Horizontal Cell

Illumination$o$photoreceptor hyperpolarization$o$horizontal cell hyperpolarization$o$photoreceptor 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 ($P\_m$) connects to the bipolar cell. The surrounding cells, which represent the outer receptive field, will be designated $P\_o$ then we can explain an off-centre arrangement as follows. If light is shown onto the $P\_m$ then

#$P\_m$ is activated by light and therefore hyperpolarizes
#$P\_m$ 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 $P\_o$is still releasing glutamate, reduction in GABA is marginal

If the light is shown onto the surrounding area then

#$P\_o$ is activated and therefore hyperpolarizes
#$P\_o$ reduce release of glutamate
#Reduction of glutamate hyperpolarizes the horizontal cell
#Horizontal cell reduces release of GABA
#Reduction of GABA depolarizes photoreceptors
#$P\_o$ not affected since they are strongly being hyperpolarized by activation
#$P\_m$ is affected and therefore depolarizes
#$P\_m$ 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 to no affects


SEE ALSO

• Photoreceptors


• Bipolar Cells


• Amacrine Cells


• Ganglion Cells

• [http://webvision.med.utah.edu/OPL2.html#horizontal Webvision

horizontal cell entry.]

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