Chapter 8: Chemical Senses

Question 1

How many receptor cell types are there? Name them and explain what they express

Answer 1

3

Type 1: glial like support cells
Type 2: “receptor cells” that express metabotropic receptors for sweet, bitter and umami
Type 3: “presynaptic cells” that express receptors for sour and salty taste

Question 2

What are two facts you can tell for both type 2 and 3 cells?

Answer 2

Type 2: - express metabotropic receptors for sweet, bitter and umami
- cells tend to be narrowly tuned to a single rate quality

Type 3: -express receptors for sour and salty
- an average type 3 cell responds to ~3 taste qualities

Question 3

What are the 3 types of receptors for type 2 cells. Name them for each taste

Answer 3

Sweet receptors: T1R2+T1R3

Umami receptors: T1R1+T1R3

Bitter receptors: the T2Rs

Question 4

What happens in a type 2 cell when receptor is activated?

Answer 4

Intercellular activation of receptor causes increase in Calcium concentration leading to opening of calcium gated Na+ channels. Depolarization leads to opening of voltage gated Na+ channels which creates action potential. This causes release of ATP as a neurotransmitter

Question 5

What is the neurotransmitter of a Cell of type 2?

Answer 5

ATP

Question 6

What are the steps of transduction for salty tastants?

Answer 6

Amiloride sensitive Na+ channels open leading to depolarization which leads to opening of voltage gated Na+ and Ca++ channels. This causes AP which causes release of serotonin

Question 7

What are the steps of transduction for sour tastants?

Answer 7

Influx of hydrogen causes closing of K+ channels which leads to depolarization. This leads to opening of Na+ and Ca++ channels which causes AP. This releases serotonin.

Question 8

What does the activation of type 2 cells release?

Answer 8

ATP

Question 9

What are the two things ATP release enables?

Answer 9

• communication with gustatory nerves

- activation of neighboring presynaptic cells (type 3 cells)

Question 10

Is the taste bud a structure of parallel independent processing units? Explain answer

Answer 10

No it is a collective unit involving extensive cell to cell interactions

Question 11

Explain what population coding is

Answer 11

Receptor cells are sensitive to a small number of Taste types, often only one. Gustatory axons the neurons they activate tend to respond more broadly (e.g. strongly to bitter, moderately to sour and salt,…). Only with a large population of taste cells with different response patterns can the brain distinguish between specific alternative tastes.
One food activates a certain subset of neurons some of them firing very strongly, some moderately and some not at all.

Question 12

What is the central taste pathway?

Answer 12

(Anterior tongue , posterior tongue, epiglottis) go to gustatory nucleus and then to ventral posterior medial nucleus of thalamus and then to primary gustatory cortex

Question 13

What’s the the steps of the odorant receptors (cilia)

Answer 13

1. GPCR binds odorant
2. Delta olf stimulates adenylyl cyclase
3. Increased cAMP opens a cAMP gated cation channel leading to increase in Na+ and Ca++
4. Increased Ca++ gates a Ca++ gated cl- gate leading to a Cl- efflux leading depolarization of receptor cell

Question 14

How do you go from receptor potentials to spikes?

Answer 14

Olfactory receptors are located on the cilia
When an odorant binds a receptor, the cell is depolarized triggering an AP in the soma and sending APs out of the olfactory nerve

Question 15

HOW MANY GPCRs does the receptor cell have?

Answer 15

1

Question 16

With what do the receptor neurons make synapses? And where?

Answer 16

2nd order neurons and in the glomeruli

Question 17

What are three advantages of the olfactory map system?

Answer 17

1) maximizes sensitivity to low concentrations of odorant ( signals from 5000 receptor neurons converge on 2 glomeruli)
2) important for stimulation of odor memories (bulb map remains constant over time)
3) the olfactory map in the bulb is identical in different individuals!

Question 18

Explain the two olfactory pathways

Answer 18

1) second order neurons project to piriform cortex in the olfactory tract. From there information is sent to other temporal lobe structures which mediate emotion, memory and memory effects of odors
2) second order neurons project to olfactory tubercle to MD thalamus then to orbifrontal neocortex. This pathway mediates perceptions of smell and odor discrimination

Question 19

What do olfactory cortical neurons require to identify a single odorant?

Answer 19

Simultaneous inputs from different odorant receptors

Question 20

What can odorant mixtures do?

Answer 20

They can activate cortical neurons that don’t respond to either odorant alone