Olfactory nerve (CN I)
Nerve · CN I Sensory
About twenty bundles of unmyelinated axons from bipolar receptor cells in the olfactory epithelium of the nasal roof pass up through the cribriform plate to synapse in the olfactory bulb. Purely sensory (smell) and the only sensory pathway that reaches cortex without a thalamic relay.
Traced from the start
- Forebrain (telencephalon and diencephalon)
- Olfactory nerve (CN I)
Detail
- Sensory
- Smell from the olfactory epithelium of the upper nasal cavity
- Pathway
- Receptor cell → cribriform plate → olfactory bulb → olfactory tract → piriform cortex, amygdala and entorhinal cortex (no thalamic relay)
- Test
- Each nostril separately with a familiar non-irritant odour (irritants such as ammonia test V, not I)
When it goes wrong
Head injury with acceleration–deceleration shearing the olfactory filaments at the cribriform plate
Anosmia, often permanent, sometimes with CSF rhinorrhoea when the plate is fractured
Medial temporal lobe seizure focus involving the uncus
Brief unpleasant olfactory hallucinations (uncinate fits) preceding a focal impaired-awareness seizure
Congenital failure of olfactory neurone migration with GnRH neurones (Kallmann syndrome)
Anosmia with hypogonadotrophic hypogonadism and delayed puberty
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