Retinal ganglion cell

The retina, the layer at the back of the eye that turns light into signals, has ten layers and six main cell types. The ganglion cell is the last one in that chain: the output neuron, the one that sends the signal out of the eye. It is the eye's output cable: everything passes through it. First it receives information from two neighbors, the bipolar cells and the amacrine cells, some of which excite it and others of which hold it back.

Its axons, the long extensions the signal travels along, all come together at the optic disc, the point where they leave the eye, and cross the lamina cribrosa, a supporting mesh. They do so without myelin, the insulating sheath nerves usually have, because it would block the light coming in.

Not all of them are alike: there are about twenty types, and between 1% and 2% detect light on their own thanks to a molecule called melanopsin. Ganglion cell axons reach places such as the suprachiasmatic nucleus, and in doing so they help regulate the circadian rhythm, the body's day and night clock, and the pupillary light reflex, the pupil narrowing in the light.