Makindo Medical Notes

Related Subjects: |Olfactory Nerve |Optic Nerve |Oculomotor Nerve |Trochlear Nerve |Trigeminal Nerve |Abducent Nerve |Facial Nerve |Vestibulocochlear Nerve |Glossopharyngeal Nerve |Vagus Nerve |Accessory Nerve |Hypoglossal Nerve

👃 Olfactory Nerve (Cranial Nerve I)

The olfactory nerve is the first cranial nerve and is responsible for the sense of smell. It is unique because its fibres are directly exposed to the external environment, making it vulnerable to injury and infection. Unlike other cranial nerves, it is considered a CNS tract (extension of the forebrain).

📍 Anatomy

• Origin: Arises from the olfactory epithelium in the upper nasal cavity.
• Course:
  • Unmyelinated axons pass through the cribriform plate of the ethmoid bone.
  • Synapse in the olfactory bulb → continue as the olfactory tract.
  • Projects to the primary olfactory cortex, amygdala, and limbic system (hence smell is strongly tied to memory & emotion).

🧠 Functions

• Sensation of smell (olfaction).
• Detection of airborne chemicals (odorants) → processed in olfactory cortex.
• Links with taste and limbic pathways → influences appetite, behaviour, and memory.

⚕️ Clinical Relevance

• Anosmia: Loss of smell.
  • Causes: URTIs (esp. COVID-19 🦠), head trauma (cribriform fracture → CSF rhinorrhoea), neurodegeneration (Alzheimer’s, Parkinson’s), nasal obstruction, toxins, medications.
  • Impact: Loss of taste (flavour perception) and safety risk (gas, smoke detection).
• Hyposmia: Reduced smell.
  • Causes: Ageing, smoking, sinonasal disease, toxins.
• Phantosmia: Olfactory hallucinations.
  • Causes: Temporal lobe seizures, tumours, trauma, infections.
• Olfactory Testing:
  • Tested with non-irritant odorants (e.g. coffee, peppermint, vanilla).
  • Avoid pungent stimuli (ammonia, alcohol) which stimulate trigeminal nerve (CN V).

📌 Summary

The olfactory nerve (CN I) mediates smell via fibres from the nasal epithelium → olfactory bulb → cortex. Damage may cause anosmia, hyposmia, or phantosmia, with links to head trauma, infections, and neurodegenerative disease. It has strong connections to the limbic system, explaining why smells trigger vivid memories. In exams, always remember: cribriform plate fracture → CSF leak + anosmia.