Ganglion cell

Overview
A ganglion cell (sometimes called a gangliocyte) is a type of neuron located in the retina of the eye that receives visual information from photoreceptors via various intermediate cells such as bipolar cells, amacrine cells, and horizontal cells. Retinal ganglion cells’ axons are myelinated. The myelinated parts are outside the eye. These axons form the optic nerve and connect mainly to the lateral geniculate nucleus (LGN) in the brain.

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, this number varies greatly with retinal eccentricity. In the fovea (center of the retina), a single photoreceptor will communicate with five ganglion cells. In the extreme periphery (ends of the retina), a single ganglion cell will receive information from multiple 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.

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.