Olfactory Nerves: Sensory Perception of Smell - Definition & Function

The olfactory nerves (Cranial Nerve I) are special sensory nerves dedicated solely to the sense of smell (olfaction). They are unique among cranial nerves: not true nerves but CNS tracts.

Anatomy

• Olfactory Epithelium: Superior nasal cavity (cribriform plate area). Contains olfactory receptor neurons (bipolar), supporting cells, basal stem cells.
• Receptor Neurons: Express odorant receptors (GPCRs). Each neuron expresses one receptor type. Axons pass through cribriform plate foramina.
• Olfactory Bulb: Axons synapse in glomeruli (one glomerulus per receptor type). Mitral/tufted cells project via olfactory tract.
• Central Pathways: Primary olfactory cortex (piriform cortex, amygdala, entorhinal cortex) → orbitofrontal cortex (conscious perception), hypothalamus/limbic (emotional/memory links).

Function

• Detection: Odorants dissolve in mucus → bind receptors → cAMP cascade → action potential.
• Discrimination: Combinatorial coding—one odorant activates multiple receptors; one receptor binds multiple odorants.
• Adaptation: Rapid (receptor) and central (cortical) – smell fades with continuous exposure.
• Flavor: Retronasal olfaction + taste = flavor. Anosmia → bland food.

Clinical Significance

• Anosmia/Hyposmia: Post-viral (COVID-19), traumatic (cribriform fracture), neurodegenerative (Parkinson's, Alzheimer's—early sign), congenital (Kallmann), sinonasal disease.
• Testing: UPSIT, Sniffin' Sticks, alcohol pad screening.
• Regeneration: Olfactory neurons regenerate from basal cells throughout life—unique in CNS.