Fig. 5. The spiking ON–OFF A1 cell shows complete polarization of synaptic inputs and outputs.

a Proximal dendritic morphology of a physiologically identified A1 amacrine cell, dye-filled (OGB + Alexa 468); 2P-confocal image; arrowhead points to the same dendritic location in a, b, h. b Ultrastructure of primary dendrites of A1 cell (tan fill) shown in a, with cell body protruding into the inner plexiform layer (IPL) confirms identification of this cell in EM tissue volume; partial view of NIRBed line at lower left (arrow). c A1 cell dendritic shaft (tan) receiving an inhibitory synapse (yellow, red arrowhead). d A1 cell dendritic spine (tan) receives a ribbon synapse (red arrowhead) from a bipolar-cell axon terminal (pink). e, f A1 cell axon-like process (teal) synapses on a bipolar-cell axon terminal (blue, red arrowheads) at 2 different locations; synapses from bipolar cells to A1 amacrine-cell axon-like processes were not observed. g A1 cell axon-like process synapses (teal, red arrowhead) on an amacrine cell process (yellow). The scale bar applies c–f. h Reconstruction of proximal dendrites and axon segment for A1 cell shown in a, illustrates that synaptic inputs from bipolar-cell ribbon contacts (red balls) were made primarily to the dendritic spines that arose from dendritic shafts (see inset lower left). By contrast, dense inhibitory synaptic inputs from other amacrine cells were made directly upon the dendritic shafts. Location of amacrine inputs to one segment of a secondary dendrite (between arrows) is indicated by the yellow ball structures. Reconstruction of a segment from the axon-like arbor (teal process at upper left) reveals synaptic output from varicosities primarily to bipolar-cell axon terminals (blue balls) but also to amacrine-cell processes (green balls) and to a ganglion cell dendrite (magenta ball). See also Supplementary Fig. 4.