The muscular system is the body's network of tissues for both voluntary and involuntary movements. Muscle cells are specialized for contraction.
Description
Body movements are generated through the contraction and relaxation of specific muscles. Some muscles, like those in the arms and legs, bring about such voluntary movements as raising a hand or flexing the foot. Other muscles are involuntary and function without conscious effort. Voluntary muscles include the skeletal muscles, of which there are about 650 in the human body. Skeletal muscles are controlled by the somatic nervous system; whereas the autonomic nervous system controls the involuntary muscles. Involuntary muscles include muscles that line the internal organs and the blood vessels. These smooth muscles are called visceral and vascular smooth muscles, and they perform tasks not generally associated with voluntary activity. Smooth muscles control several automatic physiological responses such as pupil constriction, which occurs when the muscles of the iris contract in bright light. Another example is the dilation of blood vessels, which occurs when the smooth muscles surrunding the vessels relax or lengthen. In addition to the categories of skeletal (voluntary) and smooth (involuntary) muscle, there is a third category, namely cardiac muscle, which is neither voluntary nor involuntary. Cardiac muscle is not under conscious
control, and it can also function without regulation from the external nervous system.
Smooth muscles derive their name from their appearance under polarized light microscopy. In contrast to cardiac and skeletal muscles, which have striations (appearance of parallel bands or lines), smooth muscle is unstriated. Striations result from the pattern of myofilaments, which are very fine threads of protein. There are two types of myofilaments, actin and myosin, which line the myofibrils within each muscle cell. When many myofilaments align along the length of a muscle cell, light and dark regions create a striated appearance. This microscopic view of muscle reveals that muscles alter their shape to produce movement. Because muscle cells are usually elongated, they are often called muscle fibers. Compared to other cells in the body, striated muscle cells are distinctive in shape, protein composition, and multinucleated structure.
Skeletal muscles
Skeletal muscles function as the link between the somatic nervous system and the skeletal system. Skeletal muscles carry out instructions from the brain related to voluntary movement or action. For instance, when a person decides to eat a piece of cake, the brain tells the forearm muscle to contract, allowing it to flex and position the hand to lift a forkful of cake to the mouth. But the muscle alone cannot support the weight of the fork; the sturdy bones of the forearm assist the muscles in completing the task of moving the bite of cake. Hence, the skeletal and muscular systems work together as a lever system, with the joints acting as a fulcrum to carry out instructions from the nervous system.
The somatic nervous system controls skeletal muscle movement through motor neurons. Alpha motor neurons extend from the spinal cord and terminate on individual muscle fibers. The axon, or signal-sending end, of the alpha neuron branches to innervate multiple muscle fibers. The nerve terminal forms a synapse, or junction, with the muscle to create a neuromuscular junction. The neurotransmitter acetylcholine (ACh) is released from the axon terminal into the synapse. From the synapse, the ACh binds to receptors on the muscle surface that trigger events leading to muscle contraction. While alpha motor neurons innervate extrafusal fibers, intrafusal fibers are innervated by gamma motor neurons.
