Olfactory Nerve – Cranial Nerve I/1

Introduction

• Function like indicated by name: olfaction i.e. smell.
• One of the 2 cranial nerves that do not have nuclei in the brainstem and do not enter it (the other is the Optic nerve-cranial nerve 2).

Functions in General:

• SVA (special visceral afferent): it carries the sense of smell (also called olfaction)

Route

• Nerves arise from the olfactory receptors in the olfactory mucous of the upper part of the nasal cavity (above superior concha).

Figure 1: Origin of the olfactory nerve - sagittal view

• Nerve fibers pass through openings of the cribriform plate of the ethmoid bone to enter the skull.
• The nerve fibers now synapse with secondary neurons in the olfactory bulb.
•  The secondary nerve fibers then continue in olfactory tract, which runs at the base of the frontal lobe of the cerebrum.

Figure 2: Real brain specimen showing the base of the brain with the olfactory nerves

• The olfactory tract splits into two tracts called the medial and lateral olfactory striae.
• Lateral stria: axons to olfactory area of cerebral cortex. Olfactory areas of cerebral cortex collectively called rhinencephalon.
• Medial stria: carries fibers to the olfactory bulb of the opposite side.

Figure 3: Base of the brain showing the path of the olfactory nerves

The primary olfactory cortex sends nerve fibers to other brain centres to establish emotional connections to olfactory sensations.

Entry into skull

Figure 4: Closeup of the inside of the cranial cavity showing the entry of the olfactory nerve into the skull through the cribriform plate 

Nerve	Entry	Enters
Olfactory (CN I)	Openings in cribriform plate of ethmoid bone	Anterior cranial cavity

Testing for Nerve Lesion

• Close one nostril and let the patient try to identify a familiar smell (e.g. vanilla) with the other nostril. Repeat for both nostrils.
• If patient cannot identify smell then he has anosmia.
• Cause: could be an ethmoid bone fracture. 

References

Drake, R., Vogl, A., & Mitchell, A. (2010). Chapter 8: Head and neck – Cranial Nerves. Gray's anatomy for students (2nd ed., pp. 848-957). Philadelphia: Elsevier: 

Snell, R. (2007). Chapter 15: The Cranial Nerves and Trigeminal Nerve Blocks. Clinical anatomy by systems (pp. 555-575). Philadelphia: Lippincott Williams & Wilkins.

Snell, R. (2010). The cranial nerve nuclei and their central connections and distribution. Clinical neuroanatomy (7th ed., pp. 331-370). Philadelphia: Lippincott Williams and Wilkins.

Picture references

1. Derived from: By Patrick J. Lynch, medical illustrator. (labeled by was_a_bee) [CC-BY-2.5 (http://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons
2. Derived from: By Neuropathologyblog (Took Picture) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
3. Gray's Anatomy via Wikimedia Commons
4. Unknown (if you know source please message me so I can mention it)