Functional Properties
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 dark a photoreceptor releases glutamate onto the horizontal cell, which hyperpolarizes it and prevents the release of inhibitory GABA. 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
- 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 shone 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.
Read more about this topic: Retina Horizontal Cell
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