Inner S‐cone bipolar cells provide all of the central elements for S cones in macaque retina

Synaptic terminals of cones ( pedicles ) are presynaptic to numerous processes that arise from the dendrites of many types of bipolar cell. One kind of process, a central element , reaches deeply into invaginations of the cone pedicle just below an active zone associated with a synaptic ribbon. By reconstruction from serial electron micrographs, we show that L‐ and M‐cone pedicles in macaque fovea are presynaptic to ∼20 central elements that arise from two types of inner (invaginating) bipolar cell, midget and diffuse. In contrast, S‐cone pedicles, with more synaptic ribbons, active zones/ribbon, and central elements/active zone, are presynaptic to ∼33 central elements. Moreover, all of these arise from one type of bipolar cell, previously described by others, here termed an inner S‐cone bipolar cell . Each provides ∼16 central elements. Thirty‐three is twice 16; correspondingly, these bipolar cells are twice as numerous as S cones. (Specifically, each S cone is presynaptic to four inner S‐cone bipolar cells; in turn, each bipolar cell provides central elements to two S cones.) These bipolar cells are presynaptic to an equal number of small‐field bistratified ganglion cells, giving cell numbers in 2G:2B:1S ratios. Each ganglion cell receives input from two or more inner S‐cone bipolar cells and thereby collects signals from three or more S cones. This convergence, along with chromatic aberration of short‐wavelength light, suggests that S‐cone contributions to this ganglion cell's coextensive blue‐ON/yellow‐OFF receptive field are larger than opponent L/M‐cone contributions via outer diffuse bipolar cells and that opponent L/M‐cone signals are conveyed mainly by inner S‐cone bipolar cells. J. Comp. Neurol. 457:185–201, 2003. © 2003 Wiley‐Liss, Inc.