Figure 3.

Flat contacts from midget bipolar cells form a private-line OFF pathway for S cones. A, Outlines of pedicle positions with S-cone pedicles indicated in blue, as shown in Figure 2. B, Each S-cone pedicle is densely contacted by the noninvaginating, “flat” dendritic arbors of a single OFF midget bipolar cell. Of the 17 S-OFF midget bipolar dendritic arbors reconstructed, nine complete cells were encompassed within the volume and could be reconstructed completely to their axon terminals in the outer half of the IPL. C, Axon terminals of the nine complete OFF midget bipolar cells shown in B; each forms a private-line synaptic connection with the dendritic arbor of an OFF midget ganglion cell. Seven of the nine midget ganglion cells could be reconstructed to the cell body and axon, which extends into the optic fiber layer. D, One of the OFF midget bipolar–ganglion cell private-line synapses in C (red box), shown in greater detail. The S-OFF midget bipolar–ganglion cell private-line synaptic complex is shown in partial transparency; the bipolar terminal enters from above (yellow) and the ganglion-cell dendritic arbor enters from below (orange). Twenty-five ribbon synapses in this bipolar cell were tagged and volume rendered (red), and established synaptic contact with this ganglion cell.