MUSCLE

contractile tissue found in animals, the function of which is to produce motion. Movement, the intricate cooperation of muscle and nerve fibres, is the means by which an organism interacts with its environment. The innervation of muscle cells, or fibres, permits an animal to carry out the normal activities of life. An organism must move to find food or, if it is sedentary, must have the means to bring food to itself. An animal must be able to move nutrients and fluids through its body, and it must be able to react to external or internal stimuli. Muscle cells fuel their actions by converting chemical energy in the form of adenosine triphosphate (ATP), which is derived from the metabolism of food, into mechanical energy. Muscle is contractile tissue grouped into coordinated systems for greater efficiency. In humans, the muscle systems are classified by gross appearance and location of cells. The three types of muscles are striated (or skeletal), cardiac, and smooth (or nonstriated). Striated muscle is almost exclusively attached to the skeleton and constitutes the bulk of the body's muscle tissue. The multinucleated fibres are under the control of the somatic nervous system and elicit movement by forces exerted on the skeleton similar to levers and pulleys. The rhythmic contraction of cardiac muscle is regulated by the sinoatrial node, the heart's pacemaker. Although cardiac muscle is specialized striated muscle consisting of elongated cells with many centrally located nuclei, it is not under voluntary control. Smooth muscle lines the viscera, blood vessels, and dermis, and, like cardiac muscle, its movements are operated by the autonomic nervous system and thus are not under voluntary control. The nucleus of each short, tapering cell is located centrally. Unicellular organisms, simple animals, and the motile cells of complex animals do not have vast muscle systems. Rather, movement in these organisms is elicited by hairlike extensions of the cell membrane called cilia and flagella or by cytoplasmic extensions called pseudopodia. This article consists of a comparative study of the muscle systems of various animals, including an explanation of the process of muscle contraction and an account of the human muscle system as it relates to upright posture. Warren F. Walker, Jr. Robert McNeill Alexander The Editors of the Encyclopdia Britannica contractile tissue found in animals, the function of which is to produce motion. It is by the action of muscles that animals are able to move about, digest food, focus eyes, circulate blood, and maintain body warmth, as well as perform other physiological functions. Among the higher animals, all muscles fall more or less into one of three classifications: striated, cardiac, and smooth. A more general classification applicable to all muscle-bearing creatures divides contractile tissue into phasic (which responds quickly to stimulation) and tonic (which responds gradually to stimulation). Striated muscle, so-called because the organization of its fibres makes it appear striped when viewed under a microscope, gives power of movement, maintains the posture of higher animals, and accounts for a large percentage of their body weight. It is also called skeletal muscle since most striate muscles are attached to the skeleton at both ends by tendons (somewhat elastic tissues), and their primary function is to operate the skeleton as a system of levers. Animals have voluntary control over their striate muscles through their central nervous systems. Striate muscle tissue consists of filaments of two different diameters (wide and narrow) which run parallel to the long axis of the muscle and are bundled in cords which make up multinuclear cells called myofibrils. Muscular contraction does not reflect contraction of the individual filaments of striated muscle, none of which spans the entire length of the muscle, but rather a sliding of the thin filaments between the thick filaments. A muscle will contract if the force generated in its tissues is greater than the force resisting it, in which case it is said to be in isotonic condition; if the resistance to contraction equals the force generated in the muscle tissues, the muscle will not contract (isometric condition) although the tension will be increased; too great a resistance will stretch the muscle fibres. The amount of force generated in a striate