Lateral spread of light-induced response at the cell body and axon terminal levels of external horizontal cells in the carp retina.

One type of light-induced response (photopic L-type S-potential)recorded from isolated carp (Cyprinus carpio) retinas was identified by its spectral response and later confirmed by morphological localization of the recorded sites ionophoretically marked by a fluorescent dye, Lucifer yellow. Such L-type S-potentials could be recorded from the soma and from the axon terminal of external horizontal cells. The spatial property of the S-potentials from the soma was compared with that recorded from the axon terminal by enlarging the diameter of a light spot (0.25 to 4.0 mm) and by displacing the spot (0.5 mm dia.) along a straight 4-mm line which passed over the recording point at the middle. The half-decay distance of decremental amplitude with spot displacement was significantly (P less than 0.001) shorter in recordings from the soma than from the axon terminal, indicating that the spatial summation is less in the soma than in the axon terminal. The spatial summation was found to be for the two parts of the cell. Therefore, the soma and axon terminal appear to function as a single unit with respect to spectral information, but as two separate units with respect to spatial information. Electrical and dye couplings are assumed to take place at two different (cell body and axon terminal) levels, possible being separated by the high resistance of the slender axon in the carp retina.