Muscular System (Encyclopedia of Science)
The muscular system is the body's network of tissues that controls movement both of the body and within it (such as the heart's pumping action and the movement of food through the gut). Movement is generated through the contraction and relaxation of specific muscles.
The muscles of the body are divided into two main classes: skeletal (voluntary) and smooth (involuntary). Skeletal muscles are attached to the skeleton and move various parts of the body. They are called voluntary because a person controls their use, such as in the flexing of an arm or the raising of a foot. There are about 650 skeletal muscles in the whole human body. Smooth muscles are found in the stomach and intestinal walls, vein and artery walls, and in various internal organs. They are called involuntary muscles because a person generally cannot consciously control them. They are regulated by the autonomic nervous system (part of the nervous system that affects internal organs).
Another difference between skeletal and smooth muscles is that skeletal muscles are made of tissue fibers that are striated or striped. These alternating bands of light and dark result from the pattern of the filaments (threads) within each muscle cell. Smooth muscle fibers are not striated.
The cardiac or heart muscle (also called myocardium) is a unique type of muscle that does not fit clearly into either...
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Muscular System (Encyclopedia of Nursing & Allied Health)
The muscular system is the body's network of tissues for both voluntary and involuntary movements. Muscle cells are specialized for contraction.
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 are what most people think of as muscle. Skeletal muscles are the ones that ache when someone goes for their first outdoor run in the spring after not running regularly during the winter. Skeletal muscles are also involved when someone carries heavy grocery bags, practices a difficult musical passage, or combs their hair. Exercise may increase the size of muscle fibers, but the number of fibers generally remains constant. Skeletal muscles take up about 40% of the body's mass, or weight. They also consume large amounts of oxygen and nutrients from the blood supply. Multiple levels of skeletal muscle tissue receive their own blood supplies.
GROSS ANATOMY OF STRIATED MUSCLE. At the macroscopic level, skeletal muscles usually originate at one point of attachment to a tendon (a band or cord of tough, fibrous connective tissue) and terminate at another tendon at the other end of an adjoining bone. Tendons are rich in the protein collagen, which is arranged in a wavy pattern so that it can stretch out and provide additional length at the junction between bone and muscle.
Skeletal muscles usually act in pairs, such that the flexing (shortening) of one muscle is balanced by a lengthening (relaxation) of its paired muscle or group of muscles. These antagonistic (opposite) muscles can open and close such joints as the elbow or knee. Muscles that cause a joint to bend or close are called flexor muscles, and those that cause a joint to expand or straighten out are called extensors. Skeletal muscles that support the skull, backbone, and rib cage are called axial skeletal muscles; whereas the skeletal muscles of the limbs are called distal. Several skeletal muscles work in a highly coordinated manner in such activities as walking.
Skeletal muscles are organized into extrafusal and intrafusal fibers. Extrafusal fibers are the strong, outer layers of muscle. This type of muscle fiber is the most common. Intrafusal fibers, which make up the central region of the muscle, are weaker than extrafusal fibers. Skeletal muscle fibers are additionally characterized as fast or slow according to their activity patterns. Fast or "white" muscle fibers contract rapidly, have poor blood supply, operate anaerobically (without oxygen), and tire easily. Slow or "red" muscle fibers contract more slowly, have a more adequate blood supply, operate aerobically (with oxygen), and do not fatigue as easily. Slow muscle fibers are used in sustained movements, such as holding a yoga posture or standing at attention.
The skeletal muscles are enclosed in a dense sheath of connective tissue called the epimysium. Within the epimysium, muscles are sectioned into columns of muscle fiber bundles called primary bundles or fasciculi. Each fasciculus is covered by a layer of connective tissue called the perimysium. An average skeletal muscle may have 200 fasciculi which are further subdivided into several muscle fibers. Each muscle fiber (cell) is covered by connective tissue called endomysium. Both the epimysium and the perimysium contain blood and lymph vessels to supply the muscle with nutrients and oxygen, and to remove waste products. The endomysium has an extensive network of capillaries that supply individual muscle fibers. Individual muscle fibers vary in diameter from 100 micrometers and in length from a few millimeters in the smaller muscles to about 12 in (30 cm) in the sartorius muscle of the thigh.
MICROANATOMY OF STRIATED MUSCLE. At the microscopic level, a single striated muscle cell has several hundred nuclei and a striped appearance derived from the pattern of myofilaments. Long, cylindrical muscle fibers are formed from several myoblasts in fetal development. Multiple nuclei are important in muscle cells because of the tremendous amount of activity. The two types of myofilaments, actin and myosin, overlap one another in a very precise arrangement. Myosin is a thick protein with two globular head regions. Each myosin filament is surrounded by six actin (thin) filaments. These filaments run along the length of the cell in parallel. Multiple hexagonal arrays of actin and myosin exist in each skeletal muscle cell.
Each actin filament slides along adjacent myosin filaments with the help of other proteins and ions present in the cell. Tropomyosin and troponin are two proteins attached to the actin filaments that enable the globular heads on myosin to instantaneously attach to the myosin strands. The attachment and rapid release of this bond induces the sliding motion of these filaments that results in muscle contraction. In addition, calcium ions and ATP (adenosine triphosphate, the source of cellular energy) are required by the muscle cell to process this reaction. Numerous mitochondria (organelles in a cell that produce enzymes necessary for energy metabolism) are present in muscle fibers to supply the extensive ATP required by the cell.
