NEURO‐MUSCULAR TRANSMISSION IN INVERTEBRATES

Summary 1. This article deals principally with recent experimental work on crustacean nerve‐muscle systems. A number of observations have been made which are likely to affect our views of synaptic function in vertebrates. Neuro‐muscular transmission in Crustacea, and possibly other invertebrate animals, involves three factors which in vertebrates are regarded as characteristic properties of reflex centres: convergence of excitatory impulses from different nerve fibres on to one effector cell, prolonged facilitation of motor impulses, and interplay of excitatory and inhibitory impulses. It can now be regarded as established that different nerve fibres supplying the same muscle fibres are responsible for inhibition of contraction, and for fast and slow types of facilitation (Wiersma, 1941). 2. Many invertebrate motor systems are distinguished by the extremely small number of nerve axons which supply large and powerful muscles. The question arises how the speed and strength of such muscles is regulated. In vertebrates regulation is brought about by a play of numerous motor units, involving within each unit a ‘non‐stop’ transmission from the motor nerve cell to the ends of the muscle fibres. This mechanism would not permit fine gradation in muscles which are only provided with one or two motor axons. Two types of invertebrate motor systems have been described: ( a ) Certain specialized muscles, e.g. the ‘jet‐propelling’ mantle muscle of cephalopods, which act as units, and, like the vertebrate heart or the electric organ, invariably give a maximum response. ( b ) Many other muscles, e.g. in all crustacean limbs, which are capable of extremely fine gradation, despite their sparse nerve supply. In these latter muscles, the motor unit response has been broken down into much smaller ‘quanta’ by means of a barrier system and of facilitation at the nerve‐muscle junctions. 3. The nature of neuro‐muscular facilitation in arthropods is discussed. There are certain analogies with partly curarized vertebrate muscle, in the existence of junctional barriers in both cases, which can be overcome by the summated action of several successive nerve impulses. There are, however, some important differences. In curarized vertebrate muscle, facilitation involves the recruitment of an increasing number of muscle fibres, each of which contributes a propagated maximum response. In normal crustacean muscle, facilitation can occur in the absence of any propagated muscle impulses and is then due to a progressive growth of local electrical and mechanical responses in the vicinity of the nerve endings. Electric recording reveals the existence of ‘end‐plate potentials’ (e.p.p.'s) which increase in size with each successive motor nerve impulse, and, at high rates of stimulation, summate to a plateau several times higher than their initial amplitude. These non‐propagated action potentials are accompanied by a local contraction whose rate and strength can be controlled continuously by the number and frequency of the motor impulses. 4. In addition to these graded local responses, crustacean muscle fibres can be thrown into propagated activity, both by direct and by nerve stimulation. If the frequency of the motor nerve impulses is raised, e.p.p.'s summate and at a certain level propagated spike potentials are initiated, associated with vigorous twitches of the whole muscle fibres. Thus, the motor response in Crustacea is either local or propagated, depending upon the rate of the nerve impulses. Both kinds of response have been observed in the excised limb as well as in situ. 5. There are enormous differences in the rate and power of facilitation in different muscles. Fast and slow systems have been distinguished, according as facilitation of e.p.p.'s takes several milliseconds or about one second to complete. Many muscle fibres receive branches of two motor axons, one providing a fast, the other a slow facilitation system. Thus, the rate and intensity of a muscle fibre response can be regulated, not only by the number and frequency of nerve impukes, but also by a ‘switching’ of axons. 6. If the inhibitory nerve fibre is stimulated in the absence of motor activity, no electrical or mechanical change can be detected in the muscle. The inhibitory impulse is capable of breaking the transmission of motor activity at two separate stages: (a) between the motor nerve impulse and the production of the e.p.p. (a‐action), and (V) between the e.p.p. and the local contraction of the muscle (j3‐action). The two effects can be dissociated by varying the time interval between motor and inhibitory impulses. If the inhibitory impulse precedes the motor by a few milliseconds, it interferes with the production of the e.p.p. The inhibitory impulse can be made to arrive too late to affect the e.p.p. and yet in time to prevent local contraction. In both cases, the inhibitory influence is restricted to the vicinity of the motor nerve endings. Once a propagated impulse has been initiated in the muscle fibre, it cannot be stopped by an inhibitory impulse, and a twitch invariably occurs. 7. Certain gaps and controversial matters in our present evidence are discussed. (a) The relative functional importance of local and propagated responses in invertebrate muscle remains to be cleared up. (b) The relation between structure and function of the various types of nerve‐muscle contacts in Crustacea, and their distribution within individual muscle fibres, require much further investigation, (c) In Crustacea as well as in many other invertebrate animals there is as yet no information about the nature of the processes, or chemical agents, involved in the facilitation and production of the ‘end‐plate potential’ and the two inhibitory mechanisms.