The time Course of Pacemaker Inhibition in the Hydroid Tubularia

Abstract

ABSTRACT Stimulating the DOS, a conducting system in Tubularia, inhibits spontaneous firing of the principal pacemaker systems of the polyp, the NP and HP systems. The overall inhibition is greater the greater the stimulus frequency and there is rebound from inhibition at the end of the stimulation period. The latent period for NP inhibition is 0·4–0·6 sec and the duration of inhibition is 4–5 sec. The time course and extent of NP inhibition is independent of stimulus frequency except at high frequency where there is diminishing effectiveness of inhibition with time. HP inhibition has a similarly long latent period and even longer duration, lasting 15–20 sec. HP inhibition from successive stimuli can sum to produce a more intense effect. The behavioural significance of the inhibition seems to be to reduce spontaneous polyp movements during food capture and ingestion.