Optic Nerve

The optic nerve (cranial nerve II) carries visual information from the retina to the brain — about a million fibres from each eye. The two nerves meet at the optic chiasm, where fibres from the inner half of each retina cross, so each side of the brain receives the opposite half of the visual world. Strictly, it is a tract of the brain itself, wrapped in meninges.

Key facts

Each optic nerve carries around a million nerve fibres.
It is actually part of the brain, not a true peripheral nerve.
Fibres cross at the chiasm so the left brain sees the right visual field.
The pituitary gland sits just below the chiasm — pituitary tumours can press on it.
Its head (the optic disc) can be seen with an ophthalmoscope.

What can go wrong

Inflammation of the optic nerve (optic neuritis) causes painful vision loss in one eye and is associated with multiple sclerosis. Pressure from a pituitary tumour characteristically knocks out the outer visual fields of both eyes. Glaucoma damages the nerve head slowly, and swelling of the disc can signal raised pressure in the skull.

Related procedures & tests

OCT scan (optical coherence tomography)

Related symptoms

Frequently asked questions

Why does a pituitary tumour affect the sides of vision?

The crossing fibres at the chiasm — which carry the outer (temporal) visual fields — pass directly above the pituitary gland. An enlarging tumour presses up on exactly those fibres first.

Is the optic nerve really a nerve?

By name, yes; by structure, it is brain tissue — its fibres are myelinated by the same cells as the brain's white matter, and it is wrapped in meninges — which is why diseases of the brain, like MS, can affect it.

Educational information only. This atlas explains normal anatomy and describes conditions in general terms; it is not medical advice and does not replace assessment by a qualified clinician. If you have symptoms that worry you, seek medical care.