Forebrain (telencephalon and diencephalon)
Origin · CN I CN II
The only two cranial nerves that do not arise from the brainstem. Olfactory axons synapse in the olfactory bulb, an outgrowth of the telencephalon, and the optic nerve is a tract of the diencephalon, so both are central nervous tissue (oligodendrocyte myelin, meningeal sheath) rather than true peripheral nerves.
Detail
- Gives rise to
- CN I (olfactory bulb and tract), CN II (retina to optic chiasm)
- Consequence
- Both behave as CNS tracts: optic neuritis in multiple sclerosis, papilloedema with raised intracranial pressure, no regeneration after transection
When it goes wrong
Olfactory groove (subfrontal) meningioma compressing the olfactory tract and then the optic nerve
Progressive unilateral then bilateral anosmia with personality change; ipsilateral optic atrophy and contralateral papilloedema (Foster Kennedy syndrome)
Practise this structure
4 questions in the bank tagged Cranial Nerves.
A 52-year-old woman has had two months of numbness over the right forehead, the right upper eyelid and the tip of her nose, with a gritty feeling in the right e…
Foundations · middle twist
A 44-year-old man has had 18 months of worsening hearing on the left with constant ringing in that ear. He still follows conversation in a quiet room but cannot…
Foundations · middle twist
A 58-year-old man has sudden vertigo and a hoarse voice, and chokes when he drinks. The palate elevates to the right and there is no gag response when the left…
Foundations · middle twist
A 27-year-old man was struck on the head at football a week ago. He now sees two images, one above the other, worst when reading or going down stairs. The verti…
Foundations · middle twist