NEUROMUSCULAR TRANSMISSION IN INVERTEBRATES 



241 



nerve fiber was only about i sec. and the terms 

 'slow' and 'fast' were therefore used to describe the 

 two contractions. These same terms were then ex- 

 tended to the concomitant muscle potentials and to 

 the axons themselves (e.g. 'slow a.xon'). The muscle 

 potentials showed even greater dififerences, the slow 

 potentials being very small but augmenting with 

 repetitive stimulation. Despite this increase (facilita- 

 tion) they remained smaller than a single fast muscle- 

 potential. In the crayfish closer muscle, which is per- 

 haps not typical in this respect, the fast potential 

 normally exhibited no facilitation. 



SIZE OF THE EFFERENT NERVE SUPPLY. One of the 



questions considered subsequently by these same au- 

 thors and their collaborators concerned the numbers 

 of excitatory and inhibitory axons supplying the 

 seven most distal limb mu.scles in different species of 

 decapod crustaceans. The latter included crayfish, 

 lobsters, rock lobsters, crabs, hermit crabs and various 

 other anomurans. The techniques involved isolation 

 and stimulation, separately, of as many of the axons 

 having an excitatory or inhibitory effect upon a par- 

 ticular inuscle as could be found, and an additional 

 estimate of the minimum number of efferent axons 

 using methylene-blue staining. All liinb muscles that 

 were examined received at least two axons and at most 

 five, and of the.se at least one was always an inhibitor. 

 The different groups of decapods showed diversity in 

 the numbers of inhibitory nerve fibers supplying a 

 particular muscle, but the numbers of excitatory 

 axons were constant from species to species. The fol- 

 lowing table summarizes the findings for motor deca- 

 pod axons only (74, 75, 76). 



Note that Hoffman's histological observation that 

 one axon supplied two muscles was confirmed by 

 physiological methods. In addition to the nerve fibers 

 listed in table i, each muscle receives at least one 

 inhibitory axon and in some species there are muscles 

 receiving two (78). Further, it is typical for an in- 

 hibitory axon to supply more than one muscle (up to 

 seven), and variations in the particular muscles sup- 

 plied by a common inhibitor were also found. Wiersma 

 & Ripley (85) have summarized the inhibitory in- 

 nervation of these muscles. 



POLYNEURON.'>iL INNERVATION. From the early histo- 

 logical work and the fact that inhibition can counter- 

 act excitation, it was apparent that motor and in- 

 hibitory axons both innervate the same muscle fibers. 

 Can a muscle fiber also receive more than one excita- 

 tory axon? There is now convincing evidence, both 



TABLE I. Anmlwr oj Excitatory Axons Supplying the Seven 

 Most Distal Limh Muscles of Decapod Crustaceans 



histological (73) and physiological, that this is indeed 

 possible. Although the observation is attended by 

 some difficulties, the fibers of the closer-muscle have 

 been seen to contract following stimulation of either 

 the fast or slow axon (72). Wiersma & van Harreveld 

 (84) showed that stimulation of one of the motor 

 axons to the closer-muscle augmented a contraction 

 evoked shortly afterwards by stimulation of the other 

 (heterofacilitation). No mutual influence with respect 

 to fatigue or facilitation of the muscle action poten- 

 tials, however, could be demonstrated. These results 

 suggested the presence of a pathway susceptible to 

 mutual influence .somewhere between muscle action 

 potential and contraction; and the conclusion was 

 drawn that at least .some of the contractile substance 

 was activated by both axons. More recently Fatt & 

 Katz (22) used intracellular electrodes to demon- 

 strate that muscle potentials were evoked in the 

 same fibers by both fast and slow axons, and it was 

 shown that the potentials could simimate. There is no 

 good quantitative determination of what percentage 

 of the fibers in a muscle receiving two excitatory 

 axons are innervated by both axons. 



van Harreveld & Wiersma (76) have al.so consid- 

 ered the question of polyneuronal innervation in a 

 muscle with four motor axons (and one inhibitor). 

 Here, too, they found that a test contraction set up by 

 stimulation of one of the four axons was slightly aug- 

 mented by previous stimulation of any of the other 

 three motor axons (heterofacilitation). Their experi- 

 ments did not provide direct evidence, however, con- 

 cerning the numbers of axons supplying each muscle 

 fiber. More recently, Furshpan (26) has studied the 

 same muscle (main flexor of the carpopodite in the 



