154 Muscles /8 : 6 



coordinated manner. The details of the coupling, from the free energy 

 released by splitting ATP to the mechanical energy expended by the 

 muscle, are all unknown. 



6. Summary 



Muscles are the contractile elements of animals. They act as trans- 

 ducers converting chemical energy into mechanical energy. Muscles in 

 vertebrates can be classified according to function and to morphology. 

 Of the various types, the striated muscles, usually associated with 

 voluntary motion, have been studied in greatest detail. Their efficiency, 

 the tensions developed at constant length, and the shortening produced 

 with various loads have all been measured and are well known for many 

 different muscles. 



Each striated muscle consists of bundles of small groups of individual 

 muscle fibers. These fibers make up the muscle. The single, striated 

 muscle fiber, about 10 /x, in diameter, is surrounded by a single mem- 

 brane electrically polarized in a fashion similar to that of a nerve fiber. 

 The initial step in the contraction process is an action or spike potential, 

 very similar to that of nerve fibers. This spike potential is normally 

 initiated at the muscle end plate but can also be produced by the same 

 types of stimuli which affect nerve fibers. 



Within the striated muscle fiber are many nuclei, mitochondria, 

 microsomes, and so forth, as well as long myofibrils having the same 

 striations as the muscle fiber. The myofibrils contain two types of 

 filaments which in turn are composed of helical fibers of the proteins 

 myosin, actin, and tropomyosin. The two types of filaments appear to 

 overlap in electron micrographs of extended muscles; they intermesh 

 more completely in similar electron micrographs of contracted muscles. 

 The changes during contraction are brought about at the expense of 

 chemical energy stored as ATP. 



The energy of ATP is released when the latter is split into its com- 

 ponents, ADP and phosphate. This splitting is catalyzed by enzymes 

 called ATP-ases. The protein, myosin, is an ATP-ase, but it may not 

 be active in this fashion in intact myofibrils. The molecular details of 

 how the energy is transferred from ATP to mechanical contractions are 

 not known. The details are not clear on the behavior of the protein 

 filaments within the myofibril as contraction is occurring. The con- 

 centration of ATP is "buffered" by the creatine-creatine phosphate 

 system. The net loss of organic phosphate (that is, ATP and creatine 

 phosphate) is restored by the oxidation of glucose. Oxidations in 

 muscles follow the same pathways as in other tissues. 



