Inner S-Cone Bipolar Cells Provide All of the Central Elements for S Cones in Macaque Retina STEVE HERR, 1 KARL KLUG, 2 PETER STERLING, 3 AND STAN SCHEIN 1,2 * 1 Department of Psychology, Franz Hall, University of California, Los Angeles, Los Angeles, California 90095-1563 2 Brain Research Institute, University of California, Los Angeles, Los Angeles, California 90095-1761 3 Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania 19104 ABSTRACT 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 pre- synaptic 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. Indexing terms: photoreceptors (vertebrate); presynaptic terminals; synapses; synaptic membranes; color perception; retinal ganglion cells The synaptic terminals (pedicles) of all three cone types (L, M, and S) in the fovea of macaque monkey contain 20 synaptic ribbons (Esfahani et al., 1993; Calkins et al., 1996). In cross-sectional view, such as that shown in Fig- ure 1A, each ribbon appears to be presynaptic to a triad of invaginating elements (Sjo ¨strand, 1958; Missotten, 1962, 1965): two lateral elements provided by horizontal cell dendrites (Stell, 1967) and one central element provided by an invaginating bipolar cell dendrite (Dowling and Boycott, 1966; Boycott and Dowling, 1969). [In fact, sev- eral central elements may line up in single file beneath a ribbon (Chun et al., 1996; Migdale et al., 2003; see also below).] Invaginating bipolar cells have their syn- Grant sponsor: National Institute of Mental Health; Grant number: MH15795-18; Grant sponsor: National Institute of Health; Grant number: EY11153; Grant number: EY08124. *Correspondence to: Stan Schein, Department of Psychology, Franz Hall, Mail Code 951563, University of California, Los Angeles, Los Ange- les, CA 90095-1563. E-mail: schein@ucla.edu Received 6 June 2002; Revised 24 September 2002; Accepted 31 October 2002 DOI 10.1002/cne.10553 Published online the week of January 20, 2003 in Wiley InterScience (www.interscience.wiley.com). THE JOURNAL OF COMPARATIVE NEUROLOGY 457:185–201 (2003) © 2003 WILEY-LISS, INC.