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Ganglion CellA ganglion cell (or sometimes called a gangliocyte) is a type of neuron located in the retina 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). There are about 1.2-1.5 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. Retinal ganglion cells fire action potentials at random, even when not stimulated by light. Based on their projections and functions, there are at least four main classes of retinal ganglion cells: - Parvocellular (P)
- Magnocellular (M)
- Koniocellular (K)
- Other
Parvocellular retinal ganglion cells project to the parvocellular layers of the lateral geniculate nucleus. These cells are also known as midget retinal ganglion cells, confusingly abbreviated to M, 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 only to high-contrast stimuli. 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, confusingly abbreviated to P, 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. 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 relatively 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 colour vision. Other retinal ganglion cells include cells that project to the LGN and cells that project to the suprachiasmatic nucleus (SCN). These retinal ganglion cells contain their own photopigment, melanopsin, meaning they respond directly to light in the eye. Cells projecting to the LGN include those making connections with the Edinger-Westphal nucleus (EW) for control of the pupillary light reflex and giant retinal ganglion cells. Cells projecting to the SCN go via the retinohypothalamic tract for setting and maintaining circadian rhythms.
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