How Muscles Function
Muscle is a tough contractile elastic tissue that gives form to the body and produces movement by contracting and relaxing. Muscles are responsible for every movement of the body. Working together, the muscles produce voluntary movements, such as walking and lifting, and also involuntary movements, such as breathing, circulating the blood, and opening and closing of the pupil of the eye. Muscles exist throughout the body. In an average adult male, muscles make up about 42 percent of the total body weight. In an average adult female they make up about 36 percent of the total body weight.
Striated, or skeletal, muscle accounts for most of the body's muscle tissue. In the human body there are 656 striated muscles, including all the muscles attached to the skeleton as well as the muscles of the tongue, palate, pharynx, and outer eye. Striated muscles produce a wide variety of movements, including walking, writing, and chewing. Most movements involve the action of several muscles. Some of the muscles may work in opposition to each other, one contracting while another relaxes. The coordinated action of the group of muscles allows motion to be smooth and coordinated, rather than jerky. Although striated muscles are voluntary, some also operate involuntarily. For example, the muscles that control breathing are striated and can be expanded and contracted at will, but under usual circumstances they operate automatically.
Striated muscles consist of bundles of cells, called fibers. The fibers are cylindrical in shape and range in length from 1 to 40 millimeters. (One millimeter equals one twenty-fifth of an inch.) Inside each cell are from several hundred to several thousand tightly packed strands, which are called myofibrils. Because the strands consist of alternating light and dark bands, striated muscle looks striped when seen through a microscope.
Surrounding every striated muscle is a sheath of white connective tissue, called the epimysium, which extends beyond the muscle at each end to form the attachments to the bones. Most striated muscles are attached to two bones and cross a joint. One bone usually remains fixed. The other bone is generally movable because of the joint which attaches it to the fixed bone. The attachment of the muscle to the fixed bone is called the origin and the attachment to the movable bone is called the insertion. When a striated muscle contracts, it usually brings the insertion closer to the origin.
Smooth, or visceral, muscles are present in the walls of nearly all hollow organs, including the esophagus, stomach, intestines, windpipe, bladder, ureters, blood vessels, gall bladder, and hair follicles. Smooth muscles perform most of the body's involuntary movements and are regulated by the autonomic nervous system.
The individual fibers of smooth-muscle tissue are slender spindle-shaped cells ranging from 1/67 mm. to 1/5 mm. in length. Although the cells contain myofibrils, the light and dark bands are not obvious and the cells do not appear striped. Sometimes the cells are arranged in small bundles, but generally they are grouped into dense sheets or bands. In a tubular organ, such as the esophagus, the smooth-muscle tissue usually forms two layers, one consisting of cells that encircle the tube and the other consisting of cells that lie parallel to the direction of the tube. The two layers contract and produce rhythmic waves in the walls of the tube, pushing along any material in the tube.
Cardiac muscle, also called myocardium, is present only in the heart, where it forms the thick walls of the four-chambered organ. Like smooth muscle, cardiac muscle is involuntary. It begins its rhythmic contractions during the embryo's second month of development and continues beating until death. Cardiac muscle is controlled by a special group of muscle cells that lie within the heart. In this respect, cardiac muscle differs from other muscles because its contractions are not controlled by nerves. Only the rate of heartbeat may be regulated by nerves.
Cardiac muscle is composed chiefly of thick bundles of cells that wind about the heart in spirals. The individual fibers are long slender branching cells so closely attached to each other that they seem to form a continuous network. Although the myofibrils in each cell are more conspicuous than those in smooth-muscle fibers, they are somewhat less obvious than the myofibrils in striated muscle cells.
Most research concerning the functioning of muscle cells has been conducted on striated muscle. However, it is believed that smooth and cardiac muscles work in much the same way.
Stimulation. All muscles are stimulated to contract by an electrical impulse. Generally the impulse is transmitted by a nerve cell that comes into contact with the muscle fiber. At the point of contact, known as the myoneural junction, the nerve ending transmits an impulse to the muscle fiber by secreting a special chemical, called acetylcholine. Little is known about the process, but it is believed that the acetylcholine brings about changes that cause the fiber to contract.
Contraction. The structures responsible for shortening the muscle fiber are the many tiny myofibrils inside it. The myofibrils are believed to consist of about 2,500 short filaments, called micelles. Some micelles are composed of a protein material, called actin, and some are composed of a similar material, called myosin. Just before contraction the actin and myosin filaments come into contact and form a compound called actomyosin. It is believed that the newly formed actomyosin shortens, making the fiber contract.
The energy for muscle contraction is supplied by the molecules of adenosine triphosphate (ATP) that are present in the cells. Because muscles are usually working a large part of the time, they need a fairly constant supply of ATP molecules. Generally, ATP is formed during cellular respiration, the process in which oxygen is combined with food molecules to release energy. Most of the time enough oxygen is present in the fiber to carry on cellular respiration. However, at times when the muscles are working continuously, such as during exercise, a shortage of oxygen may develop. The muscle tissue is then said to build up an oxygen debt. After the muscles stop working and relax, they need large amounts of oxygen to make up the debt. For this reason a person often pants or breathes hard after strenuous exercise. The panting supplies more oxygen to the muscles.
Muscle Fatigue. After a muscle is stimulated, it contracts for about four one-hundredths of a second. It then enters a recovery period, which lasts about five one-hundredths of a second. Sometimes, however, a muscle is stimulated repeatedly at such a rapid pace that its fibers do not have a chance to recover fully and muscle fatigue develops. The muscle's responses to stimulation become weaker and weaker, and eventually it does not respond at all. Only after stimulation has stopped and the muscle has completely recovered can it be stimulated to contract again.
Muscle Tone. All muscles, even when they are not stimulated, have some fibers in a state of contraction. This partial contraction, called muscle tone, keeps the muscles taut enough so that they can contract quickly when stimulated. The degree of muscle tone varies in different people and even in the same person at different times. It is highest when a person is exercising, cold, or tense, very low when a person is asleep, and completely lacking during unconsciousness.