09b09bSmell Smell

(also involved with taste):(also involved with taste):Cranial Nerve I, Olfactory Cranial Nerve I, Olfactory

NerveNerve

Fills Special Visceral Afferent (SVA) functionFills Special Visceral Afferent (SVA) functionSee pp. 370-372 in bookSee pp. 370-372 in book

SVA sensory cells of CN1 olfactory neurons (1st order neurons), in olfactory epithelium, transduce odor

molecules

CN1 passes through cribiform plate of ethmoid

bone

Cranial nerve I (olfactory)Cranial nerve I (olfactory)• Look at how short the 1Look at how short the 1stst order sensory neuron is! order sensory neuron is! (bipolar (bipolar

cells)cells)

• 11stst order sensory neuron = Cranial Nerve I order sensory neuron = Cranial Nerve I – Receptor in olfactory epitheliumReceptor in olfactory epithelium– Cell bodies in olfactory epithelium (PNS)Cell bodies in olfactory epithelium (PNS)– Axons travel through cribiform plate of ethmoid boneAxons travel through cribiform plate of ethmoid bone

• Synapse to second order neuron in olfactory bulb (CNS)Synapse to second order neuron in olfactory bulb (CNS)

• Cell bodies of second order neuron in olfactory bulb (CNS)Cell bodies of second order neuron in olfactory bulb (CNS)– Axons of second order neuron travel ipsilaterally Axons of second order neuron travel ipsilaterally

through olfactory tract to olfactory corticesthrough olfactory tract to olfactory cortices

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

Note facts about smell Note facts about smell pathwayspathways• No involvement of spinal cord, brainstemNo involvement of spinal cord, brainstem

or thalamus. Olfactory bulb part of or thalamus. Olfactory bulb part of

limbic systemlimbic system, which is associated with , which is associated with

memory and emotionmemory and emotion. (Remember the. (Remember the

limbic gyrus / cingulate gyrus?)limbic gyrus / cingulate gyrus?)

• Cortical smell centers strongly linked to : Cortical smell centers strongly linked to : – Hippocampus (associative learning/Hippocampus (associative learning/memorymemory))– Amygdala (which processes Amygdala (which processes emotion/moodemotion/mood))

• In your experience, smells In your experience, smells (a certain perfume, chlorine, pumpkin bread…..) (a certain perfume, chlorine, pumpkin bread…..) that that are linked to:are linked to:– memoriesmemories of event, person, moment of event, person, moment– general general moodmood (such as contentment) (such as contentment)

Many associated with childhood memories, b/c that’s when we first experience most smells

Relationship to other parts Relationship to other parts of cranial nerve systemof cranial nerve system

• Related in function to Related in function to tastetaste, which is , which is handled by other cranial nerves handled by other cranial nerves (which we’ll study later)(which we’ll study later)– VII (facial)VII (facial)– IX (glossopharyngeal)IX (glossopharyngeal)

Clinical applications: Eating, emotion, Clinical applications: Eating, emotion, memorymemory• When first order neurons are damaged, first symptom is When first order neurons are damaged, first symptom is phantosmia (phantosmia (phantom phantom

smells; due to smells; due to uncinate fitsuncinate fits) followed by ) followed by anosmia (no sense of smell)anosmia (no sense of smell). . – First-order neuron damage may result from First-order neuron damage may result from

• certain inhalants, certain inhalants,

• TBI (sheering) and associated skull fractureTBI (sheering) and associated skull fracture

– Clinical pattern: Don’t eat enough Clinical pattern: Don’t eat enough Eat too much Eat too much

• Loss of smell acuity, common in normal aging, decreases appetite… Loss of smell acuity, common in normal aging, decreases appetite… ((hyposmiahyposmia))

• Phantosmia associated 2Phantosmia associated 2ndnd-order neuron damage, or disorder in limbic system -order neuron damage, or disorder in limbic system (e.g. tumor, schizophrenia)(e.g. tumor, schizophrenia)

• Role in stimulation of low-level TBI (alertness via thalamus; memory via Role in stimulation of low-level TBI (alertness via thalamus; memory via hippocampus). However, bad smells in hospital or nursing home may also hippocampus). However, bad smells in hospital or nursing home may also worsen agitation in TBI and dementia!worsen agitation in TBI and dementia!

• Activation of memories /discussion, e.g., with people who have dementia, as Activation of memories /discussion, e.g., with people who have dementia, as long as sensory cells have not degeneratedlong as sensory cells have not degenerated

“…“…a person who loses…sense a person who loses…sense of smell, is thrown into an of smell, is thrown into an emotional crisis…more emotional crisis…more crippling…more threatening crippling…more threatening than the loss of a leg…even than the loss of a leg…even sudden blindness is less sudden blindness is less traumatic…anosmia always traumatic…anosmia always leads to depression and leads to depression and sometimes to suicide…..sometimes to suicide…..

……associated with its close associated with its close proximity to the limbic proximity to the limbic (emotional and memory) (emotional and memory) system” (p. 67)system” (p. 67)