The optic nerves are the pair of nerves that lead from the retina to the visual cortex of the brain.  They carry the sense of sight, although how they do this is poorly understood.  They are part of the central nervous system and as such are not affected by diseases that cause peripheral neuropathy.  Like most of the nerves, they cross on their way to the brain so that the left visual cortex processes information from the right eye and vice versa. 

Some diseases that do affect the optic nerve are glaucoma and multiple sclerosis.

Damage to the optic nerve can cause blindness, but the degree of blindness and whether it affects one or both eyes depends on the site of the damage.

Optic nerve at Wikipedia

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