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Horizontal cells are the laterally interconnecting neurons having cell bodies in the inner nuclear layer of the retina of vertebrate eyes. They help integrate and regulate the input from multiple photoreceptor cells. Among their functions, horizontal cells are responsible for allowing eyes to adjust to see well under both bright and dim light conditions. Horizontal cells are inhibitory interneurons that release GABA upon depolarization The four types of retinal neurons are bipolar cells, ganglion cells, horizontal cells, and amacrine cells. ==Organization== There are three basic types of horizontal cells, designated HI, HII and HIII. The selectivity of these three horizontal cells, towards one of the three cone types, is a matter of debate. According to studies conducted by Boycott and Wassle neither HI cells nor HII cells were selective towards S,M, or L cones. By contrast, Anhelt and Kolb claim that in their observations HI cells connected to all three cone types indiscriminately, however, HII cells tended to contact S cones the most. They also identified a third type of horizontal cell, HIII, which was identical to HI but did not make contact with S cones. The HII cells also make connections with rods, but do so far enough away from the horizontal cell's soma such that they do not interfere with the activities of the cones. Horizontal cells span across cones and summate inputs before feeding back onto the photoreceptor cells, hyperpolarizing them. The exact mechanism by which the horizontal cells induce this hyperpolarization is uncertain, but two mechanisms are possible. In the first, hyperpolarization is proposed to be mediated synaptically by the release of GABA by horizontal cells that acts on the photoreceptor cells. In a second option, hyperpolarization occurs non-synaptically by a form of ephaptic transmission in which hemichannels on the horizontal cell dendrites modulate the extracellular potential immediately surrounding photoreceptor terminals. It is worth noting that these two mechanisms are not mutually exclusive and hyperpolarization likely occurs through the combined effects of each of them. The horizontal cells arrangement together with the on-center and off-center bipolar cells that receive input from the photoreceptors constitutes a form of lateral inhibition, increasing spatial resolution at the expense of some information on absolute intensity. The presence of highly polarized retina horizontal cells provides the eye with contrast sensitivity and an enhanced ability to discriminate differences in intensity. There is a greater density of horizontal cells towards the central region of the retina. In cat retina it is observed that there is a density of 225 cells/mm2 near the center of the retina and a density of 120 cells/mm2 in the peripherals. Horizontal cells and other retinal interneuron cells are less likely to be near neighbours of the same subtype than would occur by chance, resulting in ‘exclusion zones’ that separate them. Mosaic arrangements provide a mechanism to distribute each cell type evenly across the retina, ensuring that all parts of the visual field have access to a full set of processing elements. MEGF10 and MEGF11 transmembrane proteins have critical roles in the formation of the mosaics by horizontal cells and starburst amacrine cells in mice. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Retina horizontal cell」の詳細全文を読む スポンサード リンク
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