Voluntary skeletal muscle movements are initiated by the motor cortex in the brain. Signals travel down the spinal cord to the alpha motor neuron to result in contraction. Not all movement of skeletal muscles is voluntary, however. Certain reflexes occur in response to such dangerous stimuli as extreme heat or the edge of a sharp object. Reflexive skeletal muscular movement is controlled at the level of the spinal cord and does not require higher brain initiation. Reflexive movements are
Cardiac muscle
Cardiac heart muscle is responsible for more than two billion beats in the course of a human lifetime of average length. Cardiac muscle cells are surrounded by endomysium like the skeletal muscle cells. The autonomic nerves to the heart, however, do not form any special junctions like those found in skeletal muscle. Instead, the branching structure and extensive interconnectedness of cardiac muscle fibers allows for stimulation of the heart to spread into neighboring myocardial cells. This feature does not require the individual fibers to be stimulated. Although external nervous stimuli can enhance or diminish cardiac muscle contraction, heart muscles can also contract spontaneously. Like skeletal muscle cells, cardiac muscle fibers can increase in size with physical conditioning, but they rarely increase in number
Smooth muscle
The concentric arrangement of some smooth muscle fibers enables them to control dilation and constriction in the blood vessels, intestines, and other organs. While these cells are not innervated on an individual basis, excitation from one cell can spread to adjacent cells through the nexuses that join neighbor cells. Multi-unit smooth muscles function in a highly localized way in such areas as the iris of the eye. Visceral smooth muscle also facilitates the movement of substances through such tubular areas as blood vessels and the small intestine. Smooth muscle differs from skeletal and cardiac muscle in its energy utilization as well. Smooth muscles are not as dependent on oxygen availability as cardiac and skeletal muscles are. Smooth muscle uses glycolysis (the breakdown of carbohydrates) to generate much of its metabolic energy.
KEY TERMS
Acetylcholine (ACh)—A short-acting neurotransmitter that functions as a stimulant to the nervous system and as a vasodilator.
Actin—A protein that functions in muscular contraction by combining with myosin.
Adenosine triphosphate (ATP)—A nucleotide that is the primary source of energy in living tissue.
Anaerobic—Pertaining to or caused by the absence of oxygen.
Angina pectoris—A sensation of crushing pain or pressure in the chest, usually near the breastbone, but sometimes radiating to the upper arm or back. Angina pectoris is caused by a deficient supply of blood to the heart.
Axial—Pertaining to the axis of the body, i.e., the head and trunk.
Axon—The appendage of a neuron that transmits impulses away from the cell body.
Cardiac muscle—The striated muscle tissue of the heart. It is sometimes called myocardium.
Distal—Situated away from the point of origin or attachment.
Dystrophy—Any of several disorders characterized by weakening or degeneration of muscle tissue
Epimysium—The sheath of connective tissue around a muscle.
Extensor—A muscle that serves to extend or straighten a part of the body.
Fasciculus (plural, fasciculi)—A small bundle of muscle fibers.
Flexor—A muscle that serves to flex or bend a part of the body.
Multinucleated—Having more than one nucleus in each cell. Muscle cells are multinucleated.
Myasthenia gravis—A disease characterized by the impaired transmission of motor nerve impulses, caused by the autoimmune destruction of acetylcholine receptors.
Myosin—The principal contractile protein in muscle tissue.
Parasympathetic—Pertaining to the part of the autonomic nervous system that generally functions in regulatory opposition to the sympathetic system, as by slowing the heartbeat or contracting the pupil of the eye.
Sarcomere—A segment of myofibril in a striated muscle fiber.
Skeletal muscle—Muscle tissue composed of bundles of striated muscle cells that operate in conjunction with the skeletal system as a lever system.
Smooth muscle—Muscle tissue composed of long, unstriated cells that line internal organs and facilitate such involuntary movements as peristalsis.
Sympathetic—Pertaining to the part of the autonomic nervous system that regulates such involuntary reactions to stress as heartbeat, sweating, and breathing rate.
Synapse—A region in which nerve impulses are transmitted across a gap from an axon terminal to another axon or the end plate of a muscle.
Tendon—A cord or band of dense, tough, fibrous tissue that connects muscles and bones.
processed at this level to minimize the amount of time necessary to implement a response.
In addition to motor neuron activity in the skeletal muscles, a number of sensory nerves carry information to the brain to regulate muscle tension and contraction. Muscles function at peak performance when they are not overstretched or overcontracted. Sensory neurons within the muscle send feedback to the brain with regard to muscle length and state of contraction.
