Olfaction and Taste

Question 1

What are turbinates?

Answer 1

Enlargements of the olfactory epithelium to improve airflow contact

Question 2

How do odorants get to receptors?

Answer 2

Diffuse in air

Dissolve in mucus film, possibly with an odorant binding protein, and interact with receptors on the olfactory cilia

Question 3

How are olfactory receptors replaced?

Answer 3

Every 60 days by basal cell division

Question 4

Describe the process of olfactory transduction

Answer 4

1. Odorant interacts with a receptor molecule
2. Receptor stim Golf
3. Stim AC
4. Produce cAMP
5. cAMP opens cyclic nucleotide-gated cation channels
6. Inward flowing receptor current carried by Ca2+ and Na+ that depolarises the cell body
7. Cell body fires an AP
8. Augmented by Cl- flowing out through a Ca2+ gated channel

Question 5

What are vomeronasal organs?

Answer 5

Blind ended tube in some species used to detect pheromones

Question 6

What is the relationship between receptor current and odorant concentration?

Answer 6

Initially steep, then shallow

Question 7

How does olfactory adaptation work?

Answer 7

Ca2+ binds to calmodulin to reduce the sensitivity of the cation channels to camp.

Question 8

How is Ca2+ that enters during depolarisation extruded?

Answer 8

Sodium calcium exchange

Ca-ATPase in some species

Question 9

What does each olfactory receptor respond to?

Answer 9

Single receptor molecule but a range of odorants

Question 10

What is the pathway from the receptor cell?

Answer 10

Olf receptor axons
Cribriform plate
Olfactory bulb
Excite mitral cells and smaller tufted cells in the olfactory glomeruli
Mitral cell axons leave the bulb in the lateral olfactory tract
Synapses on neurons in 5 regions of olfactory cortex:
1. Anterior olfactory nucleus
2. Olfactory tubercle/anterior perforated substance –> Medial dorsal nucleus of the thalamus –> orbitofrontal cortex –> conscious perception
3. Pyriform cortex –> other olfactory cortical regions
4. Amygdala –> hypothalamus (autonomic) and reticular formation (arousal)
5. Entorhinal cortex –> hippocampus
Last two form part of the limbic system, involved in the affective component of odour perception

Question 11

Describe inhibition in olfaction

Answer 11

1. Lateral inhibition: Periglomerular cells and granule cells make reciprocal dendro-dendritic synapses with mitral cells to sharpen mitral cell odour tuning so that better stimulated glomeruli inhibit lesser stimulated glomeruli. Sets up a sort of centre surround situation
2. Inhibition between two bulbs: via anterior olfactory nucleus and anterior commissure

Question 12

How many glomeruli in each bulb does an olfactory receptor cell excite?

Answer 12

Afferents from olfactory receptor cells expressing a particular receptor molecule selectively converge on just two glomeruli in each bulb (one medial, one lateral)

Question 13

How do we know that as we move up the pathways, cells respond with increasing levels of specificity?

Answer 13

Olfactory bulb cells respond to a larger number of odours on average (mode = 5) than orbitofrontal cortex (1 = mode)

Question 14

How does the vomeronasal organ project?

Answer 14

Vomeronasal organ
Accessory olfactory bulb
Amygdala Corticomedial division: olfactory input, efferents to ventromedial hypothalamus
Sexual and social behaviours in some species

Question 15

How might humans have something like pheromones?

Answer 15

Trace amine associated receptors in the main olfactory epithelium may detect volatile amines in sweat to shift mood and perhaps increase fertility

Question 16

Where are taste receptor cells?

Answer 16

Taste buds in papillae in the epithelium of the tongue

Question 17

What are the three types of papillae?

Answer 17

Circumvallate
Fungiform
Foliate

Question 18

Which nerves does taste project by?

Answer 18

Chorda tympani
Glossopharyngeal
Epiglottis - superior laryngeal

Question 19

What are the five tastes?

Answer 19

Bitter
Sweet
Sour (H+)
Salt
Umami (Glutamate)
POTENTIALLY ALSO a taste for starch

Question 20

How are the senses transduced?

Answer 20

Bitter, sweet, umami specific receptor proteins via second messenger mechanisms
Sour and salt by modulation of ion channels in taste cell surface membrane

Question 21

What are the 3 categories of taste cell?

Answer 21

Receptor cells: bitter, sweet, umami
Presynaptic cells: acid
Glial-like cells: salt, also carry out potassium and transmitter homeostasis

Question 22

Which receptor underlies umami?

Answer 22

Receptor cell
Heterodimeric GPCR
T1R1 + TIR3

Question 23

Which receptor underlies sweet?

Answer 23

Receptor cell
Heterodimeric GPCR
T1R2 + T1R3

Question 24

Which receptor underlies bitter?

Answer 24

Receptor cell
30 T2Rs
May also dimerise

Question 25

Why do we need so many receptors for ‘bitter’??

Answer 25

Danger signal
Potentially poisonous plant alkaloids taste bitter
So want to make the number of compounds that signal this danger as large as possible

Question 26

What is the transduction cascade for bitter, umami and sweet?

Answer 26

GPCR
betagamma subunit
PLCbeta PIP2 --> IP3 and DAG
IP3
IP3R3 Ca2+ release from intracellular stores
TRP5M cation channels open
Na+ enters
This + the increased Ca2+
Depolarisation
Transmitter release from receptor cells via gap junction hemichannels Panx1 ATP out
NO VESICLES

Question 27

What is the receptor for sour?

Answer 27

Intracellular acidification by a weak acid
Weak acid undissociated diffuses across plasma membrane (not strong acid as needs to get to inside)
Dissociates
H+ blocks a potassium channel
Depolarisation
Ca2+ channels open
Vesicular transmitter release - serotonin 5-HT

Question 28

What is the receptor for salt?

Answer 28

Entry of sodium ions through epithelial sodium leak channels
Depolarises glial-like cells
Protons can also enter via these channels

Question 29

Describe the higher taste pathway

Answer 29

Taste afferent VII on Vc, IX and X sup laryngeal
Synapse in solitary nuclear complex in medulla within the gustatory nucleus
Primate: uncrossed pathway projects to the ventral posterior medial nucleus of the thalamus
Gustatory neocortex in anterior insula and frontal operculum (together = primary taste cortex)
From here to secondary taste area in orbitofrontal cortex
Projections to lateral hypothalamus to modulate feeding behaviour

Question 30

What does the ventral posterior medial nucleus of the thalamus do?

Answer 30

Uncrossed pathway of taste

Pain and thermoception

Question 31

How is the orbitofrontal cortex involved in taste?

Answer 31

Units modulate discharge according to pleasantness of taste of food and amygdala signals in the affective components of taste response

Question 32

Why are rodents not a good model of human taste pathways?

Answer 32

In them, the thalamocortical pathway runs via a pontine relay in the parabrachial nucleus, where changing physiological conditions may modify feeding behaviour

Question 33

How is taste encoded? How do we know?

Answer 33

Across-fibre code
Chorda tympani fibres respond to a range of stimuli, but tend to prefer one of them
Afferent fibres receive broadly tuned input: ATP from receptor cells, also 5-HT from presynaptic cells (which themselves receive purinergic ATP input)

Question 34

What is the problem with the across-fibre code?

Answer 34

Similar to in colour we don’t know if weak green or strong red etc, if you just look at one signal in the chorda tympani that ‘prefers’ sweet, don’t know if the taste was weak sucrose or strong NaCl

Question 35

How is the problem with the across-fibre code rectified?

Answer 35

Compare across fibres to decode an ambiguous signal