1. 09b Smell (also involved with taste): Cranial Nerve I, Olfactory Nerve Fills Special Visceral Afferent (SVA) function See pp in book

2. CN1 passes through cribiform plate of ethmoid bone
SVA sensory cells of CN1 olfactory neurons (1st order neurons), in olfactory epithelium, transduce odor molecules

3. Cranial nerve I (olfactory)
Look at how short the 1st order sensory neuron is! (bipolar cells)
1st order sensory neuron = Cranial Nerve I
Receptor in olfactory epithelium
Cell bodies in olfactory epithelium (PNS)
Axons travel through cribiform plate of ethmoid bone
Synapse to second order neuron in olfactory bulb (CNS)
Cell bodies of second order neuron in olfactory bulb (CNS)
Axons of second order neuron travel ipsilaterally through olfactory tract to olfactory cortices

4. Primary and secondary cortical olfactory areas, and their relationship to limbic system

5. Note facts about smell pathways
No involvement of spinal cord, brainstem
or thalamus. Olfactory bulb part of
limbic system, which is associated with
memory and emotion. (Remember the
limbic gyrus / cingulate gyrus?)
Cortical smell centers strongly linked to :
Hippocampus (associative learning/memory)
Amygdala (which processes emotion/mood)
In your experience, smells (a certain perfume, chlorine, pumpkin bread…..) that are linked to:
memories of event, person, moment
general mood (such as contentment)
Many associated with childhood memories, b/c that’s when we first experience most smells

6. Relationship to other parts of cranial nerve system
Related in function to taste, which is handled by other cranial nerves (which we’ll study later)
VII (facial)
IX (glossopharyngeal)

7. Clinical applications: Eating, emotion, memory
When first order neurons are damaged, first symptom is phantosmia (phantom smells; due to uncinate fits) followed by anosmia (no sense of smell).
First-order neuron damage may result from
certain inhalants,
TBI (sheering) and associated skull fracture
Clinical pattern: Don’t eat enough  Eat too much
Loss of smell acuity, common in normal aging, decreases appetite… (hyposmia)
Phantosmia associated 2nd-order neuron damage, or disorder in limbic system (e.g. tumor, schizophrenia)
Role in stimulation of low-level TBI (alertness via thalamus; memory via hippocampus). However, bad smells in hospital or nursing home may also worsen agitation in TBI and dementia!
Activation of memories /discussion, e.g., with people who have dementia, as long as sensory cells have not degenerated

8. “…a person who loses…sense of smell, is thrown into an emotional crisis…more crippling…more threatening than the loss of a leg…even sudden blindness is less traumatic…anosmia always leads to depression and sometimes to suicide….. …associated with its close proximity to the limbic (emotional and memory) system” (p. 67)