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Muscle

Muscle

Muscle can be categorized into three types based on structure, function, and location in the body. The specific details of muscle, including structure, physiology of contraction, energy requirements, muscle conditioning, and disease, can be illustrated using skeletal muscle.

Three Types of Muscle

The three types of muscle are skeletal, cardiac, and smooth muscle. Skeletal muscle is attached to the skeleton and moves the body and its components. It appears striated (striped) under the microscope and is under voluntary control. The biceps of the arm is an example of skeletal muscle. Cardiac muscle is only located in the heart. Cardiac muscle is also striated, but is not normally under voluntary control. Smooth muscle surrounds blood vessels and other passageways and alters the size of openings or passageways and propels material through body tubes. Smooth muscle is distributed throughout the body. It lacks striations and is involuntary. The respiratory and digestive tracts have layers of smooth muscle in their walls.

Muscle Ultrastructure

A skeletal muscle fiber is formed from the fusion of many embryonic cells during development to form slender cells that extend from one end of the muscle to the other. Each muscle fiber normally has one nerve fiber that extends to the cell membrane, forming the neuromuscular junction. There is a 100-nanometer space, the synaptic cleft, between the nerve fiber and the muscle fiber.

The muscle cell membrane forms inward projections, the transverse tubules, associated with the cell's smooth endoplasmic reticulum (here called sarcoplasmic reticulum). The sarcoplasmic reticulum stores calcium and surrounds bundles of contractile proteins . The contractile proteins, which do the work of contraction, are parallel and arranged in an overlapping pattern that gives rise to the muscle striations. The pattern of striations is repeated many times down the length of the muscle fiber in segments called sarcomeres.

The proteins of the sarcomere are grouped in thick filaments and thin filaments. Contraction occurs when thick and thin filaments slide past each other, pulling the muscle ends closer together. A thick filament is a bundle of approximately two hundred myosin proteins. A portion of each myosin protein projects outward to form myosin heads.

Thin filaments overlap the thick filaments and are composed of three types of protein molecules. The main protein is actin. Three hundred to four hundred molecules of globular actin (G actin) link like beads in a necklace to form a strand called fibrous actin (F actin). Two such "necklaces" are then intertwined into a loose double helix. In the groove between the two F actins, much like a string, is the protein tropomysin. Each G actin contains an active site to bind the myosin head. When the muscle is at rest, tropomysin covers the active sites of actin. Attached to tropomysin is troponin, a small complex of three polypeptides . This structural arrangement allows muscle to contract.

Muscle Contraction

Muscle contraction begins when the nerve fiber releases the neurotransmitter acetylcholine into the synaptic cleft. Acetylcholine moves across the synaptic cleft and binds to receptors on the muscle fiber. This indirectly initiates an action potential , a change of electrical charge at the membrane that is similar to events in a neuron . The action potential spreads across and into the muscle fiber via the transverse tubules and triggers the release of calcium from the sarcoplasmic reticulum. Next, calcium binds to troponin, causing the troponin to change shape. Since troponin is attached to tropomysin, as troponin changes shape the tropomysin is pulled away from the active sites of actin, which become exposed. The myosin head, which was previously blocked by tropomysin, now binds to the active site of actin, forming a cross-bridge between the thick and thin filament.

In a ratchetlike movement, myosin pulls the thin filament past the myosin as the myosin head repeatedly flexes, lets go of the actin, extends and attaches to a new active site, and flexes again. As the many myosin heads continue to repeat this process, thin filaments slide past the thick filaments and the sarcomere is shortened. Shortening of all sarcomeres within the muscle fiber results in contraction of the whole fiber.

Muscle relaxes and returns to its original form when tropomysin covers up the active sites of actin, preventing the formation of cross-bridges. Relaxation also involves the destruction of acetylcholine by acetylcholinesterase in the synaptic cleft, ending muscle stimulation, and the re-uptake of calcium into the sarcoplasmic reticulum. Without calcium, troponin returns to its original shape, pulling tropomysin back over the active sites of actin. Myosin no longer forms cross-bridges, so the muscle relaxes. Note that a muscle can actively contract but cannot actively extend itself. For the releasing, muscles are usually present in pairs, each working against each other.

Energy (ATP) Requirements

The contraction of muscle fibers requires a large amount of energy in the form of adenosine triphosphate (ATP). ATP is made available through various mechanisms. A limited amount of ATP is stored in the muscle cell. ATP is also produced by a phosphate transfer from creatine phosphate to ADP ; muscles do store larger amounts of creatine phosphate. The stored ATP and the ATP created from creatine phosphate are available for immediate use and provide approximately enough ATP for about six seconds of exercise.

Additional ATP can be produced through anaerobic and aerobic metabolism . Aerobic respiration provides a larger production of ATP but depends on sufficient oxygen delivery. Myoglobin, a protein in muscle cells that binds oxygen, contributes some of the oxygen for aerobic respiration. Aerobic ATP production also requires mitochondria . Muscles packed with mitochondria give meat a darker color ("dark meat") than muscles with fewer mitochondria ("white meat"). Anaerobic fermentation provides less energy but can produce ATP in the absence of oxygen. A serious drawback of anaerobic fermentation is the production of lactic acid, a product that can alter cell pH . Both processes can use glucose released from glycogen, which is stored in muscles as a reserve fuel.

Muscle Fatigue

A decrease in the ability of muscle to contract is muscle fatigue. Muscle fatigue can result from short burst of maximum effort, such as a 50-meter swim, or sustained long-term activities such as marathon running. The cause of fatigue depends on the activity. Fatigue from short, extensive burst of activity can result from depletion of ATP or buildup of lactic acid. Muscle fatigue from sustained activities can result from depletion of fuel molecules or depletion of acetylcholine at the neuromuscular junction.

Hypertrophy and Conditioning

Through training, a muscle can become larger (hypertrophy) and have greater endurance. A muscle grows mainly by increasing the number of thin and thick filaments within the fibers. Growth results from repeated contractions of muscle, as in weight lifting. Muscle conditioning is the increased ability of the muscle to perform a task, either because of greater strength or better fatigue-resistance. Many changes in muscle performance, however, result from changes in the cardiovascular and respiratory systems, enabling them to deliver fuel and oxygen to muscle fibers more efficiently. Many changes specific to muscle fibers involve enhancing energy production, including an increase in number of mitochondria and myoglobin and greater storage of glycogen.


ARTHRITIS AND GROWTH OF CARTILAGE

Arthritis is a breakdown of articular hyaline cartilage, often increased by enzymes of inflammation. Rheumatoid arthritis is an autoimmune disease, in which one's own immune system attacks healthy tissue. Osteoarthritis may be caused or accelerated by obesity, joint injuries, defective cartilage, lack of exercise, or biomechanical defects. Defects of only 1 square centimeter will alter the functioning of the articular cartilage.

Osteoarthritis is a major cause of joint replacements. A process to harvest and grow articular cartilage outside the body, called autologous chondrocyte implantation, is under investigation as of 2001. It is expensive and not exactly like real cartilage. However, in the future, replacement may employ stimulating growth factors, cartilage cells taken from an accessible place in the patient's body, and a synthetic matrix (scaffolding).


