Nuclei and fibre types
The optic nerve carries one fibre type: special somatic afferent (SSA) fibres for vision. It has no brainstem nucleus. Its fibres are the axons of retinal ganglion cells, the third neurons in the retina after the photoreceptors (rods and cones) and the bipolar cells.
Developmentally the optic nerve is an outgrowth of the diencephalon, so it is a tract of the central nervous system rather than a true peripheral nerve. Its axons are myelinated by oligodendrocytes, not Schwann cells, and do not regenerate after section. That is why demyelinating disease such as multiple sclerosis affects it.
The nerve is sheathed by all three meninges. Its subarachnoid space is continuous with the intracranial one, so raised intracranial pressure is transmitted to the optic disc and causes papilloedema (swelling of the optic disc).
Emergence and skull foramen
CN II does not emerge from the brainstem. It leaves the eyeball at the optic disc, a few millimetres medial (nasal) to the fovea, where the absence of photoreceptors creates the physiological blind spot.
It leaves the orbit through the optic canal in the lesser wing of the sphenoid bone, together with the ophthalmic artery, which lies inferolateral to the nerve in the canal. The dural sheath fuses with the periosteum of the canal here, fixing the nerve, so a fracture through the canal can crush or shear it.
Course and branches
The optic nerve runs backward and medially from the eyeball to the optic chiasm and gives no branches. Its segments are intraocular, intraorbital, intracanalicular and intracranial; the intraorbital part is the longest and slightly S-shaped, which allows the eye to move freely.
In the orbit the nerve lies inside the cone of extraocular muscles, passes through the common tendinous ring, and has the ciliary ganglion on its lateral side. The central retinal artery, a branch of the ophthalmic artery, enters its inferior surface about 1 cm behind the globe and runs within it to the retina.
The visual pathway beyond the nerve runs as follows:
- Optic chiasm: above the diaphragma sellae and pituitary gland, in front of the pituitary stalk and below the floor of the third ventricle. Fibres from the nasal half of each retina cross; temporal fibres stay on the same side.
- Optic tract: winds round the cerebral peduncle to the lateral geniculate nucleus of the thalamus. Some fibres leave for the pretectal nucleus (pupillary light reflex) and superior colliculus.
- Optic radiation: fans through the temporal and parietal lobes. Fibres for the upper visual field loop forward into the temporal lobe as Meyer's loop.
- Primary visual cortex: on the banks of the calcarine sulcus of the occipital lobe.
Clinical testing
CN II is tested with five checks: acuity, colour, fields, pupils and fundi.
- Visual acuity: each eye separately with a Snellen chart, wearing glasses or through a pinhole.
- Colour vision: Ishihara plates; red desaturation is an early sign of optic neuritis.
- Visual fields: by confrontation, quadrant by quadrant, with a red pin to map the blind spot and central field.
- Pupillary reflexes: light in one eye constricts both pupils (direct and consensual). CN II is the afferent limb; CN III is the efferent limb. The swinging light test reveals a relative afferent pupillary defect (RAPD).
- Fundoscopy: the disc for papilloedema, pallor (optic atrophy) or cupping (glaucoma).
Lesions and palsies
The field defect localises the lesion along the visual pathway.
| Site | Defect | Typical cause |
|---|---|---|
| Optic nerve | Monocular loss or central scotoma, with RAPD | Optic neuritis, ischaemic optic neuropathy, trauma, glaucoma |
| Optic chiasm (central) | Bitemporal hemianopia | Pituitary adenoma, craniopharyngioma |
| Optic tract | Contralateral homonymous hemianopia, often incongruous | Tumour, stroke |
| Meyer's loop (temporal lobe) | Contralateral superior quadrantanopia | Temporal lobe tumour or surgery |
| Parietal radiation | Contralateral inferior quadrantanopia | Middle cerebral artery stroke |
| Occipital cortex | Contralateral homonymous hemianopia with macular sparing | Posterior cerebral artery infarct |
A pituitary adenoma presses on the chiasm from below, so the upper temporal fields go first; a craniopharyngioma presses from above and affects the lower temporal fields first. Optic neuritis causes painful eye movements, reduced acuity and colour vision, and is often the first episode of multiple sclerosis. Giant cell arteritis can blind an eye by occluding the posterior ciliary arteries that supply the optic nerve head, which makes it an emergency.