CHAPTER VIII 



Neuromuscular transmission in invertebrates 



E. J. FURS H PAN | Biophysics Department, University College, London, England 



CHAPTER CONTENTS 



Arthropods 

 Crustaceans 



Size of the efferent nerve supply 

 Polyneuronal innervation 

 Multiterminal innervation 

 Electromechanical coupling 

 The transmission process 

 The muscle spike 

 Peripheral inhibition 

 Insects 

 Molluscs 

 Coelenterates 

 Actinozoans 

 Scyphozoans 

 The Mechanism of Transmission in Coelenterates 



TO PRODUCE THE MOST EFFICIENT CONTRACTION, a 



muscle fiber must be activated along its entire length 

 almost simultaneously; otherwise, the contracting 

 portions of the fiber must lengthen inactive regions 

 before communicating their tension to the tendon. 

 That is, the active parts of the fiber will operate in a 

 less effective range of the length-tension curve. At 

 least two mechanisms are known which can achieve 

 this relatively synchronous excitation of the fiber: 

 a) a comparatively rapidly conducting muscle action 

 potential, and b) numerous motor ner\e endings along 

 the length of the muscle fiber. The term 'multi- 

 terminal innervation' (6i) will be used to describe this 

 second situation. The first device (conducted muscle 

 action potential) is most commonly found in the 

 skeletal muscle of vertebrates. The most notable 

 exception is the case of the slow muscle fibers of the 

 frog [see Kuffler & Vaughan Williams (45)] in 

 which there is multiterminal innervation and a lack 

 of a conducted muscle spike. It should be pointed 



out that vertebrate twitch muscle fibers which exhibit 

 such a spike may have more than one motor end- 

 plate (38). These cases should probably not be in- 

 cluded within the definition of multiterminal inner- 

 vation, however, for in the absence of the propagated 

 muscle spike, the density of the nerve endings is not 

 sufficient to allow an appreciable contraction. 



The second mechanism (many nerve endings) seems 

 to predominate in the somatic musculature of the 

 invertebrates. An examination of the evidence for 

 this type of innervation will serve as one of the 

 themes of this chapter. It will also be interesting to 

 consider what differences in function between the 

 two systems seem to follow from the dissimilarity in 

 the means of spreading the muscular excitation. A re- 

 lated question will also be examined — the other ana- 

 tomical specializations associated with multiterminal 

 innervation. One such feature commonly found is 

 polyneuronal innervation or the receipt by one muscle 

 fiber of more than one motor axon, and of particular 

 interest, of motor axons which elicit from the same 

 fiber responses of different strength and time course. 

 In most vertebrate muscles, by contrast, the conducted 

 action potential is all-or-nothing and produces a 

 stereotyped twitch. Thus different types of contrac- 

 tion evoked by different nerve fibers do not occur. 

 Peripheral inhibition, as found in certain inverte- 

 brate muscles, offers another example of polyneuronal 

 innervation and this topic will be considered as well. 

 Another question that will be discussed concerns the 

 number of motor neurons which innervate whole 

 mu.scles. Here, too, a contrast with vertebrate muscle 

 will be seen in many cases and again the dissimilarity 

 between the ways in which the excitation is spread 

 throughout the muscle fiber can be thought to under- 

 lie these differences. That is, in spite of the all-or- 

 nothing contractions, fine gradations of tension are 

 possible in most vertebrate twitch muscles because of 



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