An Attempt to Account for the Diversity of Crustacean Muscles

Abstract

synopsis. Crustacean muscles are known to contain muscle fibers of variable properties and to be innervated by phasic and/or tonic motoneurons which may possess synapses of diverse physiological properties. Frequently, phasic motor axons innervate shortsarcomere phasic muscle fibers and tonic motor axons innervate long-sarcomere tonic muscle fibers, but some muscles receiving a single (tonic) motor axon contain both phasic and tonic muscle fibers. Although it is not known whether neural trophic influences are involved in muscle differentiation, some neural trophic effects have been found in crustaceans, and it is reasonable to assume that such influences may be in? volved in establishing the definitive properties of the muscle. Several other postulates must be made: (1) Phasic and tonic motor axons differ in their trophic effectiveness: (2) muscle fibers innervated relatively early in development by a tonic motor axon acquire the properties of tonic muscle fibers, while those innervated later become intermediate or phasic muscle fibers; (3) the developmental stage of a growing or re? generating axon terminal plays a role in determination of synaptic properties. Studies on regenerating limb buds support the hypothesis, which can account for the genesis of all observed types of crustacean neuromuscular system. Further experimental work is necessary to test the hypothesis. Crustacean muscles come in a wide variety of different types and show amazing functional adaptations. To date, no one has ventured an explanation of how crustacean muscles came to be the way they are. True, there is little concrete data on the development of crustacean muscles from which to draw. In spite of this, it may be worthwhile (although somewhat rash) to attempt the formulation of an hy? pothesis on crustacean neuromuscular dif? ferentiation at this stage, if only to stimulate further thought about the problem. The lack of a well-defined rationale may be one reason for the fact that compara? tive studies of crustacean neuromuscular