Rod bipolar array in the cat retina: pattern of input from rods and GABA-accumulating amacrine cells

The potential and actual connections between rod and rod bipolar arrays in the area centralis of the cat retina were studied by electron microscopy of serial ultrathin sections. In the region studied there were about 378,000 rods/mm2 and 36,000-47,000 rod bipolars/mm2. The tangential spread of rod bipolar dendrites was 11.2 microns in diameter, and the "coverage factor" for the rod bipolar cell was 3.5-4.6. We estimate that about 37 rods potentially converge on a rod bipolar cell and that one rod potentially diverges to about four rod bipolar cells. The actual connections, however, are less than this by about half: 16-20 rods actually converge on a bipolar cell and one rod actually diverges to slightly less than two rod bipolar cells. The degree of convergence appears to reflect a compromise between the need to signal graded stimulus intensities (requiring wide convergence) and the need to maintain a good signal/noise ratio (requiring narrow convergence). Amacrine varicosities that provide reciprocal contact at the rod bipolar dyad were studied in serial electron microscopic autoradiograms following intraocular administration of 3H-GABA or 3H-glycine. More that 90% of the reciprocal amacrine processes accumulated GABA in a specific fashion. This information, in conjunction with Nelson's recordings from the rod bipolar and amacrine cells postsynaptic at the dyad (Nelson et al: Invest. Ophthalmol. 15:946-953, '76; Kolb and Nelson: Vision Res. 23:301-312, '83), suggests that feedback at the rod bipolar output might be positive.