Mechanical injury
Disorders of the muscular system can result from genetic, hormonal, infectious, autoimmune, poisonous, or neoplastic causes. But the most common problem associated with this system is injury from misuse. Sprains and tears cause excess blood to seep into skeletal muscle tissue. The residual scar tissue leads to a slightly shorter muscle. Muscular impairment and cramping can result from a diminished blood supply. Cramping can be due to overexertion. An inadequate supply of blood to cardiac muscle causes a sensation of pressure or pain in the chest called angina pectoris. Inadequate ionic supplies of calcium, sodium, or potassium can also affect most muscle cells adversely.
Immune system disorders
Muscular system disorders related to the immune system include myasthenia gravis and tumors. Myasthenia gravis is characterized by weak and easily fatigued skeletal muscles, one of the symptoms of which is droopy eyelids. Myasthenia gravis is caused by antibodies that a person makes against their own ACh receptors; hence, it is an autoimmune disease. The antibodies disturb normal ACh stimulation to contract skeletal muscles. Failure of the immune system to destroy cancerous cells in muscle can result in muscle tumors. Benign muscle tumors are called myomas, while malignant muscle tumors are called myosarcomas.
Disorders caused by toxins
Muscular disorders may also be caused by toxic substances of various types. A bacterium called Clostridium tetani produces a neurotoxin that causes tetanus, which is a disease characterized by painful repeated muscular contractions. In addition, some types of gangrene are caused by clostridial toxins produced under anaerobic conditions deep within a muscle. A poisonous substance called curare, which is derived from tropical plants of the genus Strychnos blocks neuromuscular transmission in skeletal muscle, causing paralysis. Prolonged periods of ethanol intoxication can also cause muscle damage.
Genetic disorders
The most common type of muscular genetic disorder is muscular dystrophy, of which there are several kinds. Duchenne's muscular dystrophy is characterized by increasing muscular weakness and eventual death. Becker's muscular dystrophy is a less severe disorder than Duchenne's, but both can be classified as X-linked recessive genetic disorders. Other types of muscular dystrophy are caused by a mutation that affects a muscle protein called dystrophin. Dystrophin is absent in Duchenne's and altered in Becker's muscular dystrophies. Other genetic disorders, including glycogen storage diseases, myotonic disorders, and familial periodic paralysis, can affect muscle tissues. In glycogen storage diseases, the skeletal muscles accumulate abnormal amounts of glycogen due to a biochemical defect in carbohydrate metabolism. In myotonic disorders, the voluntary muscles are abnormally slow to relax after contraction. Familial periodic paralysis is characterized by episodes of weakness and paralysis combined with loss of deep tendon reflexes.
BOOKS
"Muscular Disorders." Chapter 184 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.
Praemer A, et al., eds. Musculoskeletal Conditions in the United States. Rosemont, IL: American Academy of Orthopaedic Surgeons, 1999.
Vesalius, Andreas. On the Fabric of the Human Body: Book II, The Ligaments and Muscles. Tr. William Frank Richardson and John Burd Carman. San Francisco, CA: Norman Publishing, 1999.
White, Katherine. The Muscular System: The Insider's Guide to the Body. New York: Rosen Publishing Group, 2001.
PERIODICALS
Boskey, Adele L. "Musculoskeletal Disorders and Orthopedic Conditions." Journal of the American Medical Association 285, no. 5 (2001): 619-623. Full text available online at <http://jama.ama-assn.org/issues/v285n5/ffull/jsc00335.html>.
ORGANIZATIONS
National Arthritis and Musculoskeletal and Skin Diseases Information Clearinghouse. 1 AMS Circle, Bethesda, MD20892. (301) 495-4484.
National Center for Complementary and Alternative Medicine (NCCAM), 31 Center Drive, Room #5B-58, Bethesda, MD 20892-2182. (800) NIH-NCAM. Fax: (301) 495-4957. <http://nccam.nih.gov>.
National Institute of Neurological Disorders and Stroke (NINDS). Building 31, Room 8A06, 9000 Rockville Pike, Bethesda, MD 20892. (301) 496-5751. <http://www.ninds.nih.gov>.
