Cranial nerve II, made of the axons of about a million retinal ganglion cells
Optic nerve — the second cranial nerve.
Anatomy · Brain & nerves
Optic nerve
Cranial nerve II
Cranial nerves · special sense (vision)
The second cranial nerve: a cable of about a million retinal ganglion cell axons that carries visual information from the back of each eye to the brain. It leaves the eyeball at the optic disc, passes through the optic canal into the skull, and meets its partner at the X-shaped optic chiasm, where fibres from the inner half of each retina cross. Unusually, it is really an outgrowth of the brain, wrapped in meninges, and it cannot regenerate once cut.
How to spot them
- ▸A pale cord about 5 cm long running from the back of the eyeball through the bony orbit, in a sleeve of the same three membranes that wrap the brain
- ▸Leaves the eye at a small round spot, the optic disc, where there are no light receptors: the blind spot
- ▸Passes through a bony canal at the apex of the orbit and joins its fellow at an X-shaped crossing
- ▸The central retinal artery and vein run inside it for part of its length
What to know
Where it leaves the eyeball there are no light receptors, creating the physiological blind spot
Optic nerve — the optic disc.
About half of the fibres from each eye cross the midline at an X-shaped junction just above the pituitary
Optic nerve — at the optic chiasm, nasal-retina fibres cross.
Swelling of the disc where it enters the eye, seen with an ophthalmoscope, signals raised pressure inside the skull
Optic nerve — papilloedema, because it is surrounded by CSF-filled meninges.
Strictly it is a tract of the central nervous system, myelinated by oligodendrocytes and unable to regenerate after cutting
Optic nerve — an outgrowth of the diencephalon.
Inflammation of it, often the first sign of multiple sclerosis, causes painful loss of vision
Optic nerve — optic neuritis.
Passes through the optic canal, a hole at the apex of the orbit in the lesser wing of the sphenoid
Optic nerve — the ophthalmic artery runs through the canal beneath it.
Raised pressure inside the eye slowly damages its fibres, causing tunnel vision — the main danger of glaucoma
Optic nerve — glaucoma is optic neuropathy.
Key ideas
- Because the nerve is wrapped in meninges, cerebrospinal fluid surrounds it, so raised pressure inside the skull swells its head — papilloedema.
- At the chiasm, fibres from the nasal half of each retina cross, so each side of the brain receives the opposite half of the visual field.
Quick-fire facts
- •Cranial nerve II; the twelve pairs are olfactory (I), optic (II), oculomotor (III), trochlear (IV), trigeminal (V), abducens (VI), facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX), vagus (X), accessory (XI) and hypoglossal (XII)
- •Contains roughly 1 to 1.2 million axons and is about 5 cm long
- •Formed by axons of retinal ganglion cells, and part of the central nervous system in origin, so it is myelinated by oligodendrocytes and lacks a regenerative sheath
- •The optic disc is the blind spot, about 1.5 mm across; there are no rods or cones there
- •Passes through the optic canal alongside the ophthalmic artery
- •Fibres cross at the optic chiasm, sitting just above the pituitary gland; a pituitary tumour classically causes bitemporal hemianopia
- •After the chiasm, fibres form the optic tracts, ending at the lateral geniculate nucleus of the thalamus
- •Optic neuritis is a common first presentation of multiple sclerosis
- •Glaucoma is an optic neuropathy in which raised eye pressure damages its fibres
Drill it — 7 cards
1 / 7new card