Insect visceral muscle. Electrical potentials and contraction in fibres of the cockroach proctodeum

Abstract

Bioelectric potentials were recorded intracellularly from the striated, visceral fibres of the superior longitudinal muscles of the cockroach proctodeum. Maximally evoked post-synaptic potentials (PSP's) consisted of one to four component PSP's in a smooth or humped configuration, consistent with the polyaxonal innervation of these fibres. Action potentials were triggered by PSP's evoked by central motor activity and, spontaneously, by a fluctuating membrane potential of critical amplitude. Action potentials were of the plateau type with durations of 0·3 to 1 sec. Regardless of their genesis, action potentials were followed by slow contraction of the whole muscle bundle. Low-frequency motor input induced repetitive action potentials and some summation of these contractions. High-frequency motor activity evoked an action potential and a large summated PSP; the resulting contraction was considerably larger than that induced by a single action potential. These visceral fibres appear to be myogenic with central nervous system control.