Muscle Disease

Diseases affecting muscle can result from loss of neurons that stimulate the muscle, such as polio; changes in the neuromuscular junction that result in loss of ability to stimulate the muscle, such as myasthenia gravis (an autoimmune disease ); or loss of structural integrity of the muscle fiber, such as muscular dystrophy. All result in decreased ability of the muscle to contract and sometimes the complete loss of the muscle's function.

see also Autoimmune Disease; Genetic Diseases; Metabolism, Cellular; Mitochondrion; Musculoskeletal System; Neuron; Nucleotides; Synaptic Transmission

Theresa Stouter Bidle

Bibliography

Andersen, Jesper L. "Muscle, Genes and Athletic Performance." Scientific American 283, no. 3 (2000): 4855.

Bevan, James, and Richard Bayliss. A Pictorial Handbook of Anatomy and Physiology. Barnes and Noble Publishers, 1996.

LeVay, David. Teach Yourself Human Anatomy and Physiology. NTC Publishing Group, 1993.

Van Baak, Marleen A. "Relationships with Physical Activity." Nutrition Reviews 58, no.3 (2000): 5253.

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Muscular System

Muscular System

Independent movement is a unique characteristic of animals. Most animal movement depends on the use of muscles. Together, muscles and bones make up what is known as the musculoskeletal system. This combination provides protection for the body's internal organs and allows for many kinds of movement. Whether the movement is as simple as opening the eyes or as complex as flying, each is the result of a series of electrical, chemical, and physical interactions involving the brain, the central nervous system, and the muscles themselves.

Muscle is the flesh, minus the fat, that covers the skeleton of vertebrate animals. Muscles vary in size and shape and serve many different purposes. Large leg muscles such as hamstrings and quadriceps control limb motion. Other muscles, like the heart and the muscles of the inner ear, perform specialized involuntary functions. Despite the variety in size and function, however, all muscles share similar characteristics.

At the highest level, the entire muscle is composed of many strands of tissue called fascicles . These are the strands of muscle that can be seen in red meat or chicken. These strands are made up of very small fibers. These fibers are composed of tens of thousands of threadlike myofibrils, which can contract, relax, and lengthen.

The myofibrils are composed of up to ten million bands laid end-toend called sarcomeres . Each sarcomere is made of overlapping thick and thin filaments called myofilaments . The thick and thin myofilaments are made up of contractile proteins, primarily actin and myosin .

Types of Muscle Tissue

Muscles are categorized as either voluntary or involuntary. The muscles that animals can deliberately control are known as voluntary muscles . Those that cannot be controlled by the animal, such as the heart, are called involuntary muscles . Vertebrates also possess several different types of muscle tissue: cardiac, smooth, and striated or skeletal.

The muscle types are classified on the basis of their appearance when viewed through a light microscope. Striated muscle appears striped (striated) with alternating light and dark bands. Smooth muscle lacks the alternating light and dark bands.

Cardiac muscle.

Cardiac muscle makes up the wall of the heart, which is called the myocardium. In humans the heart contracts approximately seventy times per minute and can pump nearly 5 liters (4.5 quarts) of blood each minute. The fibers of the heart muscle are branched and arranged in a netlike pattern. The involuntary heart contraction is stimulated by an electrical impulse within the heart itself at the sinoatrial node.

Smooth muscle.

Smooth muscle cells are organized into sheets of muscle lining the walls of the stomach, intestines, blood vessels, and diaphragm, and parts of the urinary and reproductive systems. The smooth muscle contractions push food through the digestive system, regulate blood pressure by adjusting the diameter of blood vessels, regulate the flow of air in the lungs and expel urine from the urinary bladder. These body functions are involuntary and controlled by the autonomic nervous system .

Skeletal or striated muscle.

Skeletal muscle, which is muscle tissue attached to bones, makes up a large portion of an animal's body weight sometimes between 40 and 60 percent. Skeletal muscles move parts of the skeleton in relation to each other. They contain abundant blood vessels that transport oxygen and nutrients, nerve endings that carry electrical impulses from the central nervous system, and nerve sensors that relay messages back to the brain. Skeletal muscles are responsible for the conscious or voluntary movements of the trunk, arms and legs, respiratory organs, eyes, and mouth-parts of the animal. They are used for such actions as running, swimming, jumping, and lifting.

These distinctive muscle types can be observed throughout the evolution of vertebrates, however the arrangement of muscles varies according to differing environmental and survival needs. In fish, for example, most of the skeletal muscles fan out from either side of the backbone. Muscle makes up nearly 60 percent of the fish's body and nearly all of it is involved in moving the tail and spine.

As vertebrates evolved and adapted to life on land, the down-the-spine muscle arrangement began to change. More muscle power was needed for moving the limbs. Limb muscles became both bigger and longer. Some muscle fibers in a frog's hind legs can be nearly a quarter as long as the frog's body, which is proportionately much longer than the muscles in many fish. More muscles developed in the chest to be used for breathing, as vertebrates began spending more time on land. In mammals, this led to the development of the diaphragm, an involuntary muscle that helps to bring air into the lungs.

How Muscles Contract

Nerves connect the spinal column to the muscle. The place where the nerve and muscle meet is called the neuromuscular junction . Inside the muscle fibers, a signal from the nervous system stimulates the flow of calcium, which causes the thick and thin fibers (myofibrils) to slide across one another. When this occurs, the sarcomere shortens, which generates a force. The contraction of an entire muscle fiber results when billions of sarcomeres in the muscle shorten all at once.

The "sliding-filament theory" suggests that these thin and thick filaments become linked together by molecular cross bridges, which act as levers to pull the filaments past each other during the contraction of the muscle fiber. Myosin molecules have little pegs, called cross bridges, that protrude from the thick filament. During contraction, another molecule, called actin, appears to "climb" across these bridges.

Movement in invertebrates.

Movement occurs in all animals, including those without highly developed musculoskeletal systems. Nearly all groups of animals, including relatively simple organisms such as jellyfish and flatworms, have rudimentary muscle fibers that are specialized to move parts of the body. The number of muscles is not necessarily related to the size of the organism or the presence of a skeletal system. For example, a caterpillar may have 2,000 separate muscles compared with some 600 muscles in the human body.

Movement in invertebrates is caused by the same contractile proteins, actin and myosin, that function in the muscles of vertebrates. This primitive muscle tissue is triggered into action by nerves, hormones , or the builtin rhythm of the organism.

Simple protozoans such as the Ameoba, can either contract or extend their one-celled body in any direction. Other protozoans move by means of contractile fibers contained in cilia and flagella . Cilia are minute, hairlike, projections that stick out from the cells of some animals. Cilia allow protozoa to move freely through their aquatic environment. Another adaptation is the flagellum (pl., flagella), a whiplike structure found in sponges. A flagellum moves by a beating pattern that mimics a snakelike undulation.

Both smooth and striated muscle are present in invertebrate animals ranging from cnidarians to arthropods . Flatworms have muscle fibers in three directions, the contraction of which will move the body in multiple planes much like a human tongue. The body wall of earthworms contains both an outer and an inner layer. Contraction of the outer layer causes the body to lengthen and the action of the inner layer shortens it, producing the wiggling motion of the worm.

The only invertebrates without this layered arrangement of muscle tissue are the mollusks , crustaceans , and insects. They do, however, have many separate muscles, varied in size, arrangement, and attachments, that move the body segments and the parts of the jointed legs and other appendages. These muscles are fastened to the internal surfaces of the exoskeleton . Clams and other bivalve mollusks use strong muscle contractions to keep their shells tightly shut at high tide. Once the shell-closing muscles have contracted, they can remain tightly shut for hours without tiring.

see also Locomotion; Skeletons.

