||This article is incomplete. (February 2009)|
|Neuron: Horizontal Cell|
Plan of retinal neurons.
Horizontal cells are the laterally interconnecting neurons in the Inner Nuclear (Bipolar) layer of the retina of mammalian 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.
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 indiscriminantly, 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.
They span across cones and summate inputs from them all to control the amount of GABA released back onto the photoreceptor cells, which hyperpolarizes them. Their 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 eye is thus more sensitive to contrast and differences in intensity.
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. 
Horizontal cells are depolarized by the release of glutamate from photoreceptors, which happens in the absence of light. Depolarization of a horizontal cell causes it to release the inhibitory neurotransmitter GABA on an adjacent photoreceptor. GABA inhibits this photoreceptor, effectively hyperpolarizing it, and preventing the release of glutamate. Conversely, in the light a photoreceptor does not release or it releases less glutamate onto the horizontal cell, which hyperpolarizes it and prevents or lowers the release of inhibitory GABA on the adjecent photoreceptor. We therefore have the following negative feedback.
Illumination Center photoreceptor hyperpolarization horizontal cell hyperpolarization Surround photoreceptor depolarization
One proposed theory for facilitation by the horizontal cells proceeds as follows. Assume we have 10 photoreceptors, one hyperpolarizing (H) bipolar cell, and one horizontal cell. All ten photoreceptors connect to the horizontal cell, and the middle photoreceptor () connects to the bipolar cell. The surrounding cells, which represent the outer receptive field, will be designated then we can explain an off-centre arrangement as follows. If light is shone onto the then
If the light is shone onto the surrounding area then
To explain diffuse light, then we consider both cases together, and as it turns out, the two effects cancel each other out, and we get little or no net effect.
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