Physiological and morphological identification of two types of on-center bipolar cells in the carp retina

Abstract

Two types of on‐center bipolar cells, rod‐ and cone‐dominant bipolars have been identified in the dark‐adapted retina of the carp by means of intracellular recordings and Lucifer‐yellow dye injection. They differ physiologically and morphologically in the following respects: (1) responses of rod‐dominant cells to bright lights are characterized by a tansient depolarization followed by a smaller sustained depolarization, while those of cone‐dominant cells are approximately rectangular; (2) the cone‐dominant cells are about 1.5 log units less sensitive to light than the rod‐dominant cells; (3) the latency of the response is shorter in the cone‐dominant cells than in the rod‐dominant cells; (4) the mean diameters of the cone‐dominant cell receptive field (0.7 mm) and dendritic field (90 μm) are larger than those of the rod‐dominant cell receptive field (0.5 mm) and dendritic field (56 μm); (5) the mean diameter of the cone‐dominant cell soma (8 μm) is smaller than that of the rod‐dominant cell soma (13 μm); and (6) the terminations of the cone‐dominant cell axons form a ramification (67 μm mean diameter) in contrast to a big terminal swelling of the rod‐dominant cell axons (37 μm mean diameter). At least two ionic mechanisms are responsible for generating the depolarizing response of on‐center bipolar cells, one having a reversal at a positive potential and the other at a negative potential. Responses with a negative reversal potential only are obtained from some of cone‐dominant cells and responses with a positive reversal potential only are obtained from some other cone‐dominant cells and the rod‐dominant cells. There are a large number of bipolar cells that respond by both ionic mechanisms, although the ratio between them varied considerably in different cells.