Inhibition of Pacemaker Systems in the Hydroid Tubularia

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Abstract
Of the three conducting systems known in the stalk of Tubularia—the slow system (SS), the triggering system (TS), and the distal opener system (DOS)—the SS can be eliminated as being necessarily involved in the pacemaker system inhibition seen during continued stalk stimulation because its threshold is higher than the minimal stimulus strength required for inhibition. The TS can also be eliminated as the inhibitory pathway because (1) separating a polyp from a colony and hence from a source of continued input along the TS does not result in an increase in the spontaneous activity of the polyp above that of appropriate controls, and (2) distal tentacle stimulation which activates the DOS but not the TS is more inhibitory than is stalk stimulation which excites both the TS and the DOS. The inhibitory pathway is apparently the DOS or, less likely, a yet unidentified conducting system in the stalk. Inhibitory stimulation suppresses both single pulses and bursts in the NP pacemaker system and it is likely that the HP pacemaker system is also inhibited. The greater the background level of spontaneous activity the greater is the effectiveness of inhibitory stimulation. The period of inhibition lasts for a few seconds after each stimulus.