NERVOUS CONTROL OF INSECT MUSCLES* 



Graham Hoyle 



University of Glasgow 



Detailed studies of the nervous control of muscles have heen made 

 principally on vertebrates and crustaceans. They have been concerned 

 particularly with the elucidation of the mechanisms of neuromuscular trans- 

 mission and have left largely unsolved many of the more general problems, 

 such as the method of maintaining tone and the way antagonist muscles 

 are used (see Elftman, 1941). Ideally it should be possible to give a com- 

 plete analysis of an integrated movement in terms of all the events in- 

 volved (for both the agonists and antagonists concerned) , i.e., motor nerve 

 impulses, transmission processes, muscle fiber contractions, activation of 

 proprioceptors, sensory nerve impulses, and central nervous integrative 

 processes, all stated quantitatively. Perhaps the greatest theoretical interest 

 lies in the central processes, and it was ably demonstrated by Sherrington 

 ( 1906) half a century ago that the experimentally accessible neuromuscu- 

 lar apparatus can be used as a window to the functioning of the central 

 nervous system. An extension of these studies to other classes of animals 

 may be justified on the grounds of their intrinsic interest and also because 

 some of them may provide better experimental material for analysis of 

 some of the general problems. 



It seems likely that insects, which have been little studied in regard to 

 their neuromuscular phenomena, ofi^er excellent material. They certainly 

 ofifer some challenging problems. In all insects there are functionally im- 

 portant muscles which are microscopically small, sometimes composed of 

 no more than a dozen muscle fibers. Yet the joints operated by those 

 muscles are moved with the precision which characterizes most insect 

 movements, and the delicacy of action compares favorably with that en- 

 countered in the highest vertebrates, in which each muscle is composed of 

 thousands of muscle fibers and is innervated by hundreds of nerve fibers 

 operated by a central nervous system of immense complexity. The small 

 size of insect limbs and the simplicity of both muscles and innervation 

 ofifer peculiar advantages for a complete study of the subtler aspects of 

 nervous control as well as the special problems of their neuromuscular 

 transmission. 



* I wish to thank Dr. T. D. M. Roberts for his helpful criticism of the first draft of 

 this paper. The electrical apparatus used in making the original observations reported 

 here was purchased with the aid of an award from the Grant-in-Aid Fund of the Royal 

 Society. 



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