Retinal horizontal cells

Horizontal cells are second-order neurons in the vertebrate retina, receiving direct excitatory input from cone and rod photoreceptors. They appear in several morphological types; the most general classification is into A-type without axons and B-type with axons. Horizontal cells in all vertebrates receive direct input from photoreceptors but also interact with each other via gap junctions. They form large syncytia, each composed of cells with similar morphological and physiological properties. These syncytia provide the means for lateral spread of light signals within the retina, and form the basis for the surround component of the receptive fields of bipolar cells and ganglion cells. Thus, horizontal cells play a major role in the first stage of spatial information processing. Horizontal cells are also presynaptic to photoreceptors, exerting a negative feedback effect. This pathway underlies color opponency in horizontal cells. Such horizontal cells are called chromaticity cells (C-types), and they play a major role in the genesis of color information processing.