Computational model of the on-alpha ganglion cell receptive field based on bipolar cell circuitry.

The on-alpha ganglion cell in the area centralis of the cat retina receives approximately 450 synapses from type b1 cone bipolar cells. This bipolar type forms a closely spaced array (9 microns), which contributes from 1 to 7 synapses per b1 cell throughout the on-alpha dendritic field. Here we use a compartmental model of an on-alpha cell, based on a reconstruction from electron micrographs of serial sections, to compute the contribution of the b1 array to the on-alpha receptive field. The computation shows that, for a physiologic range of specific membrane resistance (9500-68,000 omega.cm2) and a linear synapse, inputs are equally effective at all points on the on-alpha dendritic tree. This implies that the electrotonic properties of the dendritic tree contribute very little to the domed shapes of the receptive field center and surround. Rather, these shapes arise from the domed distribution of synapses across the on-alpha dendritic field. Various sources of "jitter" in the anatomical circuit, such as variation in bipolar cell spacing and fluctuations in the number of synapses per bipolar cell, are smoothed by the overall circuit design. However, the computed center retains some minor asymmetries and lumps, due to anatomical jitter, as found in actual alpha-cell receptive fields.