Chemical senses

Question 0

General characteristics of olfaction

Answer 0

High sensitivity
- human to 10’(-14) for important smells (ex butyl mercaptan = skunk)
- due to large number of olfactory neurons (dog vs human)
Broad discrimination - over 1 trillion different smells
- can’t predict based on structure
- may be dependent on combinations, concentration
Anosmias = specific deficit (ie asparagus urine)
Decreases over time -> decreased quality of life?

Question 1

Trigeminal system

Answer 1

Free nerve endings in mouth
Detect noxious, irritating
- vinegar, ammonia, capsaicin
Ex: shared TRP (Na, Ca) channels for menthol + ice, capsaicin + heat

Question 2

Pheremones

Answer 2

? Influence in reproduction and social
Vomeronasal organ - animals, but regresses in humans

We still have similar functions!

• mother-child bonding
• synchony of menses
• male sweat, Axe body spray…

Question 3

Anatomy of olfactory system

Answer 3

Mucus layer
Olfactory epithelium - middle and upper conchae
- receptor cells (lifespan 60 days) + basal cells
- continual neurogenesis - first sign of neurodegenerative
- supporting cells - microvilli, nutrition
Axons through cribriform plate - unmyelinated
Olfactory bulb
- glomerulus - axons of receptor + dendrite of mitral cell (multiple)
- 2000 receptors: 1 mitral (convergence)
- mitral project to CNS
- peri-glomerulus - connections between glomeruli
- granule cells

Question 4

Olfactory receptors

Answer 4

Odorant -> cilia Ga protein -> AC -> cAMP ->
(some receptors act through phospholipase C -> IP3 -> Ca)
Na/Ca channel -> depolarize -> action potential

Cilia are necessary (site of ligand-gated channels)
Specificity - combinations of receptors and cells
- large range of G-protein coupled receptors (500 -> combinations)
- spatial pattern in expression (“sorting”)

Question 5

Specificity of olfaction

Answer 5

“across fiber” patterns vs “labelled line”

• integrated inputs based on
• concentration (number of receptors)
• which receptor cells (spatial organization of G proteins)
• temporal pattern
• combination of multiple receptors, cells, glomeruli

Specificity of receptors - functional groups, molecular shape
- some spatial organization - distinction via solubility, travel
Glomerulus - convergence and integration
- selective (receptor cells all express same G protein)
- not spatial
Cortex
- input from multiple glomeruli -> integration, comparison

Question 6

Central olfactory processing

Answer 6

Olfactory tubercle -> piriform cortex ->

• amygdala -> hypothalamus (emotions)
• entorhinal -> hippocampus (memory)
• thalamus -> frontal cortex (conscious discrimination)

Modulation:
SNS -> receptors (heightened with fear)
locus ceruleus -> NE fibers -> bulb (heightened with emotions/hunger)

Question 7

Papillae

Answer 7

Present on tongue, palate, epiglottis, esophagus
Types
- (circum)vallate (cylindrical, base) - most taste buds on sides -> CN IX
- at base/back of tongue
- foliate - CN IX and VII - sides of tongue
- filoform - no taste buds! - all over tongue
- fungiform - taste buds on top -> CN VII - scattered throughout
- ex: “supertasters” have higher density

Question 8

Taste bud anatomy

Answer 8

Taste/receptor cells -> project through pore -> CN VII, IX, X
Basal cells - regenerate 10-12 d
Support cells

Question 9

Taste processing

Answer 9

Taste receptors -> neurotransmitter -> action potential ->
CN VIII, CN IX, CN X ->
Nucleus of solitary tract
-> salivation, coughing, gagging
Ventral posterior medial nucleus of thalamus -> gustatory cortex = insula
- inputs combined with mechanical and smell
- overlapping system (“across fiber” pattern)
-> amygdala, hypothalamus

Question 10

Overview of taste discrimination

Answer 10

Bitter - base - circumvallate -> CN IX
- G protein -> PLC -> IP3 -> Ca
Sour - sides - folliate -> CN IX
- H+ activated cation channel
Sweet, umami - front -> CN VII
- sweet - saccharide -> Gprotein -> PLC -> IP3 -> Ca
- umami - amino acids -> G protein (altered metabotropic glutamate receptor) -> Ca
Salty - front, sides - fungiform -> CN VII
- ex “amiloride sensitive Na channel” - leak dependend on Na concentration -> Ca channel -> transmitter release (ATP, 5HT)

Each receptor has different combination -> combination of responses (“cross-fiber”)

Question 11

Processing of taste

Answer 11

Across-fiber - “taste” = specific pattern across population

• individual receptor cells may be specific or multi
• multiple inputs at solitary nucleus, thalamus, insular cortex

Modulation:
Appetite -> Serotonin (5HT), CCK -> increased sensitivity
Aldosterone, vasopressin -> increased Na sensitivity

Question 12

Pathologies of olfactory and gustatory

Answer 12

Atrophic rhinitis - reduced olfactory
Diabetes - reduced olfactory
Chronic renal failure - reduced olfactory (growth factors?)
Alzheimers - reduced olfactory (lose receptor cells)

Cystic fibrosis - enhanced olfactory and gustatory (mucus traps)
Epilepsy - enhanced olfactory and gustatory (cortical level)