of the normal positions.

More than 40 percent of the human body is composed of skeletal muscle. Skeletal muscles generate heat as they contract. As a result, skeletal muscles are of extreme importance in maintaining normal body temperature. When the body is exposed to cold temperatures, it begins to shiver. This shivering is the result of muscle contractions, which serve to generate body heat and maintain the body’s normal temperature.

Skeletal muscles are made up of
skeletal muscle cells. These cells are long and tube-shaped and therefore are referred to as skeletal muscle fibers. In some instances, these muscle fibers may be a foot long. When individual skeletal muscle fibers are viewed under a microscope, they display bands that are referred to as striations. For this reason, skeletal muscle is often called striated muscle.

Each skeletal muscle, depending upon its size, is made up of hundreds or thousands of skeletal muscle fibers. These muscle fibers are surrounded by a tough connective tissue that holds the muscle fibers together. These muscle fibers and their surrounding connective tissue form a skeletal muscle. In the human body, there are more than six hundred skeletal muscles. It is the arrangement of these muscles in the body that is referred to as the musculature, or muscle system.

Smooth muscles are often referred to as involuntary muscles because they cannot be made to contract by conscious effort. Smooth muscles are typically found in the walls of internal organs such as the esophagus, stomach, intestines, and urinary bladder. The primary function of smooth muscles in these organs is to enable the passage of material through a tube or tract. For example, the contraction of smooth muscles in the intestines helps to move digested materials through the digestive system.

Smooth muscle is composed of smooth muscle cells. These cells differ from skeletal muscle fibers in that they are short and spindle-shaped. They also differ from skeletal muscle cells in that they are not striated. Furthermore, smooth muscle cells usually are not surrounded by a tough connective tissue to form a muscle; instead, they are arranged in layers.

Cardiac muscle is found only in the heart. Like smooth muscle, cardiac muscle cannot be made to contract by means of conscious effort. Like skeletal muscle, however, cardiac muscle is striated. The contraction of cardiac muscle results in the contraction of the heart. This, in turn, results in the pumping of blood throughout the body.

Although many differences exist among skeletal, smooth, and cardiac muscle, all have one thing in common—their ability to contract. The methods by which this contraction is brought about in skeletal muscle, however, are different from those used by smooth muscle and cardiac muscle.

In order for skeletal muscles to contract, they must first be electrically stimulated. This electrical stimulation is brought about by nerves that are closely associated with the muscle fibers. Each muscle fiber has a branch of a nerve, known as an axon terminal, that lies very close to it. This axon terminal does not touch the muscle fiber, but is separated from it by a tiny space known as the synaptic cleft (or gap). An electrical impulse from the nerve causes the release of a chemical called a neurotransmitter into the synaptic cleft. The specific type of neurotransmitter for skeletal muscle is known as acetylcholine. The neurotransmitter will then pass through the synaptic cleft to the muscle fiber membrane, where it will bind to a special site known as a receptor. When the neurotransmitter binds to the receptor, it causes an electrical impulse to travel down the muscle fiber. This, in turn, causes the contraction of the muscle fiber. When most or all of the muscle fibers contract, the result is the contraction of the entire muscle.

The muscle fibers and muscle will remain in a contracted state as long as the neurotransmitter is bound to the receptor on the muscle fiber membrane. In order for the muscle fiber to relax, the neurotransmitter must be released from the receptor to which it is bound. This is accomplished by the destruction of the neurotransmitter. Another chemical, known as an enzyme, is released into the synaptic cleft. This enzyme destroys the neurotransmitter; thus, the neurotransmitter is no longer bound to the receptor. In skeletal muscle, this enzyme is called acetylcholinesterase, because it destroys the neurotransmitter acetylcholine.

