A (sometimes called a '''gangliocyte''') is a type of
Neuron located in the
Retina of the
Eye that receives visual information from
Photoreceptor s via various intermediate cells such as
Bipolar Cell s,
Amacrine Cell s, and
Horizontal Cell s. Retinal ganglion cells’
Axon s are
Myelin ated. 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 .
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 Potential s 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.
Based on their projections and functions, there are at least five main classes of retinal ganglion cells:
- Parvocellular (P)
- Magnocellular (M)
- Koniocellular (K)
- Other ganglion cells projecting to the LGN for eye movements ( Saccades )
- Photosensitive ganglion cells
Parvocellular (P) retinal ganglion cells project to the parvocellular layers of the lateral geniculate nucleus. These cells are also known as midget retinal ganglion cells, based on the small sizes of their
Dendritic Trees and cell bodies. About 80% of retinal ganglion cells are parvocellular cells. 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.
Magnocellular (M) retinal ganglion cells project to the magnocellular layers of the lateral geniculate nucleus. These cells are also 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 magnocellular cells. 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.
Koniocellular (K) retinal ganglion cells project to the koniocellular layers of the
Lateral Geniculate Nucleus . Koniocellular 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 koniocellular cells. 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 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 s 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 .
