Crustacean Neuromuscular Mechanisms: Functional Morphology of Nerve Terminals and the Mechanism of Facilitation

Abstract

Two aspects of crustacean neuromuscular physiology are discussed: (1) the ultrastructural identification of the excitatory and inhibitory nerve terminals, and (2) the characteristics of, and the possible mechanisms for, facilitation. The first problem was studied in crayfish opener muscle which has one excitatory and one inhibitory axon. One of the nerves was stimulated in the presence of DNP until synaptic transmission failed; the preparations were then fixed for electron microscopy. Whenever the excitatory nerve was stimulated, the terminals with round synaptic vesicles were depleted while nearby terminals with smaller elongate vesicles were normal. When the inhibitory nerve was stimulated, the converse was true. The possible reasons for the diversity in crustacean neuromuscular properties are discussed. Large EPSP's with a high quantal content ( m ), appear to be produced by terminals which are invaded by a propagated spike. Small EPSP's (small m ) appear to be produced by terminals which don't spike and which are depolarized by a decrementally conducted potential. There is an inverse relationship between m and the amount of facilitation. The physiological basis for facilitation is discussed; previous hypotheses are found wanting and a new one is proposed, that of slow depolarization.