Microcircuitry of the dark-adapted cat retina: functional architecture of the rod-cone network.

The structure of the rod-cone network in the area centralis of cat retina was studied by reconstruction from serial electron micrographs. About 48 rods converge on each cone via gap junctions between the rod spherules and the basal processes of the cone pedicle. One rod diverges to 2.4 cones through these gap junctions, and each cone connects to 8 other cones, also through gap junctions. A static cable model of this network showed that at mesopic intensities, when all rods converging on a cone pedicle are continuously active, the collective rod signal would be efficiently conveyed to the pedicle. At scotopic intensities sufficiently low for only one of the converging rods to receive a single photon within its integration time, the quantal rod signal would be poorly transmitted to the cone pedicle. This is because the tiny signal would be dissipated by the large network into which the individual rod diverges. Under this condition, the rod signal would also be poorly conveyed to the rod spherule. If, however, the rods are electrically disconnected from the network, the quantal signal would be efficiently conveyed to the rod spherule. This analysis suggests that the rod signal is conveyed at mesopic intensities by the cone bipolar pathway and, at scotopic intensities, by the rod bipolar pathway, in accordance with the results of Nelson (1977, 1982; Nelson and Kolb, 1985).