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.. Ganglion cells receive input from a vertical pathway (traveling via bipolar cells only) or a horizontal pathway (which includes horizontal and amacrine cells as well).
Center and surround tend to behave oppositely and in fact the surround may inhibit the center. The signals from bipolar and amacrine cells start chemically then proceed electrically. Ganglion cells are fine tuned to such factors as size, color, and movement. The brain must make sense out of this data, including changes thereof. Subtle movements can cause detection of the motion by the ganglion cell field. They typically have a preferred direction in which stimuli move, and the opposite direction is considered "null." Neurotransmitters such as acetylcholine and GABA work to produce directional selectivity.
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.
If the cell is activated by light, it is called an ON signal, and its dendrites are near ganglion cells and receive their signals from ON-center bipolar cells. When light is absent, there is an OFF signal, which derives its input via dendrites near amacrine cells and receiving signals from OFF-center bipolar cells. The ON-OFF are activated when light is switched either on or off (but quiet otherwise). These typically have bistratified dendrites.
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 , and .
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)Principles of Neural Science 4th Ed. Kandel et al.
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. They are also called tonic cells, and have slow and steady transmission of signals, and better reception of contrast and detail. About 80% of all retinal ganglion cells are midget cells in the parvocellular pathway. The cells of the central retina are typically this type, but further away there is greater input from dendrites. 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. Moreover, they may be chromatic opponent units. Their dendritic trees are the size of their cell body.
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. These are also known as phasic cells, and have faster speeds of transmission speed than the tonic cells. They are present throughout the retina, and have a larger dendritic field than tonic cells.
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.
Ganglion cells transmit a large number of signals to the brain from different locations, and objects can be found in many adjacent pathways en route to the brain. They are placed in mosaics which keeps them separate. High-frequency patterns may behave like low-frequency patterns, known as aliasing. The ganglion cell can integrate and hence attenuate these signals. Mosaics can sometimes cause interference. There is also potential for hyperacuity in the ganglion cell.
Ganglion cells share input from neighboring territory. Concurrent signals are strengthened in the brain. If both are alike (ON or OFF), they join together, if opposite, the second is shut off. These ganglion cell fields must overlap, wholly or partially. If stimuli are separate, the signal remains separate, but if it overlaps then the signal will merge, possibly wth the use of gap junctions.
Ganglion cells receive neurotransmitters across the synapse from amacrine and bipolar cells, which is converted into an electrical impulse by means of ion channels across the synapse. Excitatory neurotransmitters such as glutamate and acetylcholine depolarize the membrane by allowing positive ions to flow through the channels, whereas inhibitory ones like glycine and GABA do so with negative ions. Ganglion cells, nevertheless, go beyond merely selecting from different ion channels. They focus on specific synapses to limit neural sources, namely, amacrine and bipolar cells. Upon stimulation by light, ON-center ganglion cells will be turned on, but OFF-center cells will be shut off. The amacrine cell is the deciding factor in the control of the latter response.
Color, Scotopic, and Mesopic Vision Ganglion cells respond to color using either luminance (addition) or opponent (subtraction) mechanisms.Tonic cells react to color stimuli in center-surround opponent fields, using an ON or OFF center mechanism, hence creating six possible spectral fields. Multiple cone mechanisms may possibly lead to 12 forms. Phasic cells can achieve central sustained responses, but when surrounds are involved, these are cancelled. Summation of center and surround can give a solely central response. The main task of a phasic cell is luminance, not color. Another type of color receptor is a center only neuron, which is bistratified and has no surround action. They are either red-green or blue-yellow, and are stimulated first by one color and then by the other.
Rod (scotopic) vision is only truly experienced in a rural milieu with no moon or streetlights. A moderate environment between a photopic and scotopic one is known as mesopic. Phasic cells, the primary route of transmission, are mixed in their population of rod input; tonic cell input is disputed. Activities done at night, such as driving or sports, are impacted by the slower reaction time of rod vision. The ganglion cells have no threshold of photons entering, and in utter darkness have background noise against which light input reacts. Even just one photon is enough to expand to other ganglion cells, but the background noise interferes, requiring multiple photons. When dark-adapted, the central visual field of ganglion cells is large at the expense of the surround. It also, as expected, reacts more slowly.
Pathology Degeneration of axons of the retinal ganglion cells (the optic nerve) is a hallmark of glaucoma.
Developmental biology Retinal growth: the beginning