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Ganglion cell

It has been suggested that some sections of this article be split into a new article entitled Retinal ganglion cell. (Discuss)

Ganglion cell
Various forms of nerve cells. A. Pyramidal cell. B. Small multipolar cell, in which the axon quickly divides into numerous branches. C. Small fusiform cell. D and E. Ganglion cells
Location	Varies by type
Function	Varies but often excitatory projection
Morphology	Varies

A ganglion cell (more correctly, a retinal ganglion cell) is a type of neuron typically located near the inner surface of the retina of the eye that receives visual information from photoreceptors via two intermediate neuron types (bipolar cells and amacrine cells). Retinal ganglion cells collectively transmit visual information from the retina to several regions in the thalamus, hypothalamus, and mesencephalon, or midbrain. Retinal ganglion cells vary significantly in terms of their size, connections, and responses to visual stimulation but they all share the defining property of having a long axon that extends into the brain. These axons form the optic nerve, optic chiasm, and optic tract.

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Function

There are about 1.2 to 1.5 million retinal ganglion cells in the human retina. With about 105 million photoreceptors per retina, on average each retinal ganglion cell receives inputs from about 100 rods and cones. However, these number vary greatly among individuals and as a function of retinal location. In the fovea (center of the retina), a single photoreceptor will communicate with as many as five ganglion cells. In the extreme periphery (ends of the retina), a single ganglion cell will receive information from many thousands of photoreceptors.

Retinal ganglion cells spontaneously fire action potentials at a base rate while at rest. Excitation of retinal ganglion cells results in an increased firing rate while inhibition results in a depressed rate of firing.

Ganglion Cells also reside in the adrenal medulla. They are involved in the sympathetic nervous system's release of epinephrine and norepinephrine into the blood stream

Types

Based on their projections and functions, there are at least five main classes of retinal ganglion cells:

Midget (Parvocellular, or P pathway; A cells)
Parasol (Magnocellular, or M pathway; B cells)
Bistratified (Koniocellular, or K pathway)
Other ganglion cells projecting to the Superior Colliculus for eye movements (saccades) [reference: Principles of neural Science 4th Ed. Kandel et al.]
Photosensitive ganglion cells

Midget

Midget retinal ganglion cells project to the parvocellular layers of the lateral geniculate nucleus. These cells are known as midget retinal ganglion cells, based on the small sizes of their dendritic trees and cell bodies. About 80% of RGCs are midget cells in the parvocellular pathway. They receive inputs from relatively few rods and cones. They have slow conduction velocity, and respond to changes in color but respond only weakly to changes in contrast unless the change is great (Kandel et al., 2000). They have simple center-surround receptive fields, where the center may be either ON or OFF to one of the cones while the surround is the opposite to another cone.

Parasol

Parasol retinal ganglion cells project to the magnocellular layers of the lateral geniculate nucleus. These cells are known as parasol retinal ganglion cells, based on the large sizes of their dendritic trees and cell bodies. About 10% of retinal ganglion cells are parasol cells in the magnocellular pathway. They receive inputs from relatively many rods and cones. They have fast conduction velocity, and can respond to low-contrast stimuli, but are not very sensitive to changes in color (Kandel et al., 2000). They have much larger receptive fields which are nonetheless also center-surround.

Bistratified

Bistratified retinal ganglion cells project to the koniocellular layers of the lateral geniculate nucleus. Bistratified retinal ganglion cells have been identified only relatively recently. Koniocellular means “cells as small as dust”; their small size made them hard to find. About 10% of retinal ganglion cells are bistratified cells in the koniocellular pathway. They receive inputs from intermediate numbers of rods and cones. They have moderate spatial resolution, moderate conduction velocity, and can respond to moderate-contrast stimuli. They may be involved in color vision. They have very large receptive fields that only have centers (no surrounds) and are always ON to the blue cone and OFF to both the red and green cone.

Other retinal ganglion cells projecting to the LGN

Other retinal ganglion cells projecting to the LGN include cells making connections with the Edinger-Westphal nucleus (EW) for control of the pupillary light reflex and giant retinal ganglion cells.

Photosensitive ganglion cell

Photosensitive ganglion cells contain their own photopigment, melanopsin, which makes them respond directly to light even in the absence of rods and cones. They project to the suprachiasmatic nucleus (SCN) via the retinohypothalamic tract for setting and maintaining circadian rhythms.