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Cranial nerves · Sensory

I. Olfactory nerve

Not medically reviewed · Content last updated · 2 min read ·

This is the date the draft was written, not a medical review date.

The olfactory nerve (CN I) is the special sensory nerve of smell. It consists of many small fila olfactoria formed by the axons of olfactory receptor neurons of the nasal roof, which synapse in the olfactory bulb.

Terminologia Anatomica (Latin): Nervus olfactorius

Function

Special visceral afferent (smell) only. Olfactory information reaches the primary olfactory cortex (piriform cortex, amygdala) without a compulsory thalamic relay, which is unusual among sensory systems.

Nuclei

The first-order neurons are the bipolar receptor cells of the olfactory epithelium. Their axons end on mitral and tufted cells in the olfactory bulb, whose axons form the olfactory tract and project to the anterior olfactory nucleus and primary olfactory cortex.

Exit from the skull

The fila olfactoria pass through the foramina of the cribriform plate of the ethmoid bone to reach the olfactory bulb lying on its upper surface in the anterior cranial fossa.

Testing

Test each nostril separately with the other occluded, using a mild non-irritant familiar odour (for example coffee or soap) with the eyes closed. Avoid pungent substances such as ammonia, which stimulate the trigeminal nerve instead.

Signs of a lesion

Unilateral or bilateral loss of smell (anosmia) or reduced smell (hyposmia) ipsilateral to the lesion. Taste is often reported as impaired because flavour depends largely on smell. Foster Kennedy syndrome combines ipsilateral anosmia and optic atrophy with contralateral papilloedema from a frontal mass.

Clinical correlations

Common causes of anosmia are upper respiratory tract infection and nasal obstruction (conductive), head trauma with shearing of the fila at the cribriform plate, and neurodegenerative disease such as Parkinson disease. Anterior skull base fractures may cause CSF rhinorrhoea. Olfactory groove meningioma is a classic mass lesion.

Common exam points

  • Smell is the only sensory modality that reaches the cortex without a thalamic relay.
  • Fila pass through the cribriform plate; trauma here causes anosmia and possible CSF leak.
  • Test one nostril at a time with non-irritant odours, not ammonia.
  • Anosmia: infection, nasal obstruction, trauma, frontal tumour, Parkinson disease.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Haines DE, Mihailoff GA. Fundamental Neuroscience for Basic and Clinical Applications. Elsevier, 5th edition (2018) — Tier 3 (textbook)

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