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Vis Neurosci. 2011 Jan;28(1):3-16. doi: 10.1017/S0952523810000477.

Cone synapses in macaque fovea: I. Two types of non-S cones are distinguished by numbers of contacts with OFF midget bipolar cells.

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Department of Psychology, Franz Hall, University of California, Los Angeles, Los Angeles, California 90095-1563, USA.


L and M cones, divided into two groups by absorption spectra, have not been distinguished by structure. Here, we report what may be such a difference. We reconstructed the synaptic terminals of 16 non-S cones and the dendritic arbors of their ON and OFF midget bipolar cells from high-magnification electron micrographs of serial thin sections of a small region of macaque fovea. Each cone terminal contacted a similar number (~16) of invaginating central elements provided by its ON midget bipolar cell. By contrast, the numbers of connections between a cone terminal and its OFF midget bipolar cell were grouped into two clusters: 30-37 versus 43-50 basal contacts in the triad-associated position and 41-47 versus 61-74 Outer Densities within those basal contacts. The coefficients of variation of these distributions were all in the range of 10% or lower, characteristic of single populations. If these two clusters correspond to M- and L-cone circuits, the results reveal structural differences between M and L cones and between their corresponding OFF midget bipolar cells.

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