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 approximately 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 approximately 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 approximately 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.
Herr
Steve
S
Department of Psychology, Franz Hall, University of California, Los Angeles, Los Angeles, California 90095-1563, USA.
Klug
Karl
K
Sterling
Peter
P
Schein
Stan
S
D000818:Animals; D003712:Dendrites / Q000648:ultrastructure; D007091:Image Processing, Computer-Assisted; D008252:Macaca fascicularis / Q000502:physiology*; D008297:Male; D008854:Microscopy, Electron; D012160:Retina / Q000033:anatomy & histology*; D017949:Retinal Cone Photoreceptor Cells / Q000166:cytology* / Q000648:ultrastructure; D013569:Synapses / Q000648:ultrastructure