Neuronal diversity in the retina.

The listing of cell types present in the retina is nearing completion, the first time this can be said for any significantly complex sample of the central nervous system. Mammalian retinas contain approximately 55 separate neuronal types. The functions of 22 of them are known or can be strongly inferred. For these 22, in every instance, a cell defined as a 'type' by structural criteria carries out a distinct and individual physiological function. Electrophysiological experiments continue to reveal new features of the retina's handling of information, and there is every reason to believe that the remaining 33 types of cell will also have distinct physiological functions. Further subtleties clearly exist in both peripheral and central visual coding.