Taste (gustation)

Question 1

LO

Answer 1

• Be able to describe the specialized sense organs involved in gustation at a molecular and cellular level
• Be able to describe how gustatory information can be coded
• Be able to describe the structures of the brain that processes this type of sensory information in the CNS

Question 2

Tell me about the surface of the tongue

Answer 2

• The sense organ: the tongue. The surface of the tongue has specialised invaginations called taste papillae
• Each Fungiform has ~3 taste buds on the apical surface
• Each Circumvallate has ~250 taste buds in the trench
• Folliate are organized into parallel ridges with ~600 taste buds

Question 3

Tell me what happens when food is taken into the mouth

Answer 3

• When food is taken in it is dissolved into the saliva – the papillae concentrate the dissolved chemicals in the region of the taste buds
• Taste buds contain the sensory receptors cells

Question 4

What do chemicals stimulate?

Followed by what and towards what location?

Answer 4

Chemicals stimulate fungiform papillae, followed by foliate and circumvallate towards the base of the tongue

Question 5

What are the 5 basic tastes

Give an example of each?

Answer 5

Question 6

Tell me about how different tastes are distinguished between?

Answer 6

Taste buds are able to distinguish between different tastes through detecting interaction with different molecules or ions. Sweet, savouriness, and bitter tastes are triggered by the binding of molecules to G protein-coupled receptors on the cell membranes of taste buds.

Question 7

How many specialised epithelial cells can a single taste bud contain?

Answer 7

A single taste bud can contain up to 50 specialised epithelial cells = taste cells

Question 8

What do the tips of taste cells have and why are they there?

Answer 8

• The tips of the cells have microvilli – increases surface area, come together at the taste pore – concentrates chemicals onto receptor proteins
• Taste receptor proteins located in microvilli
• Regeneration (Basal cells)

Question 9

What are the 2 key domains in the taste cell?

Answer 9

2 key domains – apical (external environment) & basolateral (taste bud)

Question 10

Apical microvili contain receptor proteins, what are these and what tastes do they detect?

Answer 10

• ion channels: salt & sour/acids
• GPCR: sweet, bitter, umami

Question 11

What does the basolateral surface of taste cells contain?

Answer 11

Basolateral surface contains ion channels and cellular machinery required to release neurotransmitter

Question 12

What do taste cells synapse onto?

Answer 12

Synapse onto cranial nerves (the facial (VII), glossopharyngeal (IX), and vagus (X))

Question 13

What are the neurotransmitters of taste cells?

Answer 13

serotonin and ATP

Question 14

Tell me the stages to salt and sour sensing via ion channels

Answer 14

Question 15

Tell me about T1Rs and sweet and umami

Answer 15

• T1Rs can form heterodimers
• The ligands that the T1Rs bind to varies depending on the combination
• Different T1R heterodimers encode sweet and umami

Question 16

Tell me about taste blindness: bitter?

Answer 16

• 25% of the population
• Inability to sense phenylthiocarbamide (PTC)
• The gene responsible was identified as the T2R receptor gene encodes a GPCR
• T2Rs are not present in taste cells that express T1R subtypes (sweet and umami)

Question 17

Bitter

Answer 17

• There are many T2R subtypes that recognize different combinations of ligands
• Sequence diversity enables the detection of many different bitter compounds

Question 18

What are the downstream signalling molecules?

Answer 18

Question 19

Tell me about the sweet signalling pathway

And give examples of natural sugars

Answer 19

Question 20

Tell me about Umami signalling pathway

Name some food with associated high protein

Answer 20

Question 21

Tell me about the bitter signalling pathway and give an example of a taste

Answer 21

Question 22

Summary: features of gustatory transduction

Answer 22

• Taste receptor proteins are concentrated in the microvilli of taste cells
• Ion channels signal salt and sour
• GPCRs signal sweet, amino acid and bitter
• GPCRs signal through a PLCB2, TRPM5 pathway
• Graded receptor potentials spread to the basal domain
• Voltage-gated channels mediate release neurotransmitter onto the terminals of sensory afferents

Question 23

Summary of the key taste receptors

Answer 23

• Amiloride sensitive Na channel, Transient Receptor Potential (TRP), metabotropic glutamate receptor (mGluR), T2R and T1R subtypes.

Question 24

Tell me about the central processing of gustation and the cranial nerves involved

Answer 24

• Cranial nerve VII: facial nerve and Chorda Tympani (projects back from tongue and palate)
• Cranial nerve IX: lingual branch of the glossopharyngeal nerve (project from back of tongue, epiglottis)
• Cranial nerve X: superior laryngeal branch of the vagus nerve (projects back from oesophagus)
• Cranial nerves have a topographical organization in the gustatory nucleus
• Cranial nerve VII – rostral/Cranial nerve IX – mid-region/Cranial nerve X – caudal
• The nucleus of the solitary tract facilitates the integration of sensory information about taste and visceral sensory information

Question 25

Central processing of gustation

Answer 25

Question 26

Mapping of gustatory cues in the insular cortex

Answer 26

* A cartoon of the representation of tastants in the insular cortex developed by mapping responses in the insular cortex to tastants: * Active cells are in red. In the region of analysis there is a large response to NaCl, but not bitter and sweet tastants in the same region (i.e., fewer red dots) * Provides supportive evidence for a gustatory map in the insular cortex

Question 27

Taste is conveyed to the CNS using labelling line coding

Answer 27

* In the T2R rescue mice, PLCB2 has been ‘knocked in’ to PLCB2 knockout genetic background and selectively rescued back in T2R cells * What does this experiment tell us about the type of coding in the periphery