NEUROMUSCULAR TRANSMISSION IN INVERTEBRATES 



243 



"endplate potential' (e.p.p.') could be recorded fol- 

 lowing nerve stimulation. Further, they were able to 

 enter a given muscle fiber at a number of points along 

 several millimeters of its length and to measure the 

 amount of variation in the amplitudes of the e.p.p.'s. 

 This variation was comparatively small and the dura- 

 tion of the rising phase of the e.p.p.'s was practically 

 constant along the length of the fiber. These findings 

 should be compared with the situation in, for ex- 

 ample, frog twitch muscle fibers (19). The earlier 

 results (42) which indicated the presence of only lo- 

 calized e.p.p.'s were probably due to partial denerva- 

 tion of the fibers in dissecting the 'strip' preparations 

 that were used. 



Despite this clarification, major problems of crus- 

 tacean neuromuscular transmission remain. Little is 

 known about the relative effectiveness of the different 

 e.p.p.'s which can be set up in one muscle fiber, or of 

 the spike, in evoking contraction. The mechanism 

 whereby the nerve impulse leads to an e.p.p. is very 

 incompletely understood. The spike, too, has some 

 curious attributes which warrant further study. Each 

 of these problems will be considered in turn. 



ELECTROMECHANICAL COUPLING. This topic is perhaps 

 outside of the subject of neuromuscular transmission 

 in the usual sense of that phrase. In inany of the 

 experiments which have been performed, however, 

 both the electrical and mechanical responses to nerve 

 stimulation were recorded and some of the observa- 

 tions concerning the latter will be considered briefly. 

 It has been found by Wiersma & van Harreveld (83) 

 that in certain muscles (e.g. the claw-closer of 

 Blepharipoda occidentalism low-frequency stimulation (10 

 to 1 5 shocks per sec.) applied to the fast axon could 

 elicit large muscle potentials unaccompanied by any 

 visible contraction; while a stimulus of the same fre- 

 quency delivered to the slow axon evoked much 

 smaller muscle potentials which, nevertheless, set 

 up a contraction. Reasons were presented for believ- 

 ing that this seemingly paradoxical behavior could 

 occur within a single muscle fiber. Inasmuch, how- 

 ever, as the electrical recording was made with ex- 

 ternal electrodes and the contraction ol3ser\ed was 

 that of the whole muscle, it is not possiljle to conclude 

 definitely that this was the case. The unequivocal 



' There is some disadvantage in using the term e.p.p.' since 

 it suggests the presence of a particular anatomical structure, 

 which is probably absent in the crustaceans, and suggests a 

 functional similarity with the \ertebrate neuromuscular junc- 

 tion, the actual extent of which is unknown. 



demonstration of this phenomenon will probably 

 have to be made on single muscle fibers. The possible 

 presence of such a phenomenon is interesting because 

 it would seem to suggest that the transmitter has 

 some other effect aside from that which is manifested 

 in the change in muscle membrane potential. A re- 

 lated observation has been made by Kuffler (43) on 

 the muscles of the fast and slow stretch receptors of 

 the crayfish abdomen. This preparation has the ad- 

 vantage that the muscle bundle is very small and can 

 be isolated, with its nerve supply, and observed under 

 a microscope with transmitted light, and that at the 

 same time a fiber can be impaled with a micro- 

 electrode for intracellular recording of inembrane 

 potential. Ii the fast muscle bundle he observed that 

 if a nerve impulse evoked only an e.p.p. (which was 

 usually 10 to 25 mv in amplitude) no contraction was 

 visible. Only if the e.p.p. gave rise to a spike was there 

 visible contraction, and then a rapid twitch was the 

 result. In the slow muscle bundle, however, only 

 e.p.p.'s (5 to 15 mv) were observed and these were 

 accompanied by contraction. The failure of the fast 

 e.p.p.'s to bring about any visible muscle shortening 

 is puzzling in view of the fact that they are distributed 

 along the length of this muscle by numerous nerve 

 endings and attain a size which may be a considerable 

 fraction of the spike amplitude. Wiersma (80) has 

 also recently published observations made on lobster 

 closer-muscles which indicate that fast e.p.p.'s may 

 fail to bring about contraction, while spikes succeed 

 in doing so. 



THE TRANSMISSION PROCESS. It scems vcry likely that 

 the transfer of excitation across the crustacean neuro- 

 muscular junction is effected by a chemical trans- 

 mitter, rather than by the passive flow of the nerve 

 action current. While the total number of nerve 

 endings may be large (73), in any given section of 

 muscle fiber the ratio of areas of axon membrane to 

 muscle fiber meml^rane is probably always quite small. 

 Thus there is little current from the nerve terminals 

 available for charging the capacitance of the muscle 

 fiber, which is particularly large (approximately 40 /if 

 per cm-) in Crustacea (20). There are some analogies 

 with the vertebrate neuromuscular mechanism which 

 are only suggestive; during repetitive stimulation of 

 the nerve, random, often large, variations in the size 

 of the e.p.p. can be seen (22). These fluctuations 

 might be caused by intermittent failure of conduction 

 in some of the terminal nerve branches; but it is also 

 possible that they represent a quantal release mecha- 