Leslie Hutchinson

Bibliography

Hickman, Cleveland, Larry Roberts, and Frances Hickman. Integrated Principles of Zoology, 8th ed. St. Louis, MO: Times Mirror/Mosby College Publishing, 1990.

Huxley, H. E. "The Mechanism of Muscular Contraction." Science 164 (1969):1356-1365.

Randall, David, Warren Burggren, and Kathleen French. Eckert Animal Physiology: Mechanisms and Adaptations, 4th ed. New York: W. H. Freeman & Company, 1997.

Rome, L. C., and R. P. Funk. "Why Animals Have Different Muscle Fiber Types."

Nature 355 (1988):824-827.

Internet Resources

"Muscle." Encyclopedia Britannica Online. 1994-2001. <http://members.eb.com/>.

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Muscular System

Muscular system

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 of the two classes of muscle. Like skeletal muscles, cardiac muscles are striated. But like smooth muscles, they are involuntary, controlled by the autonomic nervous system.

The longest muscle in the human body is the sartorius (pronounced sar-TOR-ee-us). It runs from the waist down across the front of thigh to the knee. Its purpose is to flex the hip and knee. The largest muscle in the body is the gluteus maximus (pronounced GLUE-tee-us MAX-si-mus; buttocks muscles). It moves the thighbone away from the body and straightens out the hip joint.

Skeletal muscles

Skeletal muscles are probably the most familiar type of muscle. They are the muscles that ache after strenuous work or exercise. Skeletal muscles make up about 40 percent of the body's mass or weight. They stabilize joints, help maintain posture, and give the body its general shape. They also use a great deal of oxygen and nutrients from the blood supply.

Skeletal muscles are attached to bones by tough, fibrous connective tissue called tendons. Tendons are rich in the protein collagen, which is arranged in a wavy way so that it can stretch and provide additional length at the muscle-bone junction.

Words to Know

Autonomic nervous system: Part of the nervous system that regulates involuntary action, such as of the heart and intestines.

Extensor muscle: Muscle that contracts and causes a joint to open.

Flexor muscle: Muscle that contracts and causes a joint to close.

Myoneural juncture: Area where a muscle and a nerve connect.

Tendon: Tough, fibrous connective tissue that attaches muscle to bone.

Skeletal muscles act in pairs. The flexing (contracting) of one muscle is balanced by a lengthening (relaxation) of its paired muscle or a group of muscles. These antagonistic (opposite) muscles can open and close joints such as the elbow or knee. An example of antagonistic muscles are the biceps (muscles in the front of the upper arm) and the triceps (muscles in the back of the upper arm). When the biceps muscle flexes, the forearm bends in at the elbow toward the biceps; at the same time, the triceps muscle lengthens. When the forearm is bent back out in a straight-arm position, the biceps lengthens and the triceps flexes.

Muscles that contract and cause a joint to close, such as the biceps, are called flexor muscles. Those that contract and cause a joint to open, such as the triceps, are called extensors. Skeletal muscles that support the skull, backbone, and rib cage are called axial skeletal muscles. Skeletal muscles of the limbs (arms and legs) are called distal skeletal muscles.

Skeletal muscle fibers are stimulated to contract by electrical impulses from the nervous system. Nerves extend outward from the spinal cord to connect to muscle cells. The area where a muscle and a nerve connect is called the myoneural juncture. When instructed to do so, the nerve releases a chemical called a neurotransmitter that crosses the microscopic space between the nerve and the muscle and causes the muscle to contract.

Skeletal muscle fibers are characterized as fast or slow based on their activity patterns. Fast (also called white) muscle fibers contract

rapidly, have poor blood supply, operate without oxygen, and tire quickly. Slow (also called red) muscle fibers contract more slowly, have better blood supplies, operate with oxygen, and do not tire as easily. Slow muscle fibers are used in movements that are ongoing, such as maintaining posture.

Smooth muscles

Smooth muscle fibers line most of the internal hollow organs of the body, such as the intestines, stomach, and uterus (womb). They help move substances through tubular areas such as blood vessels and the small intestines. Smooth muscles contract automatically, spontaneously, and often rhythmically. They are slower to contract than skeletal muscles, but they can remain contracted longer.

Like skeletal muscles, smooth muscles contract in response to neurotransmitters released by nerves. Unlike skeletal muscles, some smooth muscles contract after being stimulated by hormones (chemicals secreted by glands). An example is oxytocin, a hormone released by the pituitary gland. It stimulates the smooth muscles of the uterus to contract during childbirth.

Smooth muscles are not as dependent on oxygen as skeletal muscles are. Smooth muscles use carbohydrates to generate much of their energy.

Cardiac muscle

The cardiac muscle or myocardium contracts (beats) more than 2.5 billion times in an average lifetime. Like skeletal muscles, myocardium is striated. However, myocardial muscle fibers are smaller and shorter than skeletal muscle fibers.

The contractions of the myocardium are stimulated by an impulse sent out from a small clump (node) of specialized tissue in the upper right area of the heart. The impulse spreads across the upper area of the heart, causing this region to contract. This impulse also reaches another node, located near the lower right area of the heart. After receiving the initial impulse, the second node fires off its own impulse, causing the lower region of the heart to contract slightly after the upper region.

Disorders of the muscular system

The most common muscular disorder is injury from misuse. Skeletal muscle sprains and tears cause excess blood to seep into the tissue in order to heal it. The remaining scar tissue results in a slightly shorter muscle. Overexertion or a diminished blood supply can cause muscle cramping. Diminished blood supply and oxygen to the heart muscle causes chest pain called angina pectoris.

The most common type of genetic (inherited) muscular disorder is muscular dystrophy. This disease causes muscles to progressively waste away. There are six forms of muscular dystrophy. The most frequent and most dreaded form appears in boys aged three to seven. (Boys are usually affected because it is a sex-linked condition; girls are carriers of the disease and are usually not affected.) The first symptom of the disease is a clumsiness in walking. This occurs because the muscles of the pelvis and the thighs are first affected. The disease spreads to muscles in other areas of the body, and by the age of ten, a child is usually confined to a wheelchair or a bed. Death usually occurs before adulthood.

Another form of muscular dystrophy appears later in life and affects both sexes equally. The first signs of the disease appear in adolescence. The muscles affected are those in the face, shoulders, and upper arms. People with this form of the disease may survive until middle age.

Currently, there is no known treatment or cure for any form of muscular dystrophy.

[See also Heart ]

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muscle

muscle is the body's contractile tissue. ‘Contraction’, in the physiological sense, may involve shortening and change of shape, or it may generate force without any change in length. All contraction depends on physicochemical alterations in the molecules of protein filaments within the cells, resulting in the generation of force at linkages (cross-bridges) between two different kinds of filament. The main proteins involved, in the respective filaments of all types of muscle, are actin and myosin; and in all muscles the process is powered by breakdown of adenosine triphosphate, during which chemical energy is converted by the interactions between these proteins into the mechanical energy of contraction. To initiate the process, muscle cells require excitation, which leads to contraction by a sequence that crucially involves an increase in the concentration of free calcium ions inside the cell — a sequence termed excitation– contraction coupling.

