Structural basis for on-and off-center responses in retinal bipolar cells.

Electron microscopy of Golgi preparations of goldfish retina shows that dendrites of type a (hyperpolarizing, off-center) bipolar cells make wide cleft junctions unassociated with synaptic ribbons, while those of type b (depolarizing, on-center) bioplar cells make narrow cleft junctions and synaptic ribbon contacts, with rods and cones. This suggests that wide cleft junctions are the site of sign-conserving, and narrow cleft junctions or ribbon contacts (or both) are the site of sign-inverting synaptic transmission from photoreceptors to bipolars.