GAP-JUNCTIONS WITH AMACRINE CELLS PROVIDE A FEEDBACK PATHWAY FOR GANGLION-CELLS WITHIN THE RETINA

In primates, one type of retinal ganglion cell, the parasol cell, make s gap junctions with amacrine cells, the inhibitory, local circuit neu rons. To study the effects of these gap junctions, we developed a line ar, mathematical model of the retinal circuitry providing input to par asol cells. Electrophysiological studies have indicated that gap junct ions do not enlarge the receptive field centres of parasol cells, but our results suggest that they make other contributions to their light responses. According to our model, the coupled amacrine cells enhance the responses of parasol cells to luminance contrast by disinhibition. We also show how a mixed chemical and electrical synapse between two sets of amacrine cells presynaptic to the parasol cells might make the responses of parasol cells more transient and, therefore, more sensit ive to motion. Finally, we show how coupling via amacrine cells can sy nchronize the firing of parasol cells. An action potential in a model parasol cell can excite neighbouring parasol cells, but only when the coupled amacrine cells also fire action potentials. Passive conduction was ineffective due to low-pass temporal filtering. Inhibition from t he axons of the coupled amacrine cells also produced oscillations that might synchronize the firing of more distant ganglion cells.