The contraction of cardiac muscle differs from that of skeletal muscle in that each cardiac muscle fiber does not have an axon terminal associated with it. Cardiac muscle is capable of making its own electrical impulse; it does not need a nerve to initiate the electrical impulse for every cardiac muscle fiber. An impulse is started in a particular place in the heart, called the atrioventricular (A-V) node. This impulse spreads from muscle fiber to muscle fiber. Thus, each cardiac muscle fiber stimulates those fibers next to it. The electrical impulse spreads so fast that nearly all the cardiac muscle fibers contract at the same time. As a result, the single impulse that began in the A-V node causes the entire heart to contract.

Disorders and Diseases

Any type of muscle disorder has the ability to disrupt the normal functions performed by muscles. Skeletal muscle disorders can disrupt body movement and the ability to maintain posture. If these disorders affect the diaphragm, the principal breathing muscle, they can also be fatal.

Perhaps the most common and least detrimental muscle disorder is disuse atrophy. When muscles are not used, the muscle fibers will become smaller, a process called atrophy. As a result of the decrease in the diameter of the muscle fibers, the entire muscle also becomes smaller and therefore weaker.

Disuse atrophy occurs in such circumstances as when an individual is sick or injured and must remain in bed for prolonged periods of time. As a result, the muscles are not used and begin to atrophy. Disuse atrophy is also fairly common in astronauts. This occurs as a result of the lack of gravity against which the muscles must work. If a muscle does not work against a load or force, such as gravity, it will tend to decrease in size.

In general, disuse muscle atrophy is easily treated. The primary treatment is to exercise the unused muscle. Physical activity, particularly those activities in which the muscle must work to lift or pull a weight, will result in an enlargement in the diameter of the skeletal muscle fibers, and thus of the entire muscle. The increase in the diameter of the muscle fibers and muscle is referred to as hypertrophy.

Another common muscle disorder is a muscle cramp. A muscle cramp is a spasm in which the muscle undergoes strong involuntary contractions. These involuntary contractions, which may last for as short a time as a few seconds or as long as a few hours, are extremely painful. Muscle cramps appear to occur more frequently at night or after exercise. Treatment for cramps involves rubbing and massaging the affected muscle.

Muscles are often overused or overstretched. When this is the case, it is possible for the muscle fibers to tear. When the muscle fibers are torn, the result is a muscle strain, more often referred to as a pulled muscle. Although pulled muscles may be painful, they are usually not serious. Treatment for pulled muscles most often involves the resting of the affected muscle. If the muscle fibers are torn completely apart, surgery may be required to reattach the muscle fibers.

Among the more serious skeletal muscle disorders is muscular dystrophy. The term “muscular dystrophy” is used to define those muscle disorders that are genetic or inherited. These diseases most often begin in childhood, but a few cases have been reported to begin during adult life. Muscular dystrophy results in progressive muscle weakness and muscle atrophy. The most common form of muscular dystrophy is known as Duchenne muscular dystrophy. This form of muscular dystrophy primarily affects males. In those affected with
Duchenne muscular dystrophy, muscular weakness and atrophy begin to appear at three to five years of age. There is a progressive loss of muscle strength and muscle mass such that, by the age of twelve, those individuals afflicted with the disorder are confined to a wheelchair. Usually between the ages of fourteen and eighteen, the patients develop serious and sometimes fatal respiratory diseases as a result of the impairment of the diaphragm, the primary breathing muscle. The progressive deterioration of the muscles cannot be stopped, but it may be slowed with exercise of the affected muscles.

Myasthenia gravis is also a severe muscle disorder. This disease results in excessive weakness of skeletal muscles, a condition known as muscle fatigue. Those with myasthenia gravis complain of fatigue even after performing normal everyday body movements. Although severe, myasthenia gravis is usually not fatal unless the diaphragm is affected.

Myasthenia results from a decrease in the availability of the receptors for acetylcholine. If fewer acetylcholine receptors are available on the muscle fibers, less acetylcholine binds to the muscle fiber receptors; this binding is needed for contraction to occur. As a result, fewer muscle fibers within the muscle contract. The fewer muscle fibers within the entire muscle that contract, the weaker the muscle.

