Observations On the Myo-Neural Physiology of A Polyclad Flatworm: Inhibitory Systems

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Abstract
1. The spontaneous activity displayed by Planocera preparations decreased with repetitive electrical stimulation. It was also found that the amplitude of response was related to the time since previous spontaneous activity. The response decreased as the time interval between spontaneous activity and stimulus decreased. 2. When a preparation was stimulated at a low frequency then there was an increase in the tone of the preparation. If the low-frequency stimulus was then followed by a stimulus at a slightly higher frequency then there was a drop in tone caused by a relaxation of the preparation. 3. The extent of relaxation which occurred depended on parameters or the high-frequency stimulus but could be facilitated by increasing either the frequency or the duration of the initial low-frequency stimulus. 4. Pathways involved with conduction of excitation from one side of the animal to the other pass through the brain. The brain is also required for transmission of the relaxation effect. 5. It is concluded that these relaxation and depressant effects reflect the presence of a true inhibitory system which also shows facilitation.

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