MUSCLE 



(Readings: Weisz, pp. 356-362; 446-457. Villee, pp. 48-50; 344-351. H. E. 

 Huxley, "The Contraction of Muscle," Sci. Am. 199, No. 5, 66-82, Nov. 1958, 

 Reprint No. 19. C. J. Wiggers, "The Heart," Sci. Am. 196, No. 5, 74-87, May 

 1957, Reprint No. 62.) 



The ability to move rapidly is one of the major 

 characteristics of animal life. In all but the 

 lowest animals, such motions are accomplished 

 by muscles. Throughout the animal kingdom, 

 muscle tissue is built upon a common plan, and 

 is remarkably uniform. 



During the past few years we have begun to 

 learn how muscle works. Upon excitation of a 

 muscle, through a nerve or by the same artificial 

 devices one uses to excite nerves, a wave of de- 

 polarization much like the nerve impulse passes 

 down the muscle membrane. Somehow this 

 releases ATP, which reacts with the proteins, 

 actin and myosin, of which the muscle fibrils are 

 mainly composed, causing the muscle to con- 

 tract. In this process, ATP is broken down to 

 ADP, yielding with the release of its high-energy 

 phosphate bond the chemical energy that is con- 

 verted in the muscle contraction to mechanical 

 work. Actin and myosin are long, fiber proteins, 

 arranged in alternate, overlapping sequences 

 along the muscle fiber. During contraction, the 

 actin and myosin filaments slide over one an- 

 other so that they overlap further, causing 

 shortening (see Huxley). 



How ATP causes these changes is not known; 

 nor do we know how the depolarization of the 



muscle cell membrane excites this reaction. We 

 do know, however, that the wave of excitation 

 that passes over a muscle fiber on stimulation is 

 very much like the nerve impulse. 



Three types of muscle tissue are found in 

 vertebrates: striated, cardiac, and smooth mus- 

 cle. The rapidly contracting "voluntary" mus- 

 cles of our arms, legs, and trunk are striated. 

 The cardiac muscle that forms the wall of the 

 heart, is also striated, but is otherwise inter- 

 mediate in structure and speed of contraction. 

 Smooth muscle, found in the gut and blood 

 vessels, undergoes slow, sustained contractions, 

 as for example the slow peristaltic motions of the 

 intestine. All three types of muscle, though 

 histologically and functionally distinct, owe their 

 contractility to actin and myosin. 



Since they do contain the same contractile pro- 

 teins, one might wonder why these different 

 types of muscle possess such different properties. 

 One reason is large differences in structural 

 arrangement, apparent in part under the micro- 

 scope, and persisting down to the molecular 

 level. Another factor is that the cell membranes 

 of the various types of muscle, as also of nerve, 

 have very different excitatory characteristics. 

 So, for example, the larger frog nerves conduct 



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