Chemosensory Function

Question 1

Where does oldfaction sensory transduction take place?

Answer 1

Cilia of olfactory receptor neurons

Embedded in mucus

Question 2

What type of receptor do odorant molecules bind to on the olfactory receptor neurons?

Answer 2

G-protein coupled receptors

Question 3

What is the role of cAMP in olfaction sensory transduction?

Answer 3

Binds to an intracellular site on a cation channel permeable to Na and Ca (cyclic nucleotide gated ion channel)

Creates depolarizing receptor potential

Question 4

What is the function of the Ca-gated Chloride channels in olfaction?

Answer 4

Opens in response to influx of calcium

Causes chloride EFFLUX, and further DEPOLARIZATION

Question 5

Describe the chloride equilibrium potential in olfactory receptor neurons.

Answer 5

Positive to threshold

Intracellular is high, extracellular is low (relatively)

Any membrane potential negative to E(Cl) will cause chloride to flow out of the cell, creating an inward current

Question 6

What are three mechanisms of adaptation in olfaction?

Answer 6

• Ca binds to calmodulin, Ca-calmodulin complex inhibits CNG channel
• Odorant diffusion and transport in mucus (i.e. secretion of more mucus)
• Enzymatic breakdown of odorant molecules by enzymes in the mucus

Question 7

How many receptor proteins are expressed per olfactory receptor neuron?

Answer 7

One

There are approximately 500 unique odorant receptor proteins

Question 8

How many distinct odors can humans recognize?

Answer 8

> 20,000

Question 9

What is population coding?

Answer 9

Each receptor is activated by multiple odorants to a different degree

The brain compares the pattern of firing across entire population of olfactory neurons to determine the nature of the odorant

Question 10

How is the intensity of an odor coded?

Answer 10

Frequency coding

Question 11

What is the olfactory bulb?

Answer 11

Collection of olfactory glomeruli

Question 12

What is an olfactory glomerulus?

Answer 12

Axon terminals of olfactory receptor neurons synapse onto second order neurons

All receptor neurons innervating one glomerulus contain the same olfactory receptor proteins

Question 13

What are two possible functions of olfactory bulbs?

Answer 13

Signal amplification due to the convergence of many receptor neurons

Organize chemosensory input into categories (e.g. not all flowers smell the same, but they all smell like flowers)

Question 14

What are taste receptor cells?

Answer 14

Epithelial cells contain within taste buds, which cluster into papillae on the surface of the tongue

Synapse onto different afferent axons

Question 15

What are the five basic categories of taste?

Answer 15

Salt, bitter, sour, sweet, umami (AA’s)

Individual taste receptor cells respond to stimuli belonging to one category

Question 16

What determines the afferent response to gustation?

Answer 16

The taste cells innervated by the afferent neuron

Individual afferents typically innervate multiple taste receptor cells

Question 17

Which taste modalities are ionotropic? Metabotropic?

Answer 17

Ionotropic: Salt and Sour

Metabotropic: Sweet, Bitter, and umami

Question 18

What is the pathway of salt transduction?

Answer 18

Extracellular salt concentration increase (salty foods) causes increase in Na influx via leak channels and depolarization

Depolarization opens vg Ca channels and release of transmitter

Question 19

What is the pathway of sour transduction?

Answer 19

Increase in H+ concentration stimulates opening of proton permeable TRP channel, causes influx of H+

This influx depolarizes the cell and eventual release of transmitter via v-g Ca channels

Question 20

What is the pathway for the metabotropic taste modalities?

Answer 20

Molecule binds to GPCR

G-protein stimulates PLC –> formation of IP3

IP3 activates Ca permeable TRP channel on membrane and intracellular release of Ca from smooth ER

Intracellular calcium triggers transmitter release

Question 21

How is gustation encoded?

Answer 21

Some degree of population encoding, although this is debated