NEUROMUSCULAR MECHANISMS 145 



flight muscle of higher insects are treated elsewhere in this volume, and 

 except for one reference will not be considered. 



In decapod Crustacea the use of internal electrodes has given some re- 

 markable results. Studying the electrical properties of the muscle mem- 

 branes as such, Fatt and Katz (1953a) have shown that they differ 

 markedly in several respects from those of vertebrate muscle fiber. These 

 properties of the membrane may well have a considerable influence on its 

 functional relations to contraction and inhibition. But, since at present 

 these relations are not clear, the results of their studies will not be dis- 

 cussed further. 



With the use of internal electrodes, Fatt and Katz ( 1953b) and Fursh- 

 pan and Wiersma (1954) were able to show that an impaled muscle fiber 

 of a doubly motor-innervated muscle would react on stimulation of either 

 axon. The potentials obtained depend on the axon stimulated; in "slow- 

 axon" stimulation considerable facilitation may be needed before the de- 

 polarization is evident, while single impulses in the "fast" axon usually 

 give rise to a clearly observable deflection. These results were obtained 

 with many fibers of a wide array of muscles and represent undoubtedly the 

 normal efTect. But the experiments do not necessarily prove that all muscle 

 fibers of a doubly motor-innervated muscle receive innervation from both 

 axons ( see below) . Fatt and Katz ( 1953b) have studied the distribution of 

 the potentials along the fiber, impaling one at a number of loci, and have 

 very often found insignificant dift'erences in amplitude and time relations. 

 These results are thus in sharp contrast to those obtained with the focal 

 end-plate potential of mammalian muscle fibers, and make it certain that a 

 decapod crustacean muscle fiber normally receives its excitation at the many 

 nerve endings which each axon has along the length of the fiber. 



Since there exi'sts a good deal of variation in type of innervation and 

 effects of stimulation in difiierent muscles, a new terminology, with some 

 terms already used for other preparations, has been proposed by Furshpan 

 (1955). With a slight variation in definition these terms will be presented 

 here. Instead of multiple innervation, an older term which covered at the 

 same time the fact that many nerve endings are present and that more 

 than one axon makes connection with a muscle fiber, the term multiterminal 

 innervation will be used to describe the fact that one axon has a consid- 

 erable number of endings on a muscle fiber. To indicate that more than one 

 motor fiber innervates a muscle, the term polyneiironal motor innervation 

 will be used (subdivisions like dineuronal motor innervation, etc. can be 

 derived from this). But every decapod crustacean muscle fiber may have 

 at least a dineuronal innervation, since in all known instances there is at 

 least one inhibitor, and sometimes two, present in addition to the motor 

 fiber or fibers (Wiersma, 1941 ). The types of potentials which can be ob- 



