Gap junctions between the pedicles of macaque foveal cones

Cone terminals (“pedicles”) in the fovea of macaque retina were studied in electron micrographs of serial sections. Pedicles were sheathed in glia except for small (0.2 μm 2) fenestrations, 4.8 ± 1.7 per pedicle. At each fenestration the membranes of adjacent pedicles were contiguous and marked by an adherent junction, which in turn was invariably associated with gap junctions. There were 3.2 ± 1.4 gap junctions per adherent junction and thus, about fifteen gap junctions per pedicle. The gap junctions were small, 1.6 × 10 −3 ± 1.8 × 10 −3μm 2 ( mean ± SD) and were formed indiscriminately with all neighboring pedicles. An upper bound was estimated of 170 connexons per pedicle and thus a coupling conductance of 1.7 × 10 4pS . Available psychophysical data suggest that the junctions are uncoupled at high luminance. They may couple at lower luminance where spatial averaging would improve contrast sensitivity without cost to spatial acuity.