Olfactory Nerve (CN I) and Smell

Background

• CN I/Olfactory nerve is the shortest cranial nerve and consists of special unmyelinated sensory nerves responsible for sense of smell
  • They are, however, covered in Schwann cells

The olfactory pathway

• Odourant molecules can enter your system via your nose or mouth and nasopharynx.
• They reach an area called the olfactory mucosa in the nasal cavity
  • Here, they come into contact with the olfactory receptor neurons and activate receptors to initiate an action potential
    • The axons of these cells traverse the cribiform plate of the ethmoid bone at the roof of the nasal cavity and become the olfactory bulb.
  • In the bulb, these neurons communicate with specialised mitral cells at the synaptic glomeruli.  These then pass posteriorly into the olfactory tract
    • This runs along the inferior aspect of the frontal lobe.  At the anterior perforated substance, the tract divides into medial and lateral stria.
      • Medial stria connects to the limbic system and communicates with the contralateral olfactory medial stria
      • Lateral stria continues to the primary olfactory cortex in the temporal lobe, which goes on to communicate further with the limbic system (amygdala, piriform cortex and olfactory tubercle) and orbitofrontal cortex.
• Note
  • CN I is covered by pia and arachnoid layers (i.e. continuation of the brain).  It does also not join with the brainstem.
  • The olfactory nerve is capable of regeneration.

Olfactory Dysfunction (Anosmia)

• Temporary anosmia is not uncommon in local conditions of the nose e.g. infection.
• Other causes of anosmia/abnormal sensation of smell include
  • Tumours in the olfactory groove (meningioma)
  • Head injury- damage to the cribiform plate may cause damage to the olfactory receptors
  • Neurodegenerative disease e.g. Parkinson’s Disease, Huntington’s and Alzheimer’s disease
  • Genetic conditions e.g. Kallman Syndrome, Primary ciliary dyskinesia, Foster Kennedy Syndrome
  • Partial epilepsy (pre- and post-ictal)

Testing CN I

• Ask the patient if they have noticed a change in their sense of smell.
• A more formal assessment of smell can involve using common smells