There are three main types of muscle in the body: skeletal, cardiac, and smooth. When skeletal muscles contract they either move parts of the body via their attachments to bones, or produce tension to oppose stretch or even to allow controlled lengthening. Cardiac muscle and smooth muscle, by shortening, reduce the capacity of hollow organs and tubes: thus cardiac muscle ejects blood from the heart; smooth muscle ejects urine from the bladder or the fetus from the uterus, moves the contents of the gut along, and influences the flow of blood to different regions by varying the diameter of blood vessels.

Skeletal and cardiac are together known as striated muscles, because their fibres have a striped appearance under the microscope, due to the orderly arrangement of alternating ranks of interdigitating actin and myosin filaments within their cytoplasm. Smooth (unstriated) muscle does not show this: the two types of filament are mingled throughout the cytoplasm of the cells. Whilst cardiac and skeletal muscle have a structural resemblance, skeletal muscle can be under conscious control and is therefore also known as voluntary muscle whereas cardiac muscle and smooth muscle share the designation involuntary because their actions are never under direct conscious control. (In certain contemplative regimes, the subtle influence which may be achieved — such as on the heart rate — is an indirect consequence of a profoundly disciplined emotional state.)

The voluntary/involuntary distinction implies differences also in control of the three types of muscle. Skeletal muscle is controlled through pathways in the nervous system that can be consciously activated, cardiac and smooth by the involuntary or ‘autonomic’ pathways. Each skeletal muscle fibre is called into action by release of transmitter from a terminal branch of a single axon from a motor neuron in the spinal cord; the point at which this nerve terminal contacts the muscle fibre is a specialized synapse, the neuromuscular junction. All muscle fibres controlled by this nerve are recruited together, and the grouping of a motor neuron plus its family of muscle fibres is said to comprise a ‘motor unit’. When transmitter is not being released, the muscle fibres are relaxed. Individual cardiac muscle cells by contrast are activated by electrical transmission of excitation from their neighbours; this excitation originates rhythmically at a pacemaker, even in the absence of nerve action, although normally the rate of firing is modulated by the release, close to the pacemaker site, of transmitters from autonomic nerves. Smooth muscles differ again: in some, notably in the uterus at term, excitation is electrical, starting at pacemaker sites, much as in the heart. In others, such as those controlling the diameter of a large blood vessel, excitation is by neurotransmitters released from autonomic nerve endings close to the cells, but not with structured synapses. The contraction/relaxation state of smooth muscle can also be modified by chemical agents other than neurotransmitters, released from neighbouring cells or circulating in the blood. In the autonomic control of involuntary muscle, there is at many sites the possibility of either excitatory or inhibitory neural action, according to the particular transmitter released, resulting in a two-way control system analogous to accelerator and brake. The heart, for instance, is slowed by one transmitter, yet speeded up by another; the stomach wall is contracted by one and relaxed by another.

Neil Spurway

Sheila Jennett


See musculo-skeletal system.See also autonomic nervous system; cardiac muscle; motor neurons; skeletal muscle; smooth muscle.

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muscle

muscle, the contractile tissue that effects the movement of and within the body. Muscle tissue in the higher animals is classified as striated, smooth, or cardiac, according to its structure and function. Striated, or skeletal, muscle forms the bulk of the body's muscle tissue and gives the body its general shape. It is called striated because it appears striped, in alternating bands of light and dark, when viewed under a microscope, and animals have conscious control over most of their striate muscles. Smooth muscle, which lines most of the hollow organs of the body, is not under voluntary control, but is regulated by the autonomic nervous system. Smooth muscle fibers are spindle-shaped, not striated, and generally are arranged in dense sheets. Smooth muscle lines the blood vessels, hair follicles, urinary tract, digestive tract, and genital tract. Its speed of contraction is slower than that of striated muscle, but it can remain contracted longer. Cardiac muscle is striated like skeletal muscle but, like smooth muscle, is controlled involuntarily. It is found only in the heart, where it forms that organ's thick walls. The contractions of cardiac muscle are stimulated by a special clump of muscle tissue located on the heart (the pacemaker), although the rate of contractions is subject to regulation by the autonomic nervous system.

Muscle Contraction

Skeletal muscles are attached (with some exceptions, such as the muscles of the tongue and pharynx) to the skeleton by means of tendons, usually in pairs that pull in opposite directions, e.g., the biceps (flexor) and triceps (extensor) that move the forearm at the elbow. The means by which all types of muscles contract is thought to be generally the same, although muscles are classified as phasic, or fast twitch, and tonic, or slow twitch, to differentiate between the various lengths of time a muscle may require to move in response to stimulation. Striated muscle is usually considered phasic, while cardiac and smooth muscle are thought to be tonic.

Perhaps because its action is most varied, striated muscle has been studied most extensively. This type of muscle is composed of numerous cylindrically shaped bundles of cells, each enclosed in a sheath called the sarcolemma. Each muscle fiber contains several hundred to several thousand tightly packed strands called myofibrils that consist of alternating filaments of the protein substances actin and myosin. Actin and myosin interact before muscle contraction, forming the contractile material actomyosin.

The energy required for muscle contraction comes from the breakdown of adenosine triphosphate (ATP), a substance that is present in the cells and is formed during cellular respiration. A muscle fiber is stimulated to contract by electrical impulses from the nervous system. The point of contact between nerve and muscle is the neuromuscular junction, where the chemical substance acetylcholine is secreted, initiating the changes that cause the muscle to contract. During resting states, some of the fibers in the musculature are maintained in a state of partial contraction, known as muscle tone. This permits muscles to contract quickly when stimulated without having to overcome the inertia of total relaxation.

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muscle

mus·cle / ˈməsəl/ • n. 1. a band or bundle of fibrous tissue in a human or animal body that has the ability to contract, producing movement in or maintaining the position of parts of the body: the calf muscle | the sheet of muscle between the abdomen and chest. ∎  such a band or bundle of tissue when well developed or prominently visible under the skin: showing off our muscles to prove how strong we were. 2. physical power; strength: he had muscle but no brains. ∎ inf. a person or persons exhibiting such power or strength: an ex-marine of enormous proportions who'd been brought along as muscle. ∎  power or influence, esp. in a commercial or political context: he had enough muscle and resources to hold his position on the council. • v. [tr.] inf. move (an object) in a particular direction by using one's physical strength: they were muscling baggage into the hold of the plane. ∎ inf. coerce by violence or by economic or political pressure: he was eventually muscled out of business. PHRASES: flex one's muscles give a show of strength or power. not move a muscle be completely motionless.PHRASAL VERBS: muscle in/into inf. force one's way into (something), typically in order to gain an advantage: muscling his way into meetings and important conferences | he was determined to muscle in on the union's affairs. muscle up inf. build up one's muscles.DERIVATIVES: mus·cled / ˈməsəld/ adj. [in comb.] hard-muscled. mus·cle·less adj.

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muscle

muscle Tissue that has the ability to contract, enabling movement. There are three basic types: voluntary muscle (or skeletal muscle), involuntary muscle (or smooth muscle), and cardiac muscle. Voluntary muscle is the largest tissue component of the human body, comprising c.40% by weight. It attaches by tendons to the bones of the skeleton, and is characterized by cross-markings known as striations; it typically contains many nuclei per cell. Most voluntary muscles require conscious effort for contraction. A muscle whose contraction causes a limb or a part of the body to straighten (extend) is called an extensor. A muscle whose contraction causes a limb or part of the body to bend is called a flexor. Involuntary muscle lines the digestive tract, blood vessels and many other organs. It is not striated and typically has only one nucleus per cell; it is not under conscious control. Cardiac muscle is found only in the heart. It differs from the other types of muscle in that it beats rhythmically and does not need stimulation by a nerve impulse to contract. Cardiac muscle has some striations (but not as many as in voluntary muscle) and has only one nucleus per cell.

