Three types of horizontal cells in the stingray retina: their morphology and physiology.

Histological and electrophysiological features of horizontal cells were studied in the stingray retina in which the ratio of rods to cones was about five to one. There were three distinct sublayers of horizontal cells; external, middle and internal. The type of horizontal cells recorded was identified by dye injection after recording. Relatively thick external horizontal cells sent their short dendrites into both rod and cone terminals to make synaptic contacts. The response recorded from them was not color-coded but showed a Purkinje shift depending on the adaptational state. Middle horizontal cells were flat and usually two or three of them overlying each other constituted the middle sublayer. Their response, which was difficult to record in the light-adapted retina, was characterized by a slow time course, a prolongation of the response to bright flashes and a maximum sensitivity in the green region of the spectrum without any sign of Purkinje shift. Internal horizontal cells were color-coded. They responded with hyperpolarization at shorter wavelengths and with depolarization at longer wavelengths. It was concluded that the internal horizontal cells receive inputs from cones and the middle horizontal cells from rods, whereas the external horizontal cells receive inputs from both rods and cones.