NEUROMUSCULAR MECHANISMS* 



C. A. G. WiERSMA 

 California Institute of Technology 



It has become well established that the control of muscular contraction 

 by the nervous system in the typical vertebrate striated muscle is a spe- 

 cialized case, and that other types of control are present. From a com- 

 parative viewpoint this is a logical development. For the nervous control 

 of muscular action must be a secondarily evolved process, since the con- 

 tractile elements can be brought into action without nervous structures in 

 protozoa, and most likely also in sponges. Therefore, it is likely that the 

 original function of the nerves was regulation of myogenic contractions 

 and that only subsequently the nerves obtained more complete control. 

 It may well be that in, e.g., the smooth muscles of the digestive tract in all 

 phyla the regulatory function of the nervous system has been maintained. 

 However, for the body musculature, especially the parts involved in quick 

 withdrawal reactions, nervous activation became necessarv at an early 

 stage, whereas slower and more tonic muscles may have kept a greater 

 independence. But again, for quick reaction more than one method of acti- 

 vation is used. In the vertebrate striated muscle fiber it is the development 

 of a conducted muscle action potential which spreads the excitation from 

 the end plate to the rest of the fiber, but in many arthropods conduction 

 along branches of the nerve fibers is certainly mainly responsible for this 

 spread. It will be the task of future investigations to discover to what ex- 

 tent the possibilities roughly outlined above are adequate for the explana- 

 tion of the control of the muscles in the different phyla, how far as yet un- 

 known mechanisms have been realized, and also how far the different 

 muscles of one animal are under a similar type of control. 



Muscles with fast and with slow contraction speeds are widespread. It 

 is interesting to note that in protozoa contractile fibrils differing vastly in 

 this respect are present. The types of innervation of muscle fibers of one 

 muscle are not always similar, as has been demonstrated in the frog 

 muscles, many of which have two types of muscle libers each with its own 

 type of innervation. Kufffer (1955) has reviewed the work which estab- 

 lished this. The "slow" or tonic system shows, in contrast to the conven- 

 tional fast system, multiple nerve endings on the muscle fibers and an ab- 



* This review will be largely limited to those newer papers, which have appeared 

 since the author's previous reviews (Wiersma, 1952-53) which dealt partially with 

 the same topic. 



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