Horizontal cell
Encyclopedia
Horizontal cells are the laterally interconnecting neurons in the outer plexiform layer
of the retina
of mammal
ian 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.
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.
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.
Outer plexiform layer
The outer plexiform layer is a layer of neuronal synapses in the retina of the eye. It consists of a dense network of synapses between dendrites of horizontal cells from the inner nuclear layer, and photoreceptor cell inner segments from the outer nuclear layer...
of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
of mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian 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.
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 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
Gabâ
Gabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions...
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
Lateral inhibition
In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition sharpens the spatial profile of excitation in response to a localized stimulus.-Sensory 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.
Functional Properties
When light is shone 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 anti-inhibitory effect on the photoreceptors. This reduction of inhibition leads to a depolarization of the photoreceptors. We therefore have the following negative feedbackIllumination
Center photoreceptor hyperpolarization horizontal cell hyperpolarization Surround photoreceptor depolarizationOne 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.
See also
- Photoreceptors
- Bipolar cells
- Amacrine cells
- Ganglion cells
External links
- Webvision: Horizontal Cell
- NIF Search - Horizontal Cell via the Neuroscience Information FrameworkNeuroscience Information FrameworkThe Neuroscience Information Framework is a repository of global neuroscience web resources, including experimental, clinical, and translational neuroscience databases, knowledge bases, atlases, and genetic/genomic resources.-Description:...