Myasthenia gravis affects about one in every ten thousand individuals. Unlike Duchenne muscular dystrophy, myasthenia gravis may affect any group, and, overall, women are affected more frequently than men. Myasthenia gravis is usually first detected in the facial muscles, particularly those of the eyes and eyelids. Those afflicted have droopy eyelids and experience difficulty in keeping the eyes open. Other symptoms are weakness in those muscles involved in chewing and difficulty swallowing as a result of weakening of the tongue muscles. In most patients, there is also some weakening of the muscles of the legs and arms.

The prognosis for the treatment of myasthenia gravis is very good. The most important treatment for the disorder is the use of anticholinesterase drugs. These drugs inhibit the breakdown of acetylcholine. As a result, there is a large amount of acetylcholine in the neuromuscular junction to bind with the limited number of acetylcholine receptors. This, in turn, increases the ability and number of the muscle fibers that are able to contract, resulting in an increase in muscle strength and the ability to use the muscles without fatigue.

Also of interest is the effect of pesticides and the way in which they affect muscle function. Some pesticides are classified as organic pesticides that inhibit the enzyme acetylcholinesterase. If acetylcholinesterase is inhibited, it will no longer break down the acetylcholine that is bound to the receptor on the skeletal muscle membrane. If the acetylcholine is not removed from the receptor, the muscle cannot relax and is therefore in a constant state of contraction. As a result, the respiratory muscles are unable to contract and relax, a process required for breathing. Thus, organic pesticides function to prevent the respiratory muscles from working, and an affected animal will die as a result of not being able to breathe.

Muscle fibers also require a blood supply in order to keep them alive. If the blood supply to the muscle fibers is inhibited, death of the muscle fibers can result. If enough muscle fibers are affected, death of the muscle can result. This most commonly occurs in cardiac muscle. If the blood supply to the cardiac muscle making up the heart is reduced or cut off, the result is a decrease in the ability of the cardiac muscle to contract. This, in turn, leads to heart failure.

Perspective and Prospects

The study of muscles and musculature is as old as the study of anatomy itself. The first well-documented study of muscles was done by Galen of Pergamum in the first century. Galen made drawings of muscles and described their functions. In all, Galen described more than three hundred muscles in the human body, almost half of all the muscles now known.

The first refined drawings and descriptions of the skeletal muscles of the body were made in the late fifteenth century. Among those who stood out as muscle anatomists during this period was Leonardo da Vinci, whose drawings of the skeletal muscles of the body were magnificent. His chief interest in the muscles of the body, like Galen’s, was their function. He accurately described, among many other muscles, the muscles involved in the movement of the lips and cheeks.

A major step to the understanding of muscle physiology did not occur until the late eighteenth century. Luigi Galvani in 1791 discovered the relationship between muscle contraction and electricity when he found that an electrical current could cause the contraction of a frog leg. The use of electrical stimulation to study muscle contraction and function was fully utilized in the mid-nineteenth century by Duchenne de Boulogne. The actual measurement of the electrical activity in a muscle came about in 1929, with the invention by Edgar Douglas Adrian and Detlev Wulf Bronk of the needle electrode, which could be placed into the muscle to record the muscle’s electrical activity. This recording of the electrical activity of the muscle is known as an electromyogram, or EMG. Electromyograms are important in the evaluation of the electrical activity of resting and contracting muscles. Since the discovery of EMGs, they have been used by anatomists, muscle physiologists, exercise physiologists, and orthopedic surgeons to study and diagnose muscle diseases. Furthermore, the knowledge gained from EMGs has led to the
making of artificial limbs that can be controlled by the electrical impulses of the existing muscles.

Knowledge of muscle names, muscle anatomy, and movement, as well as muscle physiology, is needed for many medical fields. These fields include kinesiology, the study of movement; physical and

occupational therapy; the treatment and rehabilitation of those who are disabled by injury;
exercise physiology and
sports medicine, in which the effects of exercise on muscle and the damage of muscle as a result of sports injuries are studied; and, finally, orthopedic surgery, which is the surgical repair of damaged bones, joints, and muscles.

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