The optic nerve is the nerve that transmits visual information from the retina to the brain.
Anatomy
The nerve is the second of twelve paired cranial nerves but is usually considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during embryonic development. Its nerve fibers mainly extend from the retina to the primary visual cortex.
The optic nerve arises from ganglion cells of the eye's retina. It leaves the orbit (eye) via the optic canal , running postero-medially towards the optic chiasm where there is a partial decussation (crossing) of fibers from the temporal visual fields of both eyes. Most of the axons of the optic nerve terminate in the lateral geniculate nucleus from where information is relayed to the visual cortex.
From the lateral geniculate body, fibers of the optic radiation pass to the visual cortex in the occipital lobe of the brain. More specifically, fibers carrying information from the contralateral superior visual field traverse Meyer's loop to terminate in the lingual gyrus below the calcarine fissure in the occipital lobe, and fibers carrying information from the contralateral inferior visual field terminate more superiorly.
Physiology
The optic nerve contains roughly one million nerve fibers. This number is low compared to the roughly 130 million receptors in the retina, and implies that substantial pre-processing takes place in the retina before the signals are sent to the brain through the optic nerve.
The blind spot of the eye lies in the part of the retina where the optic nerve leaves the eye. This is because there are no photoreceptors in this area.
Role in disease
Damage to the optic nerve typically causes an abnormal pupillary reflex and loss of vision. The type of visual field loss will depend on which optic nerve was damaged and where the damage occurred:
- Damage before the optic chiasm affect vision in one eye only.
- Damage in the chiasm causes loss of vision laterally in both eyes (bitemporal hemianopia). It may occur in large pituitary adenomata.
- Damage after the chiasm causes loss of vision on one side but affecting both eyes: the visual field affected is located on the opposite side of the lesion.
Optic neuritis is inflammation of the optic nerve. It is associated with a number of diseases, most notably multiple sclerosis.
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