CN I: Olfactory nerve - scheme

Odorant molecules
        ↓
Olfactory epithelium
(posterosuperior roof of nasal cavity)
        ↓
Bipolar olfactory receptor neurons
        ↓  Axons form fila olfactoria
        ↓  Pass through cribriform plate of ethmoid
CN I - olfactory nerve
        ↓
Olfactory bulb
        ↓  Synapse in glomeruli with mitral and tufted cells
Olfactory tract
        ↓
Olfactory trigone
        ↓
Lateral olfactory stria
        ↓
Primary olfactory cortex:
- Piriform cortex
- Periamygdaloid cortex/amygdala
- Entorhinal cortex
        ↓
Orbitofrontal cortex
(conscious identification and discrimination of odors)

Important points

• CN I is made of many small, unmyelinated axon bundles, rather than one single nerve trunk.
• Olfactory receptor neurons lie in the olfactory epithelium and are bipolar neurons.
• Unlike most sensory systems, the primary olfactory pathway reaches cortex without an initial thalamic relay.
• Head trauma can shear fibers as they cross the cribriform plate, causing anosmia. Neuroscience: Exploring the Brain, p. 783.
• Test each nostril separately using familiar, non-irritating odors such as coffee or soap. Avoid ammonia because it activates trigeminal sensory fibers rather than testing smell.