Class I and class II ganglion cells of rabbit retina: a structural basis for X and Y (brisk) cells.

Morphological studies of rabbit retina have identified ganglion cells resembling "alpha" cells but none resembling cat "beta" cells. Four distinct types of class I cell, similar to alpha cells, were identified, each narrowly stratified and differing from the other three principally in the level of dendritic branching. These four levels of dendritic branching flank the two starburst/cholinergic amacrine cell substrata that mark the middle of sublaminae a and b. Compared with the other three, class Ia2 cells are the largest in cell body and dendritic field size, are sometimes homotypically dye coupled, and have slightly broader dendritic stratification. Class Ia2 and the slightly smaller class Ib2 cells form a paramorphic pair. Compared with class I cells, class II cells have smaller dendritic fields; a greater tendency to "tufted" dendritic branching, as shown in the companion paper; branching at one of three levels of the IPL; and similarly narrow stratification. Class IIa and class Ia1 cells branch at the same level, as do class IIb1 and class Ib1 cells. Class IIb2 cells branch slightly nearer the ganglion cell layer than class Ib2 cells and costratify with "blue-ON" cone bipolar cells. The class IIa and IIb2 cells form a paramorphic pair, whereas class IIb1 cells appear to be unpaired. The four types of class I cell probably correspond to ON- and OFF-center brisk-transient, fast-movement and slow-movement cells, whereas the three types of class II cell probably correspond to ON- and OFF-center brisk-sustained and color-coded ON-center X cells. 2004 Wiley-Liss, Inc.