Chemical Senses

Question 1

Each ORN (odorant receptor nerve) expresses [one/multiple] type of olfactory receptor protein

Answer 1

Each ORN (odorant receptor nerve) expresses only one type of olfactory receptor protein

All ORNs with the same receptor protein converge on the same glomerulus in the olfactory bulb

Question 2

describe the topological organization of the olfactory bulbs

Answer 2

olfactory bulb has an odortopic organization because the response of each glomerulus is mediated by a different OR (olfactory receptor)

brain determines the identity and concentration of an odor based on the extent to which each of the glomeruli are activated

Question 3

How do you explain the following observation?
Humans have ~ 350 olfactory receptor proteins, but they can detect > 5,000 odors

Answer 3

Answer: Odor perception uses a combinatorial coding mechanism

brain determines the identity and concentration of an odor based on the extent to which each of the glomeruli are activated; Each odor elicits a distinct pattern of activation (or “fingerprint”) across the
population of glomeruli

Question 4

to where does output of the olfactory bulb travel?

Answer 4

directly to olfactory cortex (NO thalamic relay)

Note: cortical output has direct access to memory and emotional circuits

Question 5

what are the 2 types of olfactory input that occur during taste?

Answer 5

Orthonasal: occurs during inhalation, usually before ingestion

Retronasal: occurs during exhalation, usually during mastication, food manipulation and swallowing

Question 6

what are the 5 types of receptors which mediate taste?

Answer 6

1. umami = T1R1 + T1R3 (L-glutamate)
2. sweet = T1R2 + T1R3
3. bitter = T2Rs (~30)
4. sodium = ENaC
5. Sour = Otop1 (acids)

Question 7

direct diffusion pathway vs metabotropic signaling pathway of taste - which taste modalities use each?

Answer 7

Direct diffusion pathway: Na+ diffuses into taste cells and depolarizes the membrane

Metabotropic signaling pathway: sweet, bitter & umami stimuli activate taste cells via distinct receptors

Question 8

which 3 cranial nerves converge on the nucleus of solitary tract for taste?

Answer 8

1. facial nerve (CN VII) - anterior 2/3 tongue
2. glossopharyngeal (CN IX) - posterior 1/3 tongue
3. superior laryngeal nerve (branch of CN X) - root of tongue/back of throat

Question 9

how does ageusia present?

Answer 9

aka complete loss of taste

Causes loss of appetite, difficulty swallowing, inadequate salivation

[note most patients who complain of taste loss are actually experiencing olfactory loss]

Question 10

what is the role of trigeminal orosensation in taste?

Answer 10

Helps detect irritant (and potentially toxic) chemicals in foods; also helps determine the texture and temperature of foods

activated by temperature changes or chemical irritants (capsaicin, sulfur dioxide, ascetic acid, CO2, methanol/ethanol, ammonia)

elicits physiological responses (salivation, tearing, sweating, behavioral avoidance, etc)

Question 11

which cation channel mediates the “burn” of chili peppers AND heat?

Answer 11

TRPV-1: ligand- and heat-gated cation channel, expressed in trigeminal fibers

Question 12

what are the 2 brain regions which respond to food-related inputs from the sensory systems (vision, taste, olfaction, touch, auditory)?

Answer 12

1. amygdala
2. orbitofrontal cortex

These two brain regions are thought to be major areas of flavor integration