Horizontal cells are the laterally interconnecting neurons in the outer plexiform layer of the retina.
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 indiscremenantly, 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 hyperpolarises them. Their arrangement together with the on-centre and off-centre 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.
When light is shown onto a photoreceptor, the photoreceptor hyperpolarizes and reduces the release of glutamate, when this happens, horizontal cells reduce the release of GABA, which has an inhibitory affect on the photoreceptors. This reduction of inhibition leads to a depolarization of the photoreceptors. We therefore have the following negative feedback
Illuminationphotoreceptor hyperpolarizationhorizontal cell hyperpolarizationphotoreceptor depolarization
One proposed theory for facilitation by the horizontal cells proceeds as follows. Assume we have 11 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 shown onto the then
- is activated by light and therefore hyperpolarizes
- reduces release of glutamate
- Reduction of glutamate hyperpolarizes the H bipolar cell
- Reduction of glutamate hyperpolarizes the horizontal cell and it reduces release of GABA
- Since is still releasing glutamate, reduction in GABA is marginal
If the light is shown onto the surrounding area then
- is activated and therefore hyperpolarizes
- reduce release of glutamate
- Reduction of glutamate hyperpolarizes the horizontal cell
- Horizontal cell reduces release of GABA
- Reduction of GABA depolarizes photoreceptors
- not affected since they are strongly being hyperpolarized by activation
- is affected and therefore depolarizes
- releases glutamate
- H Bipolar cell is depolarized
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.
horizontal cell entry.]
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