Eye nerves are all of the cells within the retina, optic, oculomotor, trochlear and abducens nerves. These are also known as cranial nerves II, III, IV and VI. There are many kinds of specialized neurons in the retina, including photorecptors and retinal ganglion cells. Together with the optic nerve, these are the basis of the vision. The other three eye nerves control the muscles that move the eyeball.
The retina is a light-sensitive extension of the central nervous system, onto which light is projected by the lens of the eye. The photoreceptor cells of the retina, the rods and cones, are specially adapted nerves that change chemically when light strikes them. Rod cells respond to differences in brightness and are most active in darkness, while cone cells are sensitive to color information, which they derive from the wavelengths of light. Other neurons in the retina encode these changes into information that is electrically transmitted by the the ganglion cells at the back of the retina. These neurons become the optic nerve after they leave the eye.
The eye nerves develop from different tissues during pregnancy. The optic nerve originates from the same part of the embryo as much of the brain, including the forebrain and thalamus. For this reason, it is classified as a part of the central nervous system. Retinal ganglion cells and the optic nerve do more than transmit information: they actively sort it. Like neurons within the brain, they engage and process data, in this case categorizing color signals coming in from the cone cells, dividing these into several groups, called color-opponency channels.
The roughly 1 million neurons of the optic nerve run from the brain to retinal ganglion cells. Half of the the nerve fibers from each retina cross over to the other side at the optic chiasm, which is located near the pituitary gland along the path into the brain. From here the fibers are called the optic tract and pass through the thalamus before reaching the visual cortex of the parietal lobe, where the light information from the retina is processed. The eye nerves terminate in the various visual cortices.
Eye movement is controlled by cranial nerves III, IV and VI. The first of these, the oculomotor nerve, controls the eyelid, pupil constriction and much of the eye's range of motion. The trochlear nerve is smaller and controls only one muscle of the eye, the superior oblique, which allows rolling and crossing of the eyes. It is especially susceptible to skull trauma because of its long course through the brain and fragility. The abducens nerve controls only the ipsilateral lateral rectus muscle, which turns the eyeball directly to the side.