muscle depends on the length of the muscle and the amount of stimulation. Physiologically, stimulation of a striate muscle cell is initiated by an electrical impulse, which is transmitted from one of the motor nerves which innervate the striate muscle tissue, across a synapse (juncture between neurons) which occupies the area between the nerve membrane and the muscle cell membrane. A chemical reaction involving the proteins of the muscle tissue is induced by the electrical impulse and is the immediate mechanism of muscular force generation. The chemical reaction is accompanied by a release of heat, which is used to maintain body temperature; thus, muscles tend to tighten in cold atmospheres and shivering, the rapid alternating contraction and relaxation of muscle, generates a great deal of heat within the tissue. After each stimulation of muscle fibre there is a refractory period during which the fibre will not respond to further stimulation; however, repeated stimulation at a particular rate will produce a state of continuous contraction (tetanus response). Located in the muscle tissues are sensory organs called stretch receptors, which provide for close regulation and adjustment of muscular activity by feeding back to the central nervous system information about the state of the muscle, its speed, and degree of contraction at a given time. The fibres (myocardial cells) of the cardiac muscle, the muscle of the heart, are branched and arranged together in a netlike array. Heart contraction (systole) is activated within the muscle tissue itself, beginning with an electrical impulse generated in an area of the organ called the pacemaker. From there (in a mammalian heart) it spreads first to the atria (chambers for receiving blood) and then to the ventricles (pumping chambers). The membranes of myocardial cells pass electrical current with little resistance, so that one impulse from the pacemaker causes the organ to contract essentially as a unit. Vagus nerves and sympathetic nerves connect the heart to the central nervous system, serving respectively to slow down or speed up the heart rate; however, only the pacemaker can initiate heartbeat. The muscles found in the digestive and reproductive organs of vertebrates, the feet of mollusks, and the body walls of annelids (worms) are smooth. Whereas striate muscle is generally voluntary and phasic, smooth muscle is generally involuntary and tonic. Smooth muscle is more varied in its forms and characteristics than is striated muscle. In vertebrates, smooth muscle is either visceral (where the cells of the muscle are self-activating and may operate more or less collectively) or multicellular (where individual cells respond to stimulations from separate nerve endings). Cells of these muscles are most commonly spindle-, ribbon-, or rod-shaped. Clams, oysters, and scallops possess smooth catch muscles that can remain contracted for weeks at a time. Insects fly by the use of smooth asynchronous muscles, so called because the rate of alternating contractions and relaxations of these wing muscles is greater (up to 1,000 cycles per second in gnats) than the rate of nerve impulse stimulations to the muscle. In primitive life forms, such as the jellyfish, contractile tissues develop from the cells of the epithelial (surface, or skin) layer. Mammalian muscle, however, develops from the mesoderm, or middle layer, of the embryo. Muscles evolved from simple contractile systems as life functions became more complex, and well-developed muscle tissue is held by all life forms above jellyfish in the evolutionary scale. Among the higher animals, evolution has been characterized by increasing control of the central nervous system over muscular activity. The voluntary muscles of humans are subject to various disorders, symptoms of which include weakening, atrophy, pain, and twitching. A number of systemic diseases, e.g., dermatomyositis and polymyositis, may cause inflammation of muscles. Muscular dystrophies are hereditary developmental diseases. Myasthenia gravis, a condition in which transmission of nerve impulses to muscles is incomplete, is an autoimmune disease. Additional reading General features of muscles and muscle systems Basic information at various levels of difficulty may be found in Geoffrey H. Bourne (ed.), The Structure and Function of Muscle, 2nd ed., 4 vol. (197273), a collection of important