| Abstract: | We investigated in Hydra attenuata the possibility of altering more or less permanently and in different environmental conditions, the frequency of Contraction Pulse Trains (CPT's) associated with the rhythmic spontaneous contraction activity, by repetitive light stimuli of variable duration, frequency and amplitude. The CPT's activity of various pieces of Hydra has been also investigated in indisturbed conditions and under stimulation. The following observations have been performed. 1. A transient effect, consisting of an increase or a decrease of CPT's frequency, occurs respectively after an abrupt decrease or increase of the light level. 2. If Hydra is stimulated by repetitive light pulses of 0.5-10 sec duration, at a frequency different from the CPT's average one, the CPT's frequency modifies; if the stimulation frequency is included in a range not too much up or below that of CPT's the new CPT's frequency equals exactly that of stimulation; close to this range the CPT's frequency is a multiple or submultiple of that of stimulation. 3. No habituation to such repetitive stimulation was found. 4. The phase relation between CPT's at the new frequency and light stimuli is a function of the difference between CPT's and stimulation frequencies. 5. Stimulation with repetition of light and darkness periods of some minutes duration induces activity only or mainly during darkness. 6. Modification of CPT's frequency by means of repetitive light stimulation of the type mentioned either in 2) or 5)] has been observed also with hypostomal preparations. 7. With cessation of the light stimulation, the new CPT's frequency of the whole animal lasts in darkness for a time (10-85 min) that is about 5-10 times longer than that necessary to obtain CPT's syncronization with stimulation. 8. The influence of the light intensity level on transient CPT's frequency variation (see 1), CPT's inhibition and stimulation, promptness of entrainment, range of entrainability, phase relation between entrained CPT's and stimuli, retention time of entrained rhythm has been examined, together with the influence of the reversal of polarity of light transitions on CPT's inhibition and entrainment. |
