Excitatory and Inhibitory Control of Inherent Contractions in the Sea Anemone Calliactis Parasitica

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Abstract
Bursts of nerve-net activity are always followed by a contraction cycle involving parietal and circular muscle contractions in isolated preparations of Calliactis parasitica. Both muscle groups can, however, also contract in the absence of nerve-net activity. These contractions, termed inherent, seem to follow periods of reduced activity in the endodermal slow conduction system (SS2). Electrical stimulation of the SS2 inhibits inherent contractions of parietal and circular muscle preparations. Electrical stimulation of the nerve net excites parietal muscles but seems to have both excitatory and inhibitory effects on circular muscles. A model for control of parietal and circular muscle contractions proposes that both the nerve net and the SS2 are responsible for directing the inherent muscular activity into the observed contraction cycle. It is suggested that when the action of these antagonistic muscles is strongly opposed the SS2 pulse frequency rises, resulting in inhibition of further muscular activity.

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