Tuesday, December 21, 2010

Retinal Ganglion Cells

A retinal ganglion cell (RGC) is a type of neuron (nerve cell), which is situated in the retina of the eye. Forming a thin layer, retinal ganglion cells receives visual information from photoreceptors (cones and rods) via two intermediate neuron types: bipolar cells and amacrine cells. The long axons of the retinal ganglion cells join together to make up the optic nerve and transmit image-forming and non-image forming visual information from the retina to several regions in the thalamus, hypothalamus, and mesencephalon, or midbrain.

In the human retina there are about 1.2 to 1.5 million retinal ganglion cells. With about 125 million photoreceptors per retina, on average each retinal ganglion cell receives inputs from about 100 rods and cones. Based on their projections and functions, there are at least five main classes of retinal ganglion cells: 1) midget cells, which project to the parvocellular layers of the lateral geniculate nucleus; 2) parasol cells; which project to the magnocellular layers of the lateral geniculate nucleus; 3) bistratified cells, which project to the koniocellular layers of the lateral geniculate nucleus; 4) photosensitive ganglion cells, which send their axons to the suprachiasmatic nucleus (SCN) via the retinohypothalamic tract for setting and maintaining circadian rhythms; 5) other ganglion cells extend their axons to the superior colliculus for eye movements (saccades).