Download Cranial nerve of smell, plus olfactory pathway

09b
Smell
(also involved with taste):
Cranial Nerve I, Olfactory Nerve
Fills Special Visceral Afferent (SVA) function
See pp. 370-372 in book
CN1 passes
through
cribiform
plate of
ethmoid bone
SVA sensory cells of CN1 olfactory neurons (1st order
neurons), in olfactory epithelium, transduce odor molecules
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
Primary and
secondary cortical
olfactory areas,
and their
relationship to
limbic system
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
associated with childhood
– general mood (such as contentment) Many
memories, b/c that’s when we first
experience most smells
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)
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
“…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)