Inhibition in the Slow Muscle of the Scallop,Pecten circularis aequisulcatus Carpenter

Abstract

The adductor muscle of Pecten consists of two parts, the larger of which is responsible for the rapid movements during swimming, and the smaller, which contracts slowly, is responsible for keeping the valves closed. The peculiarities of this muscle have been studied repeatedly. 1 , 2 , 3 A satisfactory answer has not been given to the question of how the slow muscle can withstand rather large tensions for indefinite periods without appreciable fatigue and at the same time be able to relax very quickly when necessary.

Reflex Relaxation. In Pecten stimulation of the mouth in a normal animal leads to rapid relaxation of the slow muscle. In our experience the sensitive area for this reflex is much wider than hitherto reported, and includes all parts in the vicinity of, and innervated by the cerebral ganglion. Stimulation of the branchial nerves also leads to relaxation, although they do not connect directly with the cerebral ganglion, but with the visceral ganglion. That they nevertheless find their reflex center in the cerebral ganglion is shown by the fact that after cutting both cerebro-visceral commissures their stimulation elicits no relaxation of the slow muscle. As long as either commissure is intact, the reflex relaxation takes place. We were not able to find in this species the reported difference in action of the two commissures. 1 , 2 As long as the nerve bands between the visceral ganglion and the slow muscle are intact, mechanical stimulation of these bands results in relaxation. This, too, is most likely reflex as this kind of stimulation becomes totally ineffective after cutting the bands. 2

Peripheral Inhibition. The relaxation following a contraction produced by direct stimulation of isolated slow muscle is much slower than the relaxation observed when preganglionic mechanisms are stimulated.