This pathway operates as a high gain circuit that facilitates the transmission of single-photon responses from rods. Finally, the ON and OFF cone bipolar cells relay signals to their respective ON and OFF retinal ganglion cells. In turn, AII amacrine cells split this signal via sign-conserving gap junctions with ON cone bipolar cells and inhibitory glycinergic synapses with OFF cone bipolar cells, or sometimes directly with OFF ganglion cells. In the canonical primary rod pathway, rods signal to rod bipolar cells which then synapse onto AII amacrine cells. Rod signals can reach retinal ganglion cells via several different pathways ( Bloomfield and Völgyi, 2009). Rods are specialized for high sensitivity in dark conditions whereas cones operate in daylight and support color vision. In part, this is accomplished by the use of two types of photoreceptors, rods, and cones, which operate in different intensity ranges. The mammalian retina can process signals over a vast range of intensities, approximately 10 log units, from starlight to sunlight. In contrast, our results do not support the presence of a tertiary rod ON pathway in the rabbit retina. The tertiary rod OFF pathway operates via direct contacts between rods and OFF cone bipolar cells. We conclude that there is some limited cone input to rod bipolar cells, but we could find no evidence for rod contacts with ON cone bipolar cells. Imaging the AII-coupled ON cone bipolar dendrites in this way showed that they contact cones exclusively. Dye-filling AII amacrine cells, combined with subtractive labeling, revealed most of the ON cone bipolar cells to which they were coupled, including the occasional blue cone bipolar cell, identified by its contacts with blue cones. We report that approximately half the rod bipolar cells receive one or two cone contacts. To image their rod and cone contacts, we have dye-filled individual rod bipolar cells in the rabbit retina. ![]() Physiological recordings suggested that rod bipolar cells receive input from cones, and ON cone bipolar cells can receive input from rods, in addition to the established pathways. Recently, it has been proposed that there is even more crossover between rod and cone pathways. However, there is an exception to this specific wiring in that rods also contact certain OFF cone bipolar cells, providing a tertiary rod pathway. Rod and cone pathways are segregated in the first stage of the retina: cones synapse with both ON- and OFF-cone bipolar cells while rods contact only rod bipolar cells.
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