Olfactory nerve

Special sensory function

Afferent system for the sense of smell.

Anatomy

1. Transethmoidal segment

The unmyelinated central processes of bipolar receptor cells in the nasal epithelium form ~20 fascicles (fila olfactoria) on each side of the nasal cavity. The fascicles traverse the cribriform plate and synapse at the olfactory bulb.

2. Olfactory bulb and tract

Bipolar processes synapse at the olfactory bulb with the second order neurons (mitral cells and tufted cells). The mitral cell axons form the olfactory tract, which is an extension of the brain and not an actual nerve.

The olfactory tract trifurcates into medial, intermediate and lateral stria thus forming the olfactory trigone.

3. Central pathways

The medial olfactory stria terminates in the parolfactory area of Broca in front of the subcallosal gyrus. Some fibers terminate in the subcallosal gyrus and in the anterior perforated substance. A few fibers cross in the anterior commissure to the opposite olfactory tract.

The intermediate olfactory stria (smallest of the three) terminates in the anterior perforated substance.

The lateral olfactory stria carries the majority of the fibers. It terminates in the prepiriform area anterior to the uncus and amygdala. It gives off collaterals to subfrontal and frontal olfactory cortex as well as subthalamic nuclei, thalamus and stria medullaris.

Clinical Implications

• Most causes of anosmia affect the nerves bilaterally.
• Commonest cause of bilateral anosmia is upper respiratory tract infection. Less common causes include tumors affecting olfactory groove or ethmoid (eg meningioma, esthesioblastoma), trauma, pituitary surgery, nasal polyps, neurodegenerative (eg Parkinson’s and Alzheimer’s disease) and congenital (eg Kallman’s syndrome).
• Causes of unilateral anosmia include trauma and tumors affecting olfactory groove.