Optic nerve (CN II)
Nerve · CN II Sensory
Axons of retinal ganglion cells leave the orbit through the optic canal with the ophthalmic artery, then partially decussate at the chiasm to form the optic tracts. It is a CNS tract wrapped in meninges and myelinated by oligodendrocytes, so it shares CNS diseases and does not regenerate.
Traced from the start
- Forebrain (telencephalon and diencephalon)
- Optic nerve (CN II)
Detail
- Sensory
- Vision; afferent limb of the pupillary light reflex (to the pretectal nucleus) and of accommodation
- Pathway
- Retina → optic canal → chiasm (nasal fibres cross) → optic tract → lateral geniculate nucleus → optic radiation → calcarine cortex
- Sheath
- Dura, arachnoid and pia continuous with the brain; the subarachnoid space around the nerve transmits intracranial pressure to the disc
When it goes wrong
Optic neuritis (demyelination, often the first attack of multiple sclerosis)
Subacute monocular visual loss with pain on eye movement, impaired colour vision and a relative afferent pupillary defect; disc normal (retrobulbar) or swollen
Complete transection or infarction of one optic nerve
Monocular blindness: no direct response when that eye is lit, but its pupil still constricts consensually when the other eye is lit
Pituitary adenoma pressing on the crossing nasal fibres at the chiasm
Bitemporal hemianopia, typically starting in the upper temporal fields
Raised intracranial pressure transmitted along the optic nerve sheath
Bilateral papilloedema with an enlarged blind spot and transient visual obscurations; acuity preserved until late
Practise this structure
4 questions in the bank tagged Cranial Nerves.
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