Optic nerve

The optic nerveis the second (cranial nerve II, a.k.a. CN II) is the second cranial nerve. It is a purely sensory nerve that conveys visual information from the eye to the brain. 

The nerve arises from the back of the globe exiting the orbit via the optic canal. It joins the contralateral optic nerve at the optic chiasm where medial fibres decussate before continuing as the optic tracts. 

The cells of origin consist of the ganglion cells of the retina with the main central connections consisting of the lateral geniculate nucleus of the thalamus,  and the pretectal area of the midbrain.

Gross anatomy

Along with the olfactory nerve (CN I), is really an extension of the central nervous system. It is not surrounded by Schwann cells with the first sensory bipolar cell body located peripherally in the retina. Their central processes synapse on ganglion cells on the vitreous surface of the retina and their central processes pass via the optic disc out of the globe and form the optic nerve proper.The optic nerve is divided into four segments:

Intraocular segment

Lies within the retina and emerges through a scleral opening (lamina cribrosa)

Intraorbital segment

Passes posteriorly and centrally within the orbit and is surrounded by dural lining and CSF; hence it directly communicates with the subarachnoid space and therefore allows transmission of increased pressure from hydrocephalus to manifest as papilloedema; additionally, the dural covering can develop a meningioma

Intracanalicular segment

Where the optic nerve exits through the tendinous ring and optic canal superior to the ophthalmic artery

Intracranial segment

Also known as the cisternal segment,​ enters the middle cranial fossa and passes within the suprasellar cistern with the anterior cerebral artery at its superolateral aspect joining the contralateral optic nerve at the optic chiasm

At the optic chiasm, the nasal fibres of each optic nerve (fibres carrying light impulses from the nasal side of the retina) decussate while the temporal fibres do not (partial decussation). From the optic chiasm arise two optic tracts, each one containing nasal fibres of the contralateral optic nerve and temporal fibres from the ipsilateral optic nerve. The optic tract courses around the cerebral peduncle to relay in the lateral geniculate body of the thalamus.

Arising from the lateral geniculate body of the thalamus, the optic radiations are divided into superior and inferior bundles. The superior bundle carries information from the superior retinal quadrant that represents the inferior visual field and ends at the superior aspect of calcarine sulcus (cuneus). The inferior bundle (including Meyer loop) carries information from the inferior retinal quadrant which represents the superior visual field. It passes anteriorly in the temporal lobe and forms the lateral wall of the inferior horn of lateral ventricle before passing posteriorly to end inferior to the calcarine sulcus (lingual gyrus).

Arterial supply

Intraocular, intraorbital, and intracanalicular segments are supplied by the ophthalmic artery and its branch, the central retinal artery.

Small branches of the anterior cerebral artery (ACA) and the superior hypophyseal artery supply the intracranial segment of the optic nerves and optic chiasm.

The optic tracts are supplied by small branches of the anterior choroidal and PCOM arteries.

Variant anatomy

According to a study by Delano et al., the course of the optic nerve in relation to the sphenoid sinus was classified according to four types ⁶:

• type 1: most common (76%): the optic nerve is immediately adjacent to the lateral or superior wall of the sphenoidal sinus, without impression on the sinus wall
• type 2: (15%): nerve causes an impression on the lateral sphenoidal sinus wall
• type 3: (6%): nerve courses through the sphenoidal sinus rather than simply running adjacent to the sinus
• type 4: (3%) nerve courses immediately lateral to the posterior ethmoidal and sphenoidal sinuses

Related pathology

• optic nerve glioma
• optic nerve meningioma
• optic nerve enlargement