Inhibition and Rhythmic Activity of the Circular Muscles of Calliactis Parasitica (Couch)

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Abstract
1. The responses to electrical stimulation of isolated rings of the column and pedal disk of Calliactis are described. Such rings make slow spontaneous contractions which are frequently rhythmical, the interval between contractions normally being 7-20 min. 2. Continuous low-frequency stimulation inhibits spontaneous activity of rings from the pedal disk and also of fresh rings from more adoral regions of the column. Older rings from the mid-column respond to such stimulation by a tetanic contraction. 3. The latency of response to electrical stimulation of pedal rings is of the order of 120 sec. This latency is not affected by stimulation frequency but is prolonged by increase in the number of shocks applied. 4. Stimulation of a pedal ring at the onset of a contraction prevents the further development of this contraction, while stimulation as a contraction reaches its maximum is followed by more rapid relaxation than in unstimulated controls. 5. Mid-column rings when freshly prepared show a latency of the order of 120 sec. As the preparation ages, a double response to stimulation appears; the first response has a latency of about 30-40 sec. and presently becomes the only type of response shown. 6. If two sets of stimuli are applied to a mid-column ring, the magnitude of response to the second set increases as the time between stimulations increases. With long intervals an almost total contraction is obtained in response to a single shock. 7. The effect of intercalated stimuli upon the rhythm of spontaneous activity is studied. The effect is very variable and it is suggested that this is the result of electrical stimulation having both an excitatory and an inhibitory effect. 8. The very long latent periods characteristic of pedal rings and the rhythmic activity of these preparations are interpreted as interactions of excitation and inhibition.

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