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muscle

muscle The contractile cellular unit of skeletal muscle is the cylindrical fibre, composed of many myofibrils. Chemically, muscle consists of three main proteins, actin, myosin, and tropomyosin. Contraction is achieved by formation of a complex between actin and myosin.

The muscle fibre is surrounded by a thin membrane, the sarcolemma; within the muscle fibre, surrounding the myofibrils, is the sarcoplasm. Individual fibres are separated by a thin network of connective tissue, the endomysium, and bound together in bundles by thicker sheets of connective tissue, the perimysium.

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muscle

muscle (mus-ŭl) n. a tissue whose cells have the ability to contract, producing movement or force. The major functions of muscles are to produce movements of the body and of structures within it and to alter pressures or tensions of internal organs. There are three types of muscle (see cardiac muscle, smooth muscle, striated muscle). See illustration.
muscular (mus-kew-ler) adj.

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muscle

muscle A tissue consisting of sheets or bundles of cells (muscle fibres) that are capable of contraction, so producing movement or tension in the body. There are three types of muscle. Voluntary muscle produces voluntary movement (e.g. at joints); involuntary muscle mainly effects the movements of hollow organs (e.g. intestine and bladder); and cardiac muscle occurs only in the heart.

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muscle

muscle Tissue consisting of cells that form fibres, arranged as sheets or bundles, which are able to contract and thus produce tension. There are two types of muscle in vertebrates: smooth (involuntary) and striated (voluntary). Smooth muscle is derived from the splanchic mesoderm, striated or skeletal muscles are derived from the myotomes of the mesoderm.

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muscle

muscle contractile fibrous bundle producing movement in an animal body. XVI. — (O)F. — L. mūsculus, dim. of mūs MOUSE, the form and movements of some muscles suggesting those of a mouse.
Hence muscular XVII. musculo-, comb. form of L. mūsculus.

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muscle

musclehassle, Kassel, passel, tassel, vassal •axel, axle •cancel, hansel, Hänsel, Mansell •transaxle •castle, metatarsal, parcel, tarsal •chancel • sandcastle • Newcastle •Bessel, nestle, pestle, redressal, trestle, vessel, wrestle •Edsel • Texel •intercensal, pencil, stencil •pretzel • staysail • mainsail • Wiesel •abyssal, bristle, epistle, gristle, missal, scissel, thistle, whistle •pixel • plimsoll •tinsel, windsail •schnitzel, spritsail •Birtwistle •paradisal, sisal, trysail •apostle, colossal, dossal, fossil, glossal, jostle, throstle •consul, proconsul, tonsil •dorsal, morsel •council, counsel, groundsel •Mosul • fo'c's'le, forecastle •bustle, hustle, muscle, mussel, Russell, rustle, tussle •gunsel • corpuscle •disbursal, dispersal, Purcell, rehearsal, reversal, succursal, tercel, transversal, traversal, universal •Herzl

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Muscular System

Muscular System

Definition

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 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 20-40 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 10-60 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—a bicep for example—makes this region anatomically visible.

Cardiac muscle

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 5-15 micrometers in diameter and 20-30 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

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. Multiunit 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 multiunit 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 5-15 micrometers, but they are long, typically 15-500 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.

Function

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 signalsending 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 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 muscle

The 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 musclediffers 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

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.

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.

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.

Resources

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., ed. 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, MD 20892. (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〉.

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Muscular System

Muscular System

Skeletal Muscles

Cardiac Muscles

Smooth Muscles

Disorders of the Muscular System

Resources

The muscular system is the bodys network of tissues for both conscious and unconscious movement. Movement is generated through the contraction and relaxation of specific muscles. Some muscles, like those in the arms and legs, are involved in voluntary movements such as raising a hand or flexing the foot. Other muscles are involuntary and function without conscious effort.

Voluntary muscles include skeletal muscles and total about 650 in the whole human body. Skeletal muscles are controlled by the somatic nervous system; whereas the autonomic nervous system controls involuntary muscles.

Involuntary muscles include muscles that line internal organs. These smooth muscles are called visceral muscles, and they perform tasks not generally associated with voluntary activity throughout the body even when it is asleep. Smooth muscles control several automatic physiological responses such as pupil constriction when iris muscles contract in bright light and blood vessel dilation when smooth muscles around them relax, or lengthen. In addition to skeletal and smooth muscle, which are considered voluntary and involuntary, respectively, there is cardiac muscle, which is considered neither. Cardiac muscle is not under conscious control, and it can also function without external nervous system regulation.

Smooth muscles derive their name from their appearance when viewed in 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 the 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 the striated appearance. This microscopic view of muscle reveals some hint of how muscles alter their shape to induce movement. Because muscle cells tend to be elongated,

they are often called muscle fibers. Muscle cells are distinct from other cells in the body in shape, protein composition, and in the fact that they are multi-nucleated (have more than one nucleus per cell).

Skeletal Muscles

Skeletal muscles are probably the most familiar type of muscle to people. Skeletal muscles are the ones that ache when someone goes for that first outdoor run in the spring after not running much during the winter. And skeletal muscles are heavily used when someone carries in the grocery bags. Exercise may increase muscle fiber size, but muscle fiber number generally remains constant. Skeletal muscles take up about 40% of the bodys mass, or weight. They also use a great deal of oxygen and nutrients from the blood supply. Multiple levels of skeletal muscle tissue receive their own blood supplies.

Like all muscles, skeletal muscles can be studied at both a macroscopic and a microscopic level. At the macroscopic level, skeletal muscles usually originate at one point of attachment to a tendon 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 way so that it can stretch out and provide additional length at the muscular-bone junction.

Skeletal muscles act in pairs where the flexing (shortening) of one muscle is balanced by a lengthening (relaxation) of its paired muscle or a group of muscles. These antagonistic (opposite) muscles can open and close joints such as the elbow or knee. Muscles that contract and cause a joint to close are called flexor muscles, and those that contract to cause a joint to stretch out are called extensors. Skeletal muscle that supports the skull, backbone, and rib cage are called axial skeletal muscles; whereas, skeletal muscles of the limbs are called distal. These muscles attach to bones via strong, thick connective tissue called tendons. Several skeletal muscles work in a highly coordinated manner in activities such 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 muscles fibers are additionally characterized as fast or slow based on their activity patterns. Fast, also called white, muscle fibers contract rapidly, have poor blood supply, operate anaerobically, and fatigue rapidly. Slow, also called red, muscle fibers contract more slowly, have better blood supplies, operate aerobically, and do not fatigue as easily. Slow muscle fibers are used in movements that are sustained, such as maintaining posture.

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, which are each covered by connective tissue called the perimysium. An average skeletal muscle may have 2040 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 remove waste products, respectively. The endomysium has an extensive network of capillaries that supply individual muscle fibers. Individual muscle fibers vary in diameter from 10-60 micrometers and in length from a few millimeters to about 12 in (30 cm) in the sartorius muscle of the thigh.

At the microscopic level, a single 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 in muscle. The myofilaments, actin and myosin, overlap one another in a very specific arrangement. Myosin is a thick protein with two globular head regions. Each myosin filament is surrounded by six actin (thin) filaments. These filaments run longitudinally 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 which result in muscle contraction. In addition, calcium ions and ATP (cellular energy) are required by the muscle cell to process this reaction. Numerous mitochondria 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 the 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. A sarcomere can induce muscle contraction the way a paper towel roll holder can be pushed together before inserting it into a dispenser. The actin and myosin filaments slide over one another like the outer and inner layers of the roll holder. Muscle contraction creates an enlarged center region in the whole muscle. The flexing of a bicep makes this region anatomically visible. This large center is called the belly of the muscle.

Skeletal muscles function as the link between the somatic nervous system and the skeletal system. One does not move a skeletal muscle for the sake of moving the muscle unless one is a bodybuilder. Skeletal muscles are used to carry out instructions from the brain so that someone can accomplish something. For instance, someone decides that they would like a bite of cake. Unless the cake will come to the mouth by itself, the person needs to figure out some way to get that cake to their mouth. The brain tells the muscle to contract in the forearm allowing it to flex so that the hand is in position to get a forkful of cake. But the muscle alone cannot support the weight of a fork; it is the sturdy bones of the forearm that allow the muscles to complete the task of obtaining the cake. Hence, the skeletal and muscular systems work together as a lever system with 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 neurons branch 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 which triggers 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. Then signals travel down the spinal cord to the alpha motor neuron to result in contraction. However, not all movement of skeletal muscles is voluntary. Certain reflexes occur in response to dangerous stimuli, such as extreme heat. Reflexive skeletal muscular movement is controlled at the level of the spinal cord and does not require higher brain initiation. Reflexive movements are processed at this level to minimize the amount of time necessary to implement a response.

In addition to motor neuron activity in skeletal muscular activity, a number of sensory nerves carry information to the brain to regulate muscle tension and contraction to optimize muscle action. Muscles function at peak performance when they are not over-stretched or overcontracted. Sensory neurons within the muscle send feedback to the brain with regard to muscle length and state of contraction.

Cardiac Muscles

Cardiac muscles, as is evident from their name, make 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 cardiac vessels before tapering off completely. The heart muscle is also called the myocardium. The heart muscle is responsible for more than two billion beats in a lifetime. The myocardium has some properties similar to skeletal muscle tissue, but it is also unique. Like skeletal muscles, myocardium is striated; however, the cardiac muscle fibers are smaller and shorter than skeletal muscle fibers averaging 5-15 micrometers in diameter and 20-30 micrometers in length. In addition, cardiac muscles align lengthwise more than side-by-side compared to skeletal muscle fibers. The microscopic structure of cardiac muscle is also unique in that these cells are branched such that they can simultaneously communicate with multiple cardiac muscle fibers.

Cardiac muscle cells are surrounded by an endomysium like the skeletal muscle cells. But innervation of autonomic nerves to the heart do not form any special junction like that 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 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 making them myogenic. Like skeletal muscle cells, cardiac muscle fibers can increase in size with physical conditioning, but they rarely increase in number.

Smooth Muscles

Smooth muscle falls into two general categories, visceral smooth muscle and multi-unit smooth muscle. Visceral smooth muscle fibers line internal organs such as the intestines, stomach, and uterus. They also facilitate the movement of substances through tubular areas such as blood vessels and the small intestines. Multi-unit smooth muscles function in a highly localized way in areas such as the iris of the eye. Contrary to contractions in visceral smooth muscle, contractions in multi-unit smooth muscle fibers do not readily spread to neighboring muscle cells.

Smooth muscle is unstriated with innervations from both sympathetic (flight or fight) and parasym-pathetic (more relaxed) 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 which are more haphazardly arranged than they are in skeletal muscles. The sympathetic neurotransmitter, Ach, and parasympathetic neurotransmitter, norepinephrine, activate this type of muscle tissue.

The concentric arrangement of some smooth muscle fibers enables them to control dilation and constriction in the intestines, blood vessels, and other areas. While innervation of these cells is not individual, excitation from one cell can spread to adjacent cells through nexuses which join neighbor cells. Smooth muscle cells have a small diameter of about 5-15 micrometers and are long, typically 15-500 micro-meters. 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 hollow organs, such as the uterus, smooth muscle cells are arranged into two layers. The outer layer is usually arranged in a longitudinal fashion surrounding the inner layer which is arranged in a circular orientation. Many smooth muscles are regulated by hormones in addition to the neurotransmitters of the autonomic nervous system. In addition, contraction of some smooth muscles are myogenic or triggered by stretching as in the uterus and gastrointestinal tract.

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 to generate much of its metabolic energy.

Disorders of the Muscular System

Disorders of the muscular system can be due to genetic, hormonal, infectious, autoimmune, poisonous, or cancerous causes. But the most common problem associated with this system is injury from misuse. Skeletal muscle sprains and tears cause excess blood to seep into the tissue in order to heal it. The remaining 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. Poor blood supply to the heart muscle causes chest pain called angina pectoris. And inadequate ionic supplies of calcium, sodium, or potassium can adversely effect most muscle cells.

Muscular system disorders related to the immune system include myasthenia gravis (MG) and tumors. MG is characterized by weak and easily fatigued skeletal muscles, one of the symptoms of which is droopy eyelids. MG is caused by antibodies that a person makes against their own Ach receptors; hence, MG 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.

KEY TERMS

Cardiac muscle Narrow, long, striated muscle tissue of the heart.

Skeletal muscle Also called voluntary or striated muscle, it is a muscle under conscious control by the individual. Striated muscles flex or extend the leg or arm, curl the fingers, move the jaw during chewing, and so forth.

Smooth muscle Long, unstriated muscle cells which line internal organs and facilitate involuntary movements such as peristalsis.

Tendon A strap of tissue that connects muscle to bone, providing for movement.

Contamination of muscle cells by infectious substances and drugs can also lead to muscular disorders. A Clostridium bacteria can cause muscle tetanus, which is a disease characterized by painful repeated muscular contractions. In addition, some types of gangrene are due to bacterial infections deep in a muscle. Gangrene is the decay of muscle tissue in varying degrees; it can involve small areas of a single muscle or entire organs. The poisonous substance, curare, blocks neuromuscular transmission in skeletal muscle causing paralysis. And large doses of prolonged alcohol consumption can cause muscle damage, as well.

The most common type of muscular genetic disorder is muscular dystrophy of which there are several kinds. Duchennes muscular dystrophy is characterized by increasing muscular weakness and eventual death. Beckers muscular dystrophy is milder than Duchennes, but both are X-linked recessive genetic disorders. Other types of muscular dystrophy are caused by a mutation that affects the muscle protein dystrophin, which is absent in Duchennes and altered in Beckers muscular dystrophies. Other genetic disorders (such as some cardiomyopathies) can affect various muscle tissues.

Resources

BOOKS

Gray, Henry. Grays Anatomy. Philadelphia: Running Press, 1999.

Marieb, Elaine Nicpon. Human Anatomy & Physiology. 5th Edition. San Francisco: Benjamin/Cummings, 2000.

Netter, Frank H., and Sharon Colacino. Atlas of Human Anatomy. Yardley, PA: Icon Learning Systems, 2003.

OTHER

Bartleby.com. Anatomy of the Human Body <http://www.bartleby.com/107> (accessed October 14, 2006).

Intellimed. Human Anatomy Online <http://www.innerbody.com/htm/body.html> (accessed October 14, 2006).

MedLinePlus. Anatomy <http://www.nlm.nih.gov/medlineplus/anatomy.html> (accessed October 14, 2006).

Louise Dickerson

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Muscular System

Muscular system

Definition

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 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 20–40 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 10–60 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—a bicep for example—makes this region anatomically visible.

Cardiac muscle

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 5–15 micrometers in diameter and 20–30 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

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 5–15 micrometers, but they are long, typically 15–500 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.

Function

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


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.

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.

Common diseases and disorders

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.

Resources

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>.

Crystal Heather Kaczkowski, MSc.

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Muscular System

Muscular system

The muscular system is the body's network of tissues for both conscious and unconscious movement. Movement is generated through the contraction and relaxation of specific muscles. Some muscles, like those in the arms and legs, are involved in voluntary movements such as raising a hand or flexing the foot. Other muscles are involuntary and function without conscious effort. Voluntary muscles include skeletal muscles and total about 650 in the whole human body. Skeletal muscles are controlled by the somatic nervous system ; whereas the autonomic nervous system controls involuntary muscles. Involuntary muscles include muscles that line internal organs. These smooth muscles are called visceral muscles, and they perform tasks not generally associated with voluntary activity throughout the body even when it is asleep. Smooth muscles control several automatic physiological responses such as pupil constriction when iris muscles contract in bright light and blood vessel dilation when smooth muscles around them relax, or lengthen. In addition to skeletal and smooth muscle, which are considered voluntary and involuntary, respectively, there is cardiac muscle, which is considered neither. Cardiac muscle is not under conscious control, and it can also function without external nervous system regulation.

Smooth muscles derive their name from their appearance when viewed in 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 the 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 the striated appearance. This microscopic view of muscle reveals some hint of how muscles alter their shape to induce movement. Because muscle cells tend to be elongated, they are often called muscle fibers. Muscle cells are distinct from other cells in the body in shape, protein composition, and in the fact that they are multi-nucleated (have more than one nucleus per cell).


Skeletal muscles

Skeletal muscles are probably the most familiar type of muscle to people. Skeletal muscles are the ones that ache when someone goes for that first outdoor run in the spring after not running much during the winter. And skeletal muscles are heavily used when someone carries in the grocery bags. Exercise may increase muscle fiber size, but muscle fiber number generally remains constant. Skeletal muscles take up about 40% of the body's mass , or weight. They also use a great deal of oxygen and nutrients from the blood supply . Multiple levels of skeletal muscle tissue receive their own blood supplies.

Like all muscles, skeletal muscles can be studied at both a macroscopic and a microscopic level. At the macroscopic level, skeletal muscles usually originate at one point of attachment to a tendon 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 way so that it can stretch out and provide additional length at the muscular-bone junction.

Skeletal muscles act in pairs where the flexing (shortening) of one muscle is balanced by a lengthening (relaxation) of its paired muscle or a group of muscles. These antagonistic (opposite) muscles can open and close joints such as the elbow or knee. Muscles that contract and cause a joint to close are called flexor muscles, and those that contract to cause a joint to stretch out are called extensors. Skeletal muscle that supports the skull, backbone, and rib cage are called axial skeletal muscles; whereas, skeletal muscles of the limbs are called distal. These muscles attach to bones via strong, thick connective tissue called tendons. Several skeletal muscles work in a highly coordinated manner in activities such 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 muscles fibers are additionally characterized as "fast" or "slow" based on their activity patterns. Fast, also called "white," muscle fibers contract rapidly, have poor blood supply, operate anaerobically, and fatigue rapidly. Slow, also called "red," muscle fibers contract more slowly, have better blood supplies, operate aerobically, and do not fatigue as easily. Slow muscle fibers are used in movements that are sustained, such as maintaining posture.

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, which are each covered by connective tissue called the perimysium. An average skeletal muscle may have 20–40 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 remove waste products, respectively. The endomysium has an extensive network of capillaries that supply individual muscle fibers. Individual muscle fibers vary in diameter from 10-60 micrometers and in length from a few millimeters to about 12 in (30 cm) in the sartorius muscle of the thigh.

At the microscopic level, a single 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 in muscle. The myofilaments, actin and myosin, overlap one another in a very specific arrangement. Myosin is a thick protein with two globular head regions. Each myosin filament is surrounded by six actin (thin) filaments. These filaments run longitudinally 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 which result in muscle contraction. In addition, calcium ions and ATP (cellular energy ) are required by the muscle cell to process this reaction. Numerous mitochondria 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 the 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. A sarcomere can induce muscle contraction the way a paper towel roll holder can be pushed together before inserting it into a dispenser. The actin and myosin filaments slide over one another like the outer and inner layers of the roll holder. Muscle contraction creates an enlarged center region in the whole muscle. The flexing of a bicep makes this region anatomically visible. This large center is called the belly of the muscle.

Skeletal muscles function as the link between the somatic nervous system and the skeletal system . One does not move a skeletal muscle for the sake of moving the muscle unless one is a bodybuilder. Skeletal muscles are used to carry out instructions from the brain so that someone can accomplish something. For instance, someone decides that they would like a bite of cake. Unless the cake will come to the mouth by itself, the person needs to figure out some way to get that cake to their mouth. The brain tells the muscle to contract in the forearm allowing it to flex so that the hand is in position to get a forkful of cake. But the muscle alone cannot support the weight of a fork; it is the sturdy bones of the forearm that allow the muscles to complete the task of obtaining the cake. Hence, the skeletal and muscular systems work together as a lever system with 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 neurons branch 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 which triggers 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. Then signals travel down the spinal cord to the alpha motor neuron to result in contraction. However, not all movement of skeletal muscles is voluntary. Certain reflexes occur in response to dangerous stimuli, such as extreme heat . Reflexive skeletal muscular movement is controlled at the level of the spinal cord and does not require higher brain initiation. Reflexive movements are processed at this level to minimize the amount of time necessary to implement a response.

In addition to motor neuron activity in skeletal muscular activity, a number of sensory nerves carry information to the brain to regulate muscle tension and contraction to optimize muscle action. 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 muscles

Cardiac muscles, as is evident from their name, make 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 cardiac vessels before tapering off completely. The heart muscle is also called the myocardium. The heart muscle is responsible for more than two billion beats in a lifetime. The myocardium has some properties similar to skeletal muscle tissue, but it is also unique. Like skeletal muscles, myocardium is striated; however, the cardiac muscle fibers are smaller and shorter than skeletal muscle fibers averaging 5-15 micrometers in diameter and 20-30 micrometers in length. In addition, cardiac muscles align lengthwise more than side-by-side compared to skeletal muscle fibers. The microscopic structure of cardiac muscle is also unique in that these cells are branched such that they can simultaneously communicate with multiple cardiac muscle fibers.

Cardiac muscle cells are surrounded by an endomysium like the skeletal muscle cells. But innervation of autonomic nerves to the heart do not form any special junction like that 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 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 making them myogenic. Like skeletal muscle cells, cardiac muscle fibers can increase in size with physical conditioning , but they rarely increase in number.


Smooth muscles

Smooth muscle falls into two general categories, visceral smooth muscle and multi-unit smooth muscle. Visceral smooth muscle fibers line internal organs such as the intestines, stomach, and uterus. They also facilitate the movement of substances through tubular areas such as blood vessels and the small intestines. Multi-unit smooth muscles function in a highly localized way in areas such as the iris of the eye . Contrary to contractions in visceral smooth muscle, contractions in multi-unit smooth muscle fibers do not readily spread to neighboring muscle cells.

Smooth muscle is unstriated with innervations from both sympathetic (flight or fight) and parasympathetic (more relaxed) 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 which are more haphazardly arranged than they are in skeletal muscles. The sympathetic neurotransmitter, Ach, and parasympathetic neurotransmitter, norepinephrine, activate this type of muscle tissue.

The concentric arrangement of some smooth muscle fibers enables them to control dilation and constriction in the intestines, blood vessels, and other areas. While innervation of these cells is not individual, excitation from one cell can spread to adjacent cells through nexuses which join neighbor cells. Smooth muscle cells have a small diameter of about 5-15 micrometers and are long, typically 15-500 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 hollow organs, such as the uterus, smooth muscle cells are arranged into two layers. The outer layer is usually arranged in a longitudinal fashion surrounding the inner layer which is arranged in a circular orientation. Many smooth muscles are regulated by hormones in addition to the neurotransmitters of the autonomic nervous system. In addition, contraction of some smooth muscles are myogenic or triggered by stretching as in the uterus and gastrointestinal tract.

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 to generate much of its metabolic energy.


Disorders of the muscular system

Disorders of the muscular system can be due to genetic, hormonal, infectious, autoimmune, poisonous, or cancerous causes. But the most common problem associated with this system is injury from misuse. Skeletal muscle sprains and tears cause excess blood to seep into the tissue in order to heal it. The remaining 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. Poor blood supply to the heart muscle causes chest pain called angina pectoris. And inadequate ionic supplies of calcium, sodium , or potassium can adversely effect most muscle cells.

Muscular system disorders related to the immune system include myasthenia gravis (MG) and tumors. MG is characterized by weak and easily fatigued skeletal muscles, one of the symptoms of which is droopy eyelids. MG is caused by antibodies that a person makes against their own Ach receptors; hence, MG 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.

Contamination of muscle cells by infectious substances and drugs can also lead to muscular disorders. A Clostridium bacteria can cause muscle tetanus , which is a disease characterized by painful repeated muscular contractions. In addition, some types of gangrene are due to bacterial infections deep in a muscle. Gangrene is the decay of muscle tissue in varying degrees; it can involve small areas of a single muscle or entire organs. The poisonous substance, curare , blocks neuromuscular transmission in skeletal muscle causing paralysis. And large doses of prolonged alcohol consumption can cause muscle damage, as well.

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 milder than Duchenne's, but both are X-linked recessive genetic disorders . Other types of muscular dystrophy are caused by a mutation that affects the muscle protein dystrophin, which is absent in Duchenne's and altered in Becker's muscular dystrophies. Other genetic disorders (such as some cardiomyopathies) can affect various muscle tissues.


Resources

books

Becker W., and D. Deamer, eds. "Cellular Motility and Contractility." In The World of the Cell. 2nd ed. New York: The Benjamin/Cummings Publishing Company, Inc., 1991.

Rhoads R., and R. Pflanzer, eds. "The Motor System." and "Muscle." In Physiology. New York: Saunders College Publishing, 1992.


Louise Dickerson

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cardiac muscle

—Narrow, long, striated muscle tissue of the heart.

Skeletal muscle

—Also called voluntary or striated muscle, it is a muscle under conscious control by the individual. Striated muscles flex or extend the leg or arm, curl the fingers, move the jaw during chewing, and so forth.

Smooth muscle

—Long, unstriated muscle cells which line internal organs and facilitate involuntary movements such as peristalsis.

Tendon

—A strap of tissue that connects muscle to bone, providing for movement.

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Muscular System

Muscular System


The muscular system is composed of all those body tissues capable of contracting and relaxing and, therefore, of producing movements in its body parts. Muscles are able to produce movement by converting chemical energy into mechanical energy. This conversion occurs due to the ability of muscles to contract and relax. Animals use muscles not only to move about but to operate their many necessary internal processes such as blood circulation.

Since animals cannot make their own food as plants do, they must be able to move about to locate things to eat. Movement is therefore a trait shared by all animals, from a one-celled amoeba to a killer whale. Animals are able to move because they have a support framework (a jointed skeleton) that is moved by muscles, which push bones together and pull them apart. Muscles are made up of long fibers of contractile tissue, meaning they have the ability to contract or grow shorter by means of a chemical change, as well as relax or return to their normal length. Muscles are rich in blood vessels that bring them food and oxygen and take away their wastes. The harder the muscles work, the more blood is transported to them.

TYPES OF MUSCLES

There are three types of muscle found in the muscular system: skeletal muscle, smooth muscle, and cardiac muscle. Although all three contract in basically the same manner, they look very different when examined under a microscope.

Skeletal Muscle. Skeletal muscle is attached to bones and can be said to make up the flesh of an animal. It is also called striated muscle since it is made up of long, striped muscle fibers. Skeletal muscle is a voluntary muscle because it can be consciously controlled. A person can, therefore decide when and how to move his or her arms or legs to walk or run, or to move facial muscles to smile or frown. Most skeletal muscles are attached to bones and are connected at both ends by a tough, connecting tissue called a tendon. Tendons can be felt in the forearms near the wrist, in back of the leg near the knee, and in the back of the ankle. Skeletal muscles move bone by acting in pairs. One muscle, called the flexor, contracts and pulls the bones connected by a joint together. Another muscle, called the extensor, pulls, or moves the bones apart. Muscles, such as the flexor and extensor that work together but in opposing ways, are called an antagonistic pair.

Smooth Muscles. Smooth muscle is a second type of muscle, which actually appears smooth under a microscope. It makes up the walls of many

organs inside the body, including the digestive tract, reproductive organs, bladder, arteries, and veins. Since it does not respond to a person's will or command—that is, it cannot deliberately be controlled—it is called involuntary muscle. Blood is pushed through the veins, and the lungs expand whether a person wants them to or not. Smooth muscles contract like any muscle, and although they do so more slowly than skeletal muscle, they can maintain these contractions for a longer period of time.

Cardiac Muscle. A third type of muscle is cardiac muscle. As its name implies, it is found only in the heart and it is responsible for the strong, regular contractions known as the heartbeat. Like smooth muscle, cardiac muscle is involuntary, but unlike any other type of muscle, cardiac muscle contracts independently of any nerve supply. This means that cardiac muscle contracts and relaxes automatically according to its own, built-in rhythm. Cardiac muscle is amazingly strong and resilient, given that it has to beat continuously over an individual's lifetime. Unlike skeletal muscle, which requires regular periods of rest, cardiac muscle neither requires—nor is allowed—to take a rest.

WHY MUSCLES CONTRACT

The actual contraction or shortening of any muscle occurs because a muscle contains two proteins, actin and myosin, that form long threads called filaments. These filaments are arranged in parallel stacks, like over-lapping decks of cards. When a muscle gets a nerve signal to contract or shorten, the actin filaments physically slide over the myosin filaments and overlap them. The more they overlap, the more the muscle shortens. When the filaments slide back again, the muscle relaxes.

The human body has more than 600 muscles, and in the adult male, muscles make up about 40 percent of the total body mass. Although skeletal muscle fiber cannot replace itself by cell division after an organism is born, it can increase in size through exercise, which is important for healthy, strong muscles. Regular physical activity creates an increase in the number of blood vessels, which means the muscle receives more nutrients and oxygen. Muscles that are not used for a long period of time undergo a wasting process called atrophy.

[See alsoHeart ]

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