A labeled ON parasol ganglion cell from a macaque retina was
analyzed in serial, ultrathin sections. It received 13% of its
input from diffuse bipolar cells. These directed a large proportion
of their output to amacrine cells but received a relatively
small proportion of their amacrine cell input via feedback
synapses. In these respects, they were similar to the DB3 bipolar
cells that make synapses onto OFF parasol cells. Bipolar cell
axons that contacted the ON parasol cell in stratum 4 of the
inner plexiform layer always made synapses onto the dendrite,
and therefore, the number of bipolar cell synapses onto these
ganglion cells could be estimated reliably by light microscopy
in the future. Amacrine cells provided the majority of inputs
to the ON parasol cell. Only a few of the presynaptic amacrine
cell processes received inputs from the same bipolar cells as
the parasol cells, and most of the presynaptic amacrine cell
processes did not receive any inputs at all within the series.
These findings suggest that most of the inhibitory input to
the ON parasol cell originates from other areas of the retina.
Amacrine cells presynaptic to the parasol ganglion cell interacted
very infrequently with other neurons in the circuit, and therefore,
they would be expected to act independently, for the most part.