NEUROMUSCULAR TRANSMISSION IN INVERTEBRATES 



247 



the contraction elicited by F, and Hoyle ascrilies to Si 

 the function of raising the membrane potential briefly 

 before a maximum efTort is required by the animal. 

 The unusual features of the Sl, and S^ responses 

 would certainly warrant further study of the mecha- 

 nism of their generation. Concerning the various 

 combinations of the responses which were found in a 

 single muscle fiber, F could apparently occur with 

 any of the others since that axon inner\ated all of the 

 fibers. Respon.ses S2 and Sia were seen together in one 

 fiber, but none was found which showed both Sn, and 

 So effects. 



Aside from this demonstration of poKneuronal in- 

 nervation, the use of intracellular electrodes has also 

 provided evidence for multiterminal innervation. 

 Working with the flexor of the tibia, del Castillo et al. 

 (15) found that cooling the muscle reduced the size of 

 the e.p.p. so that it did not give rise to a spike. They 

 then found that the height of the e.p.p. did not \ary 

 b>- more than 10 to 15 per cent when the recording 

 was made from different points along the muscle 

 fiber. The spikes recorded in the.se experiments over- 

 shot zero potential (i.e. the inside of the fiber became 

 relatively positive) in good preparations, but the 

 magnitude of the overshoot was always less than 20 

 mv (cf. 27). Information was also obtained concerning 

 the mechanism of neuromuscular transmission. It was 

 found that the amplitude of the e.p.p. was propor- 

 tional to the size of the resting potential as experi- 

 mentally altered by passing current with a second 

 microelectrode. This is the result which had previ- 

 ously been obtained from work with frog muscle (19) 

 where it was found that the transmitter released by 

 the nerve impulse seemed to act by causing an in- 

 crease in the conductance of the endplate membrane.' 

 There is no evidence that the presumed transmitter 

 in insects is acetylcholine. Roeder & Weiant (62) were 

 unable to affect neuromuscular transmission with 

 curarine in a dilution of lo^''. 



MOLLUSCS 



Much of the preceding information on the neuro- 

 muscular mechanism of arthropods was obtainable 

 because of several fortunate characteristics of those 



'Such evidence is not conclusive by itself, howe\er. It has 

 recently been shown by E. Furshpan & D. D Potter (J. Phy- 

 siol., in press) that even at an 'electrical' synapse the amplitude 

 of the postsynaptic response can vary with the level of mem- 

 brane potential. 



systems. The few large motor axons, which run in 

 comparatively long nerves, can often be dissected free 

 and stimulated separately, and the muscle fibers are 

 most often large and can be impaled with micro- 

 electrodes. The absence of these features from most 

 other invertebrates makes experiments with them 

 considerably more diflicult to perform. Thus, one 

 finds interesting phenomena which are difficult to 

 interpret because it is not known whether they reflect 

 properties of the nerves, muscles, neuromuscular 

 junctions or neural synap.ses. 



A good example of these difficulties is provided by 

 a number of studies on the anterior byssus retractor 

 muscle (ABRM) of Mytilus edulis. One of the more in- 

 teresting properties of this and mans- other lamelli- 

 branch muscles is the ability to maintain considerable 

 tension for very long periods of time. Some muscles 

 may remain contracted for more than ten days (46) 

 while durations of a few hours are common (4, 37). 

 Several explanations of this ability have been put for- 

 ward. On the one hand there is the idea of a molecular 

 'catch-mechanism' (71, 77). According to this hy- 

 pothesis, one set of nerves would bring about a change 

 in the contractile mechanism so that, following con- 

 traction, the mu.scle could reinain shortened without 

 expending additional energy. Another set of nerves 

 would bring about an active reversal of this state and 

 relax the muscle. On the other hand, an explanation 

 has been sought in terms of already known properties 

 of nerve-muscle systeins. According to the tetanus 

 hypothesis, the muscle would remain contracted 

 only as long as there were periodic depolarizations of 

 the muscle fiber membranes, relaxation ensuing at 

 the cessation of .such activity (12, 37, 46, 48). It has 

 also been pointed out by proponents of this hypothesis 

 that the passive tension decay in these muscles, follow- 

 ing activation, is very slow and that this factor would 

 contribute considerably to the economy of contrac- 

 tion (2). Molluscan muscle would then differ from 

 other muscles only in the slowness of its relaxation. 



Intermediate between the 'catch-mechanism' and 

 tetanus hypothesis is one in which the 'viscosity' (and 

 thus the rate of passive tension decay) of the muscle 

 would be variable, depending upon the way in which 

 the muscle had been activated. When prolonged con- 

 traction was required the muscle could be put into 

 the "high viscosity' state and then infrequent activa- 

 tion would suffice to maintain a tetanus (39, 87). This 

 idea was suggested by Winton (87) following a study 

 of the Mytilus ABRM. He found that the muscle 

 responded differently to alternating and to direct 

 current stimulation. Following the cessation of an 



