Overview
This lecture covers the anatomy of the optic nerve, detailing its origin, course, divisions, and key anatomical relationships.
Visual Pathway Overview
- The visual pathway starts at the retina, then passes through the optic nerve, optic chiasma, optic tracts, lateral geniculate body, optic radiations, and ends at the visual cortex.
- The optic nerve extends from the optic disc to the optic chiasma as a continuation of the retina's nerve fiber layer.
Basic Features of the Optic Nerve
- The optic nerve is a sensory nerve responsible for vision, including light sense, color sense, contrast sensitivity, form sense, and visual fields.
- Unlike peripheral nerves, the optic nerve lacks neurilemma (does not regenerate), contains finer fibers, and is covered by meninges.
Divisions of the Optic Nerve
- The optic nerve is about 50 mm and has four parts: intraocular (1 mm), intraorbital (25 mm), intracanalicular (9 mm), and intracranial (16 mm).
Intraocular Part
- Present inside the eye, passes through the sclera’s fenestrations called lamina cribrosa to form the optic disc.
- Diameter in eye: ~1.5 mm; outside sclera: ~3 mm due to myelination.
- Subdivided into surface nerve fiber layer, prelaminar region, laminar region, and retrolaminar region (myelination starts in the retrolaminar region).
Intraorbital Part
- Extends from the eye to the optic foramen, is about 25 mm, and is sinuous to allow eye movement.
- Surrounded by the annulus of Zinn, from which several extraocular muscles originate.
- Painful ocular movements (e.g., in retrobulbar neuritis) are due to involvement of adjacent muscles.
Vascular and Neural Relationships
- The central retinal artery, from the ophthalmic artery, enters the optic nerve from the inferomedial aspect.
- The ophthalmic artery changes position relative to the optic nerve from inferior, to lateral, to medial.
- The intraorbital part is close to the ciliary ganglion and branches of the oculomotor, abducens, nasociliary, and sympathetic nerves.
Intracanalicular Part
- Located within the optic canal, about 9 mm long.
- Close to the sphenoid and ethmoid sinuses, separated by a thin bony lamina, making it susceptible to trauma and sinus infections.
Intracranial Part
- About 16 mm long, located above the cavernous sinus, ends at the optic chiasma (site of fiber crossing).
- Related to the internal carotid artery (below and lateral) and anterior cerebral arteries (above).
- Contains only the pia mater; intraorbital and intracanalicular parts have all meningeal layers.
Meningeal Coverings
- Subarachnoid and subdural spaces of the optic nerve are continuous with those of the brain, explaining papilledema in raised intracranial pressure.
- Meninges end and fuse with the sclera at the eye; dura mater splits at the apex of the orbit.
Key Terms & Definitions
- Optic Disc — Starting point of the optic nerve in the eye.
- Lamina Cribrosa — Fenestrated scleral region allowing passage of optic nerve fibers.
- Annulus of Zinn — Tendinous ring giving origin to extraocular muscles.
- Papilledema — Swelling of the optic disc due to raised intracranial pressure.
- Retrobulbar Neuritis — Inflammation of the optic nerve behind the eyeball causing pain with eye movement.
Action Items / Next Steps
- Review the visual pathway in more detail in a future lecture.
- If unclear, submit any questions on the lecture content in the comments section.