Horizontal cells in the retina of the rabbit.

The light responses, morphology, and connections of horizontal cells (HCs) were studied in the retina of the rabbit using intracellular recordings and the injection of visible markers. Two types of HCs were identified, axonless and axon-bearing HCs. Axonless HCs and the somatic end of axon-bearing HCs respond to white light of varying intensity with graded hyperpolarizations; both display a transient superimposed on the sustained hyperpolarization at stimulus initiation and a small rod aftereffect at the cessation of high intensity stimuli. Anatomically, both are connected to cones, but their responses also suggest rod influence. Both summate stimuli from a retinal area which is much larger than their respective fields. However, only axonless HCs transfer a fluorescent, low molecular weight dye to adjoining, homologous cells. The axon terminal of axon-bearing HCs has response properties different from those of the cell body: the transient at stimulus initiation is absent; furthermore, at high levels of illumination, the rod aftereffect becomes equal in amplitude to the primary hyperpolarization. Anatomically, it is connected to rods, but its responses also suggest cone influence. Its receptive field approximates in diameter its anatomical spread and it does not transfer fluorescent dye to its neighbors.