Optic Nerve: Anatomy and Function in Vision

The optic nerve (CN II) is a paired cranial nerve, but developmentally a tract of the central nervous system. Each nerve contains ~1 million axons from retinal ganglion cells.

Pathway

1. Retina → optic nerve → optic chiasm (nasal fibers cross).
2. Optic tract → lateral geniculate nucleus (LGN) of thalamus.
3. Optic radiations → primary visual cortex (V1) in occipital lobe.
4. Higher processing in ventral (what) and dorsal (where) streams.

Function

Transmits visual information (light intensity, color, contrast, motion) from photoreceptors to the brain for perception. Also carries pupillary light reflex afferents (pretectal nucleus → Edinger-Westphal → CN III).

Clinical Significance

• Lesions cause visual field defects (e.g., bitemporal hemianopia at chiasm).
• Optic neuritis (demyelination) → pain with eye movement, vision loss — often first sign of MS.
• Papilledema (disc swelling) → increased intracranial pressure.
• Glaucoma: axonal loss → cupping, peripheral field loss.