The system of myofilaments within muscle fibers are divided into units called sarcomeres. Each skeletal muscle cell has several myofibrils, long cylindrical columns of myofilaments. Each myofibril is composed of myofilaments that interdigitate to form the striated sarcomere units. The thick myosin filaments of the sarcomere provide the dark, striped appearance in striated muscle, and the thin actin filaments provide the lighter sarcomere regions between the dark areas. Muscle contraction creates an enlarged center region called the belly of the muscle. The flexing of a muscle bicep for exampleakes this region anatomically visible.
Cardiac muscle, as is evident from its name, makes up the muscular portion of the heart. While almost all cardiac muscle is confined to the heart, some of these cells extend for a short distance into the cardiac vessels before tapering off completely. Heart muscle is also called myocardium. The myocardium has some properties similar to skeletal muscle tissue, but it also has some unique features. Like skeletal muscle, the myocardium is striated; however, the cardiac muscle fibers are smaller and shorter than skeletal muscle fibers. Cardiac muscle fibers average 55 micrometers in diameter and 200 micrometers in length. In addition, cardiac muscles align lengthwise more than they do in a side-by-side fashion, compared to skeletal muscle fibers. The microscopic structure of cardiac muscle is also distinctive in that these cells are branched in a way that allows them to communicate simultaneously with multiple cardiac muscle fibers.
Smooth muscle falls into three general categories: visceral smooth muscle, vascular smooth muscle, and multi-unit smooth muscle. Visceral smooth muscle fibers line such internal organs as the intestines, stomach, and uterus. Vascular smooth muscle forms the middle layer of the walls of blood and lymphatic vessels. Arteries generally have a thicker layer of vascular smooth muscle than veins or lymphatic vessels. Multi-unit smooth muscle is found only in the muscles that govern the size of the iris of the eye. Unlike contractions in visceral smooth muscle, contractions in multi-unit smooth muscle fibers do not readily spread to neighboring muscle cells.
Smooth muscle is innervated by both sympathetic and parasympathetic nerves of the autonomic nervous system. Smooth muscle appears unstriated under a polarized light microscope, because the myofilaments inside are less organized. Smooth muscle fibers contain actin and myosin myofilaments that are more haphazardly arranged than their counterparts in skeletal muscles. The sympathetic neurotransmitter, ACh, and parasympathetic neurotransmitter, norepinephrine, activate this type of muscle tissue.
Smooth muscle cells are small in diameter, about 55 micrometers, but they are long, typically 1500 micrometers. They are also wider in the center than at their ends. Gap junctions connect small bundles of cells which are, in turn, arranged in sheets.
Within such hollow organs as the uterus, smooth muscle cells are arranged into two layers. The cells in the outer layer are usually arranged in a longitudinal fashion surrounding the cells in the inner layer, which are arranged in a circular pattern. Many smooth muscles are regulated by hormones in addition to the neurotransmitters of the autonomic nervous system. Moreover, the contraction of some smooth muscles is myogenic or triggered by stretching, as in the uterus and gastrointestinal tract.
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
Acetylcholine (ACh) short-acting neurotransmitter that functions as a stimulant to the nervous system and as a vasodilator.
Actin protein that functions in muscular contraction by combining with myosin.
Adenosine triphosphate (ATP) nucleotide that is the primary source of energy in living tissue.
Anaerobicertaining to or caused by the absence of oxygen.
Angina pectoris 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.
Axialertaining to the axis of the body, i.e., the head and trunk.
Axonhe appendage of a neuron that transmits impulses away from the cell body.
Cardiac musclehe striated muscle tissue of the heart. It is sometimes called myocardium.
Distalituated away from the point of origin or attachment.
Dystrophyny of several disorders characterized by weakening or degeneration of muscle tissue
Epimysiumhe sheath of connective tissue around a muscle.
Extensor muscle that serves to extend or straighten a part of the body.
Fasciculus (plural, fasciculi) small bundle of muscle fibers.
Flexor muscle that serves to flex or bend a part of the body.
Multinucleatedaving more than one nucleus in each cell. Muscle cells are multinucleated.
Myasthenia gravis disease characterized by the impaired transmission of motor nerve impulses, caused by the autoimmune destruction of acetylcholine receptors.
Myosinhe principal contractile protein in muscle tissue.
Parasympatheticertaining 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 segment of myofibril in a striated muscle fiber.
Skeletal muscleuscle tissue composed of bundles of striated muscle cells that operate in conjunction with the skeletal system as a lever system.
Smooth muscleuscle tissue composed of long, unstriated cells that line internal organs and facilitate such involuntary movements as peristalsis.
Sympatheticertaining to the part of the autonomic nervous system that regulates such involuntary reactions to stress as heartbeat, sweating, and breathing rate.
Synapse 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 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.
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
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.
Common diseases and disorders
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.
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.
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National Institute of Neurological Disorders and Stroke (NINDS). Building 31, Room 8A06, 9000 Rockville Pike, Bethesda, MD 20892. (301) 496-5751. <<a href="http://www.ninds.nih.gov">http://www.ninds.nih.gov>.
Crystal Heather Kaczkowski, MSc.