NEUROMUSCULAR MECHANISMS 147 



primary role in the distribution of the excitation along the muscle fiber 

 and that the spike is not the indication of conduction. 



While it is thus quite possible that under normal conditions different 

 parts of one fiber react difi^erently to stimulation, it may be considered 

 certain that dift'erent muscle fibers of one muscle react rather independently. 

 In the closer muscles of crabs it was regularly observed that a single im- 

 pulse in the "fast" axon would give a large spike in part of the muscle fibers, 

 in others an abortive spike or large junctional potential, while in a different 

 part of the muscle only small junctional potentials would be present in the 

 fibers (Furshpan and Wiersma, 1954). Sampling in these cases has not 

 yet been extensive enough to make certain that there is a constant gradient 

 in this respect from the back to the front of the muscle, although this was 

 often observed. Fatt and Katz (1953b) also report large differences be- 

 tween the responses of muscle fibers of one muscle. 



Polyneuronal innervation is, according to the results of Furshpan 

 (1955), also variable from muscle fiber to muscle fiber in one muscle. 

 In many muscles with double motor innervation the indications are at 

 present that the great majority of fibers, perhaps all, do indeed receive 

 branches from both axons. But in the main flexor of the rock lobster, which 

 has four motor axons supplying it, it was found that, when single penetra- 

 tions of numerous muscle fibers were used, only a relatively small per- 

 centage (7%) of the muscle fibers responded definitely to all four motor 

 axons. Response to three axons was obtained in 29%, to two in 26%, and 

 to only one axon in 38% of the fibers. In the last case it is known that, of 

 these, 90% responded solely to stimulation of the particular axon, which 

 elicits the fastest contraction. Since none of the fibers tested failed to re- 

 spond to at least one motor axon, these figures may represent a fair approxi- 

 mation of the distribution. A further significant observation of Furshpan 

 is that it is possible to obtain spike potentials by combining the stimulation 

 of two axons, when each gives only a junctional potential by itself. This 

 proves that the junctional potentials caused by any of the motor axons 

 must be considered equivalent with regard to their relation to the mem- 

 brane changes. 



In insects a rather similar picture has been reported by Hoyle (1955 

 and this volume) for locust muscles. Here only certain of the muscle fibers 

 of a given muscle receive a polyneuronal motor innervation, the majority 

 being supplied by a single axon. 



A functional differentiation between parts of a crustacean muscle, which 

 must be due to unequal distribution of axon types or number of their end- 

 ings, has been reported (Wiersma and Ripley, 1954). It was observed 

 that, in contrast with the great majority of leg joints, one joint in the 

 walking legs of a hermit and of a dromid crab can rotate as well as bend at 



