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Stan Schein; Neuroanatomy of S cone ON/OFF pathways. Journal of Vision 2005;5(12):16. doi: https://doi.org/10.1167/5.12.16.
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© ARVO (1962-2015); The Authors (2016-present)
Ultrastructural reconstruction shows that S cones in macaque retina drive both ON and OFF S-cone specific pathways (1). Psychophysical evidence suggests that humans are similar (2,3).
ON pathway: In central retina of macaque monkey each S cone distributes ∼30 ribbon synapses to ∼4 S-cone ON bipolar cells. [These cells had been called “blue cone bipolar cells” (4).] Each of these ON bipolar cells receives a total of ∼15 ribbon synapses from ∼2 S cones. Thus, there are 2 S-cone ON bipolar cells for each S cone (5). Each S-cone ON bipolar cell distributes ∼40 ribbon synapses to 2–3 small bistratified (“blue ON - yellow OFF”) ganglion cells. Each of these BY ganglion cells receives ∼40 ribbon synapses from 2–3 of these bipolar cells. Thus, there is one BY ganglion cell for each S-cone ON bipolar cell, and correspondingly, 2 BY ganglion cells for each S cone (5). Due to the uneven distribution of synapses in this pathway, one of these two BY ganglion cells is likely to be narrow field, dominated by just one S cone, and one is likely to be wide field, driven strongly by several S cones (6).
OFF pathway: Each S cone is also the source of a private (“midget”) OFF pathway (7). Each S cone makes ∼50 “outer” synaptic densities (within ∼35 “triad-associated” basal contacts) and a similar number of “inner” synaptic densities with a single OFF midget bipolar cell. The OFF midget bipolar cell makes ~40 ribbon synapses with its OFF midget ganglion cell.
Relation to color vision: It has been suggested (8) that efficient coding of color signals from three spectrally overlapping (and thus correlated) cone signals requires three postreceptoral pathways, a spectrally unimodal one (achromatic), a spectrally bimodal one (blue-yellow), and a spectrally trimodal one (red-green-red) (9). I speculate as follows:
1. S cones appear to supply blueness via the ON pathway (2), the anatomically special S-cone ON bipolar cell and perhaps the wide-field BY ganglion cell (10).
2. S cones supply short-wavelength redness (11) via both ON and OFF pathways (2). Perhaps S-cone OFF midget ganglion cells collaborate with L-cone OFF midget ganglion cells to oppose M-cone ON midget cells, whereas narrow-field BY ganglion cells collaborate with L-cone ON midget ganglion cells to oppose M-cone OFF midget ganglion cells.
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