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Cranial nerves · Sensory

II. Optic nerve

Not medically reviewed · Content last updated · 2 min read ·

This is the date the draft was written, not a medical review date.

The optic nerve (CN II) carries visual information from the retina to the brain. Developmentally it is an outgrowth of the diencephalon, so it is a CNS tract covered by meninges and myelinated by oligodendrocytes rather than a true peripheral nerve.

Terminologia Anatomica (Latin): Nervus opticus

Function

Special somatic afferent (vision). Retinal ganglion cell axons convey light information; the nasal retinal fibres cross at the optic chiasm, so each optic tract carries the contralateral visual hemifield. Afferent limb of the pupillary light reflex.

Nuclei

Origin: retinal ganglion cells. Main termination: lateral geniculate nucleus of the thalamus (relay to the primary visual cortex via the optic radiations). Other targets: pretectal nuclei (pupillary reflex), superior colliculus (visual reflexes) and suprachiasmatic nucleus (circadian rhythm).

Exit from the skull

The nerve leaves the orbit through the optic canal of the lesser wing of the sphenoid, accompanied by the ophthalmic artery and the meningeal sheaths, and joins its fellow at the optic chiasm above the pituitary gland.

Testing

Test visual acuity (Snellen chart, with correction), colour vision (Ishihara plates), visual fields by confrontation, the direct and consensual pupillary light reflexes, the swinging-light test for a relative afferent pupillary defect, and fundoscopy of the optic disc.

Signs of a lesion

Optic nerve lesion: monocular visual loss with a relative afferent pupillary defect and reduced colour vision. Chiasmal lesion: bitemporal hemianopia. Optic tract or radiation lesion: contralateral homonymous hemianopia; temporal lobe (Meyer loop) lesions give a superior quadrantanopia, parietal lesions an inferior one.

Clinical correlations

Optic neuritis, often associated with multiple sclerosis, presents with subacute painful monocular visual loss. Papilloedema (bilateral disc swelling from raised intracranial pressure) usually preserves acuity early. Pituitary adenomas compress the chiasm. Ischaemic optic neuropathy and glaucoma are other important causes of optic nerve damage.

Common exam points

  • The optic nerve is a CNS tract; it is part of the diencephalon, hence affected by multiple sclerosis.
  • Afferent limb of the light reflex; a relative afferent pupillary defect indicates optic nerve disease.
  • Chiasmal lesion: bitemporal hemianopia; optic tract or radiation lesion: contralateral homonymous hemianopia.
  • Passes through the optic canal with the ophthalmic artery.
  • Papilloedema indicates raised intracranial pressure; examine the disc by fundoscopy.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Kandel ER, Koester JD, Mack SH, Siegelbaum SA. Principles of Neural Science. McGraw-Hill, 6th edition (2021) — Tier 3 (textbook)
  4. Bickley LS. Bates' Guide to Physical Examination and History Taking (Bickley). Wolters Kluwer, 13th edition (2020) — Tier 3 (textbook)

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