Receptor Potential of Dermal Photoreceptor Cells and Spike Responses in a Small Papillar Nerve in the Mantle of Onchidium verruculatum (Gastropoda, Mollusca)(Issue Commemorating the Directorship of Prof. Yoko HASHIMOTO, at the Department of Physiology)

Depolarizing responses to light of dermal photoreceptor (DP) cells which were scattered under the epidermis of Onchidium verruculatum were recorded by intracellular electrodes, and spike responses were recorded with suction electrodes from a small papillar nerve of a mantle which includes axons from the DP cells. When the DP cell was dark-adapted, the resting membrane potential was -40 to -60 mV, and a depolarizing response to light appeared. Amplitude of the depolarizing response increased with increasing light intensity. The depolarizing response was associated with an increase in membrane conductance. The depolarization was accompanied by a huge afterhyperpolarization which reached -100 mV and continued for 10～15 minutes. During the afterhyperpolarization, the amplitude of the depolarizing response reached 100 mV to strong light stimulation. Dynamic range of the receptor potential in dark adaptation reached -6.0 log units. The spectral sensitivity curve obtained from the threshold criteria of the receptor potential of DP cells appears to be that of a single Dartnall pigment having maximum absorption at about 520 nm. Two types of spikes elicited to light from a small papillar nerve of mantle were the sustaining-type and the delayed off-type. Both types showed the same spectral sensitivity to the DP cell. On the basis of electrophysiological findings it is suggested that visual information from DP cells is conducted via the small papillar nerve to the central nervous system