An Electrophysiological Study of the Photo-Excitative Neurones of Onchidium Verruculatum in Situ

The distribution of the axons of the photo-excitative neurones in Onchidium verruculatum has been traced by intracellular stimulation of the soma and extracellular stimulation of the axon. They send axon branches mainly into the pleuro-parietal and abdominal nerves in both sides. In the whole-animal preparation, photo-excitative spikes could be recorded from neither the soma nor the nerves of inherently photo-excitative neurones during light stimulation. ‘On’ and ‘off’ spikes were initiated only immediately after the beginning and the cessation of illumination of a whole animal. ‘Off’ spikes originated from dorsal eyes and stalk eyes to which shadow stimuli were applied. Those spikes were not the direct response of photo-excitative neurones to light. The excitation of the inherently photo-excitative neurones in situ was suppressed by inhibitory inputs coming through the right and left pleuro-parietal nerves. Cutting one (or some) of the pleuro-parietal nerves was the only condition that diminished the inhibitory inputs to the photo-excitative neurones in the present work. Adequate electrical stimulation of the pleuro-parietal nerves inhibited spikes of photo-excitative neurones due to photo-excitator or spontaneous discharge.