A retinal ganglion cell (RGC) is a type of neuron located near the inner surface (the ganglion cell layer) of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and retina amacrine cells. Retina amacrine cells, particularly narrow field cells, are important for creating functional subunits within the ganglion cell layer and making it so that ganglion cells can observe a small dot moving a small distance. Retinal ganglion cells collectively transmit image-forming and non-image forming visual information from the retina in the form of action potential 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. A small percentage of retinal ganglion cells contribute little or nothing to vision, but are themselves photosensitive; their axons form the retinohypothalamic tract and contribute to circadian rhythms and pupillary light reflex, the resizing of the pupil.
Function There are about 0.7 to 1.5 million retinal ganglion cells in the human retina. With about 4.6 million cone cells and 92 million rod cells, or 96.6 million photoreceptors per retina, on average each retinal ganglion cell receives inputs from about 100 rods and cones. However, these numbers vary greatly among individuals and as a function of retinal location. In the fovea (center of the retina), a single ganglion cell will communicate with as few as five photoreceptors. In the extreme periphery (edge 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.
Types There is wide variability in ganglion cell types across species. In primates, including humans, there are generally three classes of RGCs:
W-ganglion: small, 40% of total, broad fields in retina, excitation from rods. Detection of direction movement anywhere in the field. X-ganglion: medium diameter, 55% of total, small field, color vision. Sustained response. Y-ganglion: largest, 5%, very broad dendritic field, respond to rapid eye movement or rapid change in light intensity. Transient response. The W, X and Y retinal ganglion types arose from studies of the cat. These physiological types are closely related to the respective morphological retinal ganglion types γ {\displaystyle \gamma } , β {\displaystyle \beta } and α {\displaystyle \alpha } . Based on their projections and functions, there are at least five main classes of retinal ganglion cells:
Midget cell (parvocellular/P pathway; P cells) Parasol cell (magnocellular/M pathway; M cells) Bistratified cell (koniocellular/K pathway; K cells) Photosensitive ganglion cells Other ganglion cells projecting to the superior colliculus for eye movements (saccades)
P-type P-type 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 all retinal ganglion cells 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. They have simple center-surround receptive fields, where the center may be either ON or OFF while the surround is the opposite.
M-type M-type 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 all retinal ganglion cells are parasol cells, and these cells are part of 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. They have much larger receptive fields which are nonetheless also center-surround.
K-type BiK-type retinal ganglion cells project to the koniocellular layers of the lateral geniculate nucleus. K-type 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 all retinal ganglion cells are bistratified cells, and these cells go through the koniocellular pathway. They receive inputs from intermediate numbers of rods and cones. 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.
Photosensitive ganglion cell Photosensitive ganglion cells, including but not limited to the giant retinal 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, among other areas, the suprachiasmatic nucleus (SCN) via the retinohypothalamic tract for setting and maintaining circadian rhythms. Other retinal ganglion cells projecting to the lateral geniculate nucleus (LGN) include cells making connections with the Edinger-Westphal nucleus (EW), for control of the pupillary light reflex, and giant retinal ganglion cells.
Physiology Most mature ganglion cells are able to fire action potentials at a high frequency because of their expression of Kv3 potassium channels.
Pathology Degeneration of axons of the retinal ganglion cells (the optic nerve) is a hallmark of glaucoma.
Developmental biology
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