NEUROMUSCULAR TRANSMISSION IN INVERTEBRATES 



249 



59, 70). The difference would seem to lie in the fact 

 that on the one hand (Fletcher and others) the 

 synchronized response of a large part of the muscle 

 fibers was recorded immediately following the stimu- 

 lus; while on the other hand (Hoyle & Lowy) record- 

 ing, at much higher gain, was made from two asyn- 

 chronously active regions of the muscle some time 

 after cessation of the stimulus. (The potentials re- 

 corded by Fletcher were more prolonged than those 

 seen by other workers; but this might have been due to 

 his recording apparatus.) Nevertheless, the significant 

 point would seem to be that there is electrical activity 

 in the muscle during the prolonged tonic contractions; 

 and this was found to be true whether shortening 

 was brought about by d.c. stimulation or addition 

 of ACh. (The origin of these potentials, which persist 

 for so long after a stimulus, will be considered below.) 

 .Since these muscles do .seem, therefore, to require 

 periodic activation during tonic contractions, the 

 main support for the catch-mechanism' hypothesis is 

 removed. It will be recalled that Twarog (71) had 

 found that the muscle could repolarize during the 

 tonic contractions induced by ACh. Her recording 

 apparatus would not, however, have detected the 

 potentials observed by Lowy and others. It was al.so 

 found by these latter authors that when the muscle 

 relaxed, following subthreshold stimulation or the 

 application of 5-HT, the electrical activity ceased. 

 Further, it has been found by Abbott & Lowy (i) 

 that the heat production of the ABRM measured 

 during either an ACh-induced tonus or during a 

 tetanus (stimulating at 2 per sec.) is the same, al- 

 though the value obtained in both cases is very 

 small compared, for example, to that of frog muscle. 

 Thus, the most attractive interpretation of the ability 

 of molluscan muscles to maintain tensions for pro- 

 longed times would seem to be the one given by 

 Abbott & Lowy (2). It is based on the observation 

 that once the contractile elements of these muscles 

 shorten, they return to the rest length only very slowly, 

 and thus infrequent activation suffices for the main- 

 tenance of tension. During a d.c. -induced tonus this 

 repeated activation is being supplied by some means, 

 as evidenced by the recorded electrical activity. 

 What then is the source of these potentials, inasmuch 

 as they are present in muscles which are isolated from 

 the central nervous system for long times after the 

 cessation of a stimulus and often in the apparent 

 ab.sence of stimulation? To study this Bowden & Lowy 

 (9) have examined histologically the intramuscular 

 nerve supply of a number of lamellibranch muscles, 

 including the ABRM. Histochemical methods for the 



detection of the presence of cholinesterase revealed a 

 dense ple.xus of nerve fibers and structures which they 

 interpreted to be nerve cell bodies. These findings 

 provide a possible explanation of the 'spontaneous' 

 potentials and disclose an additional factor which 

 must be considered in interpreting experiments on 

 these muscles, namely the possible presence of 

 peripheral interneuronal synapses. For example, 

 Schmandt & Sleator (70) found that the large syn- 

 chronous muscle potentials which they observed in 

 the ABRM were conducted decrementally (at a rate 

 of about 20 cm per sec). One possible interpretation 

 of their results is that the muscle fibers show no 

 conducted response and that the apparent conduction 

 is carried out by synapsing intramuscular nerve 

 elements. The decrement could then arise from the 

 failure of transmission at some of these synapses. 



The finding of electrical activity during the d.c- 

 induced tonus also complicates the interpretation 

 of most of VVinton's results and diminishes the neces- 

 sity for an hypothesis of the type that he propo.sed. 

 Nevertheless it is still possible that the mu.scle can 

 relax at different rates depending upon the means 

 by which it was activated (possibly by different motor 

 nerves). 



COELENTERATES 



As experimental objects for the study of neuro 

 muscular transmission, the coelenterates present some 

 of the same difficulties as those found in the molluscs. 

 The motor axons are supplied by a net of synapsing 

 neurons. The muscle fibers are very fine, usually being 

 several to less than one micron in diameter when 

 extended. They are arranged in sheets or 'fields,' 

 although there are places where the arrangement is 

 more compact and discrete muscles can be distin- 

 guished. 



Aciinozoans 



Pantin was the first to have stimulated these 

 animals electrically rather than mechanically and 

 thus had some idea of the number of impulses set up 

 in the nerve net. Much of the work has been devoted 

 to the properties of this net, but there are a number of 

 phenomena which are pertinent here. Most of the 

 earlier work was done with the sea anemone Calliactis 

 parasitica, and one of the responses studied was the 

 contraction of the sphincter muscle (at the top of 

 the column) following stimuli applied to the side of 



