Olfactory Nerve Pathway (CN I)

Step 1 - Olfactory Epithelium (1st-order neurons)

• Located in the roof of the nasal cavity (lateral and septal walls)
• Composed of pseudostratified columnar epithelium containing three cell types:
  • Olfactory receptor neurons (bipolar neurons) - the actual sensory cells; each has cilia bearing G protein-coupled receptor proteins (>1000 types) that bind odorant molecules
  • Sustentacular cells - supporting cells
  • Basal cells - stem cells that regenerate receptor neurons throughout life
• Bowman's glands secrete mucus to dissolve odorant molecules

Step 2 - Cribriform Plate

• Axons of olfactory receptor neurons (unmyelinated, called fila olfactoria) bundle together
• They pass through the foramina of the cribriform plate of the ethmoid bone to enter the anterior cranial fossa

Step 3 - Olfactory Bulb (2nd-order neurons)

• Axons synapse onto mitral cells in structures called glomeruli
• ~1000 olfactory receptor axons converge onto 1 mitral cell (massive convergence)
• Also contains periglomerular cells and granule cells (interneurons for lateral inhibition and processing)
• The olfactory bulb lies in the olfactory groove of the anterior cranial fossa

Step 4 - Olfactory Tract

• Mitral cell axons travel posteriorly along the inferior surface of the frontal lobe as the olfactory tract
• At the anterior perforated substance (near the optic chiasm), the tract divides into two striae:

Step 5 - Primary Olfactory Cortex (no thalamic relay!)

• Piriform (prepiriform) cortex - main conscious olfactory perception
• Amygdala - emotional responses to smell, fear/memory associations
• Olfactory tubercle - integration
• Entorhinal cortex - connects to hippocampus for memory

Step 6 - Secondary Connections

• The medial forebrain bundle (MFB) connects olfactory cortex to the hypothalamus and brainstem
• Regulates autonomic responses: salivation, gastric contraction, arousal via the reticular formation

Key Distinguishing Features

Clinical Correlate - Anosmia

• Loss of smell (anosmia) most commonly results from: head trauma shearing the fila olfactoria at the cribriform plate, viral infections (e.g., post-COVID), nasal polyps, or anterior cranial fossa tumors (e.g., olfactory groove meningioma)

Olfactory Nerve Pathway

Odorant molecules
        ↓
Olfactory receptor neurons (bipolar cells)
— in olfactory epithelium (roof of nasal cavity)
        ↓
Fila olfactoria (unmyelinated axons)
— pierce cribriform plate of ethmoid bone
        ↓
Olfactory bulb → synapse on Mitral cells (glomeruli)
— in anterior cranial fossa
        ↓
Olfactory tract
— runs on inferior surface of frontal lobe
        ↓
        ├── Lateral stria ──────────────────────────────→ Piriform cortex (primary olfactory cortex)
        │                                                  Amygdala, Olfactory tubercle
        │
        └── Medial stria ──→ Septal nuclei
                           → Anterior commissure → Contralateral olfactory bulb

Key points to remember:

• 1st neuron: Bipolar receptor cell (nasal epithelium)
• 2nd neuron: Mitral cell (olfactory bulb)
• No thalamic relay - unique among all sensory pathways
• Fibers are unmyelinated
• Olfactory receptor neurons are CNS-derived and can regenerate