Optic nerve (CN II)
Nervus opticus
Also known as: CN II, second cranial nerve
The nerve of vision (purely sensory, SSA). It is formed by about 1.2 million retinal ganglion cell axons running from the optic disc through the optic canal to the optic chiasm. Developmentally it is a CNS tract, not a peripheral nerve.
Overview
The optic nerve is made of the axons of retinal ganglion cells. They converge on the optic disc, which is the physiological blind spot, and pierce the sclera through the lamina cribrosa. Behind the globe the axons become myelinated by oligodendrocytes, not Schwann cells. The nerve is surrounded by all three meningeal layers, and the subarachnoid space around it is continuous with the intracranial subarachnoid space. This explains three things:
• the optic nerve is affected by CNS demyelinating disease such as multiple sclerosis;
• it cannot regenerate after injury;
• raised intracranial pressure is transmitted to the optic nerve head and causes papilloedema.
The nerve is about 4–5 cm long and has four parts: intraocular, intraorbital, intracanalicular and intracranial. The orbital part is slightly S-shaped, which allows eye movement. The nerve passes through the optic canal of the lesser wing of the sphenoid together with the ophthalmic artery. At the optic chiasm, above the pituitary gland, the fibres from the nasal half of each retina, which see the temporal visual fields, cross to the opposite side. Each optic tract therefore carries the opposite visual hemifield to the lateral geniculate nucleus of the thalamus. From there the optic radiations run to the primary visual cortex around the calcarine sulcus. A small number of fibres leave to reach the pretectal nuclei (pupillary light reflex), the superior colliculus and the suprachiasmatic nucleus.
- Location
From the back of the eyeball, through the orbit within the cone of extraocular muscles and the optic canal, to the optic chiasm above the sella turcica.
Parts & subdivisions
- Functional componentsSensory only, SSA (vision). It also carries the afferent limb of the pupillary light and accommodation reflexes.
- Clinical testing- Visual acuity (Snellen chart, each eye, corrected with glasses or pinhole). • Colour vision (Ishihara plates; red desaturation in optic neuritis). • Visual fields by confrontation. • Pupillary reflexes, including the swinging-light test for a relative afferent pupillary defect (Marcus Gunn pupil). • Fundoscopy of the optic disc (papilloedema, optic atrophy, cupping).
- - Optic nerve: ipsilateral monocular visual loss and a relative afferent pupillary defect. • Optic chiasm (central compression): bitemporal hemianopia. • Optic tract: contralateral homonymous hemianopia (incongruous). • Meyer's loop (temporal lobe): contralateral superior quadrantanopia, "pie in the sky". • Parietal radiation: contralateral inferior quadrantanopia, "pie on the floor". • Occipital cortex (PCA stroke): contralateral homonymous hemianopia with macular sparing.
Anatomy
- Arterial supply
- Ophthalmic artery: pial branches, plus the central retinal artery for the inner retina and the anterior part of the nerve.
• Short posterior ciliary arteries: the optic nerve head (circle of Zinn–Haller).
• Intracranially, the anterior cerebral, anterior communicating and superior hypophyseal arteries supply the nerve and chiasm.- Course
Retinal ganglion cells → optic disc → lamina cribrosa → intraorbital nerve (inside the muscle cone; the central retinal artery enters it about 1 cm behind the globe) → optic canal (with the ophthalmic artery) → optic chiasm (nasal fibres decussate) → optic tract → lateral geniculate nucleus → optic radiation (inferior fibres loop through the temporal lobe as Meyer's loop; superior fibres run through the parietal lobe) → primary visual cortex (area 17).
- Relations
- In the orbit: surrounded by the four rectus muscles; the ciliary ganglion lies laterally.
• In the optic canal: the ophthalmic artery lies inferolaterally, and the sphenoid sinus lies medially (risk during sinus surgery).
• At the chiasm: the pituitary gland lies below, the third ventricle behind, and the internal carotid arteries laterally.
Function
- Conveys visual information from the retina to the lateral geniculate nucleus and visual cortex
- Afferent limb of the pupillary light reflex (to the pretectal nuclei)
- Afferent limb of the accommodation (near) reflex
- Retinohypothalamic input to the suprachiasmatic nucleus that entrains the circadian rhythm
Clinical significance
Bitemporal hemianopia
A pituitary macroadenoma growing up out of the sella compresses the crossing nasal fibres in the middle of the chiasm. This causes loss of both temporal fields, often starting as a superior bitemporal defect because the inferior crossing fibres are compressed first. Craniopharyngioma, which presses from above, tends to cause an inferior defect first.
Optic neuritis
Inflammatory demyelination of the optic nerve causes subacute, painful (pain on eye movement) monocular loss of vision, reduced colour vision (red desaturation), a central scotoma and a RAPD. The disc may look normal at first (retrobulbar neuritis). It is the presenting event in about 20% of patients with multiple sclerosis and is also seen in NMOSD (AQP4) and MOG antibody disease.
Papilloedema
Swelling of the optic disc from raised intracranial pressure (tumour, idiopathic intracranial hypertension, venous sinus thrombosis). Pressure in the subarachnoid sheath impairs axoplasmic flow. It is usually bilateral, with an enlarged blind spot and preserved acuity early on. Its presence is a contraindication to lumbar puncture until imaging has been done.
Mnemonics
Meyer's loop
"PITS": Parietal → Inferior quadrant, Temporal → Superior quadrant (contralateral homonymous quadrantanopias).
Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.