articles on most aspects and types of muscles; Graham Hoyle, Muscles and Their Neural Control (1983), which includes a survey of the diverse kinds of muscle found in different animals; Thomas A. McMahon, Muscles, Reflexes, and Locomotion (1984), a book that ranges from basic muscle mechanics to the mechanics of walking and running; R.B. Stein, Nerve and Muscle: Membranes, Cells and Systems (1980), a comprehensive treatment of the biophysics of muscles; and Design and Performance of Muscular Systems, Journal of Experimental Biology, 115:1412 (1985), an entire volume devoted to the proceedings of an important conference. Invertebrate muscle systems R. McNeill Alexander, The Invertebrates (1979), is a survey that emphasizes the mechanics of movement. E.R. Trueman, The Locomotion of Soft-Bodied Animals (1975), is mainly about worms and mollusks. Vertebrate muscle systems Sources include Alfred Sherwood Romer and Thomas S. Parsons, The Vertebrate Body, 6th ed. (1986), a textbook of comparative anatomy that describes the embryonic development and evolution of the vertebrate muscular system; and Warren F. Walker, Vertebrate Dissection, 7th ed. (1986), a laboratory manual for comparative anatomy that gives a thorough description of the muscular systems of the dogfish, mud puppy, cat, and rabbit. Milton Hildebrand, Analysis of Vertebrate Structure, 3rd ed. (1988); and Alfred Sherwood Romer, The Vertebrate Story, 4th ed. (1959, reprinted 1971), are reference texts on vertebrate muscle. Human muscle systems Information on the unique human musculature can be found in Ernst Huber, Evolution of Facial Musculature and Facial Expression (1931, reprinted in Evolution of Facial Expression, 1972), detailed presentations of the evolution of two important groups of muscles; Arthur Keith, Man's Posture: Its Evolution and Disorders, British Medical Journal, 1:451454, 499502, 545548, 587590, 624626, 669672 (March 17April 21, 1923), a classic summary of the adaptation of musculature to upright posture; and Paul E. Klopsteg et al. (eds.), Human Limbs and Their Substitutes (1954, reprinted 1968), a comprehensive review of normal human limbs and their artificial replacements. Muscle types Striated Bernard Katz, Nerve, Muscle, and Synapse (1966), is an excellent brief work. Gerald H. Pollack, The Cross-Bridge Theory, Physiology Reviews, 63(3):10491113 (July 1983), is a good summary of minority views suggesting alternatives to the cross-bridge theory. See also Eric R. Kandel and James H. Schwartz, Principles of Neural Science, 2nd ed. (1985).Works on muscle contraction include J.R. Bendall, Muscles, Molecules and Movement: An Essay in the Contraction of Muscles (1969), with numerous references to original papers; Graham Hoyle, Comparative Physiology of the Nervous Control of Muscular Contraction (1957), a review of the literature for the general reader of biology; Andrew Huxley, Reflections on Muscle (1980), rich in history and in identifying unsolved problems; and John Squire, The Structural Basis of Muscular Contraction (1981). Several useful journal articles are found in Scientific American: Carolyn Cohen, The Protein Switch of Muscle Contraction, 223(3):3645 (November 1975); Graham Hoyle, How Is Muscle Turned On and Off? 222(4):8493 (April 1970); H.E. Huxley, The Mechanism of Muscular Contraction, 213(6):1827 (December 1965); Keith R. Porter and Clara Franzini-Armstrong, The Sarcoplasmic Reticulum, 212(3):7380 (March 1965); and David S. Smith, The Flight Muscles of Insects, 212(6):7688 (1965). Cardiac Diseases of the heart are discussed in Norman R. Alpert (ed.), Myocardial Hypertrophy and Failure (1983); Eugene Braunwald, John Ross, Jr., and Edmund H. Sonnenblick, Mechanisms of Contraction of the Normal and Failing Heart, 2nd ed. (1976), providing information on structure, metabolism of heart muscle, operation of the heart, and heart failure; and Harry A. Fozzard et al., The Heart and Cardiovascular System: Scientific Foundations, 2 vol. (1986). Smooth Studies of smooth muscle include David F. Bohr, Andrew P. Somlyo, and Harvey V. Sparks, Jr. (eds.), Vascular Smooth Muscle, vol. 2 in Handbook of Physiology: A Critical, Comprehensive Presentation of Physiological Knowledge and Concepts, section 2, The Cardiovascular System (1980); and Marion J. Siegman, Andrew Somlyo, and Newman L. Stephens (eds.), Regulation and Contraction of Smooth Muscle (1987).