Modulation of Hydra attenuata rhythmic activity. IV. The mechanism responsible for rhythmic activity.

It has been hypothesized that the mechanism responsible for the rhythmic contraction-relaxation activity of Hydra attenuata is oscillatory. This nature is confirmed by the possibility of obtaining the suppression of overt behaviour by high frequency photic stimulation, which entrains the behaviour at a phase different from that giving rise to overt activity. Such a triggering mechanism should be structured as two different subsystems, mutually inhibiting. Also each one rebounds after the received inhibition in such a way that the active phase of each of them corresponds to the inhibited phase of the other one. This functional structure is argued from: 1. the existence of two triggering zones responsible for the contraction and relaxation phases; 2. the existence of a difference between the bioelectric potentials which reflect the activity of such triggering zones; 3. the opposite trend of these two bioelectric potentials at all phases in undisturbed conditions, and 4. the opposite direction of the variation of such bioelectric potentials as post-effects of either photic or electric stimulation, which follow the effects in the same direction undergone by the potentials themselves.