Crustacean Neuromuscular Mechanisms

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Abstract
Properties of crustacean muscle fibers and neuromuscular synapses are discussed, with particular reference to the problems of fast and slow contraction, synaptic diversity, and peripheral inhibition. Electrical and mechanical responses of crustacean muscle fibers are variable, and govern to a large extent the muscle's performance. Fast and slow contractions are often mediated by distinct “phasic” and “tonic” muscle fibers, as in abdominal muscles, in which such fibers are segregated into two parallel sets of muscles. In leg muscles the fibers are often heterogeneous in properties and innervation. In doubly-motor-innervated muscles of crabs the axons producing fast and slow contractions preferentially innervate rapidly and slowly contracting fibers, respectively. Crustacean neuromuscular synapses vary greatly in electrical behavior and in ultrastructural characteristics. Some motor axons possess both facilitating and nonfacilitating synapses. The proportion of the different types of synapse associated with a motor axon probably determines in large measure the properties of the postsynaptic potentials evoked by that axon. Pre-synaptic and post-synaptic inhibition both occur, sometimes in the same muscle. The latter type is more common. Pre-synaptic inhibition is thought to be mediated by the action of an inhibitory transmitter-substance on receptors of the motor nerve terminals.

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