Cellular Neuronal Control of Molluscan Heart

Abstract

Bivalve and gastropod molluscs undergo large changes in external environmental conditions, as well as in internal state. Cardiac responses to these changing conditions have been recorded in a variety of species. There is a general tendency for heart rate, and presumably cardiac output, to increase in response to situations that would increase the load on respiratory and excretory systems. Changes in molluscan heart function in many cases appear not to be mediated directly by cardiac nerves, but rather by such indirect mechanisms as changes in blood constituents or mechanical, hemodynamic effects on heart muscle. Three types of cardiac response in Aplysia have been shown to be mediated, at least in part, by the heart regulator nerves. The neural circuits that regulate heart rate in Aplysia and in Helix have been partially described in cellular detail. In Mercenaria, Aplysia and other molluscan species there is evidence that cyclic adenosine monophosphate has a role in mediating the excitatory effects of serotonin on heart muscle. There appears to be, in fact, a general tendency in the Aplysia nervous system for neurons that exert tonic, modulatory effects within neural networks that control a variety of behaviors to use serotonin for a transmitter. In each case there is evidence to suggest that changes in cyclic adenosine monophosphate levels may mediate some of the modulatory effects of serotonin.