Neuromuscular physiology of the longitudinal muscle of the earthworm, Lumbricus terrestris. II. Patterns of innervation.

Abstract

ABSTRACT Patterns of innervation of the longitudinal muscle of the earthworm, Lumbricus terrestris, were examined electrophysiologically. The longitudinal musculature of a segment is innervated by relatively few axons, a fast and slow axon being present in segmental nerve I and in the double nerve, segmental nerve II–III. Single-pulse stimulation of the fast axon produces large external muscle potentials and small twitch-like contractions, which with repetitive stimulation are antifacilitating. Repetitive stimulation of the slow axon produces large, slowly developing and sustained mechanical responses, with electrical and mechanical responses showing summation and facilitation. The amplitude and time course of slow mechanical responses are related to the frequency of stimulation. Individual longitudinal muscle fibres are innervated by either the fast or slow axon in a segmental nerve, or by both fast and slow axons. No evidence was found for peripheral inhibitory innervation of the longitudinal muscle.