Overview
The optic nerves (CN II) are paired, cylindrical structures extending from the posterior part of the eyeball to the suprasellar space. It contains only afferent sensory fibres and is responsible for transmitting visual information from the retina to the primary visual cortex of the brain. The extraorbital segments are surrounded by all three cranial meninges, rather than epi-, peri- and endoneurium like most other nerves. Additionally, the extraorbital segments are myelinated by oligodendrocytes, not Schwann cells like the axons of peripheral nerves.
Origin
Originates from rods and cones from the retina which activate bipolar cells which synapse with ganglia cells.
Segments & Course
Intraocular segment
- Unmyelinated, deep to the sclera of the eye
- Formed by the convergence of axons from retinal ganglion cells at the optic disc
- Exit the eye through scleral fenestrations termed lamina cribrosa
Intraorbital segment
- Myelinated and encased by all three meningeal layers and cerebral spinal fluid
- Lies inferior to the ophthalmic artery as it passes medially
- Passes posteromedially through intraconal space, along the axis of orbit, into optic canal
- Central retinal artery & vein enters the optic nerve 1 cm posterior to the globe
Intracanalicular segment
- The portion within the bony optic canal, formed by the lesser wing and body of the sphenoid bone
- Lies superior to the ophthalmic artery (previously inferiorly)
- Dura surrounding optic nerve fuses with the orbital periosteum
Intracranial segment
- Enters middle cranial fossa at the lateral border of tuberculum sellae (anterior part of the hypophyseal fossa)
- Passes posteromedially, inferior to A1 segments of anterior cerebral arteries
- Converges with contralateral optic nerve to form optic chiasm in suprasellar cistern anterior to the pituitary infundibulum, between the terminal internal carotid arteries
- Forms floor of the third ventricle
- Ophthalmic artery is inferolateral to the nerve
Optic chiasm
- Nasal retinal fibres decussate to the contralateral optic tract
- Temporal fibres do not decussate
Termination
Each optic tract passes posteriorly around the cerebral peduncle to relay to 3 destinations:
- Lateral geniculate nucleus – located within the body of the thalamus. Axons from the lateral geniculate nucleus then carry visual information via a pathway known as the optic radiation
- Upper optic radiation – carries fibres from the superior retinal quadrants (corresponding to the inferior visual field quadrants). It travels through the parietal lobe to reach visual cortex.
- Lower optic radiation – carries fibres from the inferior retinal quadrants (corresponding to the superior visual field quadrants). It travels through the temporal lobe via a pathway known as Meyers’ loop to reach the visual cortex.
- Superior colliculus
- Pretectal nuclei
Relations
- Intraocular segment: adjacent to oculomotor nerve, trochlear nerve and the nasociliary nerve. Superomedial to the ophthalmic artery.
- Intracanalicular: Earlier in its course, orbital fat separates the optic nerve from the surrounding extraocular muscles. However, the nerve becomes more closely related to the annulus of Zinn, where the four recti originate.
Blood supply
- Ophthalmic artery (first branch of the internal carotid arteries, arising from the ophthalmic (C6) segment) – supplies the retina and extracranial segment of the optic nerve
- Anterior cerebral, superior hypophyseal and anterior communicating arteries – supply the intracranial segment and optic chiasm
- Medial lenticulostriate arteries (branch of anterior cerebral artery) – supplies the dorsal aspect of the optic chiasm
- Posterior communicating and anterior choroidal arteries – optic tract
- Anterior choroidal artery – most distal branch of the internal carotid artery, originates just after the origin of the posterior communicating artery
- Anterior and posterior choroidal arteries – supply the lateral geniculate nucleus
- Posterior choroidal artery – branch of the posterior cerebral artery
- Middle cerebral artery and posterior cerebral artery – supply the otic radiation
- Posterior cerebral artery – supplies the primary visual cortex
Lymphatics
- Followed by the lymphatic system to drain a part of the cerebrospinal fluid
Venous Drainage
- Ophthalamic veins drain the back, top and bottom of the orbit
- Central retinal vein drains into superior ophthalmic vein
Variants
- Onodi air cells surround intracanalicular segment
- Pneumatised lesser wing of sphenoid
- Middle meningeal artery origin of ophthalmic artery
- Absent ACOM
Development
- Developed from the optic vesicle, an outpocketing of the forebrain.
- At week three of gestation, the appearance of optic grooves from the developing forebrain marks the first sign of eye development. As the neural folds fuse, the optic grooves evaginate, forming the optic vesicle.
- Subsequently, the optic vesicle invaginates and forms the optic cup at about four weeks of gestation; this will become the retina. The inner and outer layers are due to the invagination process. They form the pigmented and the neural layers, respectively.
- Axons of the neural layer then proliferate into the optic stalk, causing obliteration of the lumen; this forms the optic nerve.
Surgical considerations
- Optic nerve is commonly injured during posterior ethmoidectomy and sphenoid dissection