Convergence and divergence of cones onto bipolar cells in the central area of cat retina

Abstract

In the central area of cat retina the cone bipolar cells that innervate sublamina b of the inner plexiform layer comprise five types, four with narrow dendritic fields and one with a wide dendritic field. This was shown in the preceding paper (Cohen & Sterling 1990a) by reconstruction from electron micrographs of serial sections. Here we show by further analysis of the same material that the coverage factor (dendritic spread $\times $ cell density) is about one for each of the narrow-field types (b$_{1}$, b$_{2}$, and b$_{4}$). The same is probably true for the other narrow-field type (b$_{3}$), but this could not be proved because its dendrites were too fine to trace. The dendrites of types b$_{1}$, b$_{2}$, and b$_{4}$ collect from all the cone pedicles within their reach and do not bypass local pedicles in favour of more distant ones. The dendrites of type b$_{5}$, the wide-field cell, bypass many pedicles. On average 5.1 $\pm $ 1.0 pedicles converge on a b$_{1}$ bipolar cell; 6.0 $\pm $ 1.2 converge on a b$_{2}$ cell and 5.7 $\pm $ 1.5 converge on a b$_{4}$ cell. Divergence within a type is minimal: one pedicle contacts only 1.2 b$_{1}$ cells, 1.0 b$_{2}$ cells, and 1.0 b$_{4}$ cells. Divergence across types is broad: each pedicle apparently contacts all four types of the narrow-field bipolar cells that innervate sublamina b. Each pedicle probably also contacts an additional 4-5 types of narrow-field bipolar cell that innervate sublamina a. There are several possible advantages to encoding the cone signal into multiple, parallel, narrow-field pathways. These include: tuning of pathways to transmit different temporal frequencies, use of ion channels with widely separated equilibrium potentials (to increase gain), and formation of different regulatory circuits in the inner plexiform layer. The latter possibility would permit different operations (e.g linear or nonlinear) to be performed on the visual signal on its way towards different types of ganglion cell.