Inputs to bipolar cell dendrites in goldfish retina.

Goldfish mixed bipolar cells, which contact both rods and cones, comprise two types; Ma, with axon terminals in the distal sublamina(a) and Mb, with axon terminals in the proximal sublamina(b) of the inner synaptic layer. Type Ma cells make only wide-cleft junctions (WCJ), whereas type Mb cells make only narrow-cleft junctions (NCJ) with rods and cores. It follows that WCJs mediate sign-conserving synaptic transmission to the center hyperpolarizing Ma cells, whereas NCJs mediate sign-inverting transmission to the center depolarizing Mb cells. Bipolar cell (BC) surrounds may be mediated largely through horizontal cell (HC) pathways. Type Ma BCs appear to be influenced primarily through HC feedback at cones and both feedback and nonribbon feedforward contacts at rods; in contrast most Mb BCs may be influenced by both HC feedback and ribbon-associated feedforward contacts at rods and cones, accounting for the greater complexity of Mb BC surrounds. BC subtypes Mal and Mbl, which contact only red-sensitive cones (+ rods), are likely to have red-opponent (non-color-coded) surrounds mediated by H1 (L-type) cone HCs; while subtypes Ma2 and Mb2 and 3, which contact also green-sensitive cones, are likely to have red + green-opponent (color-coded) surrounds mediated by H1 and H2 (R/G C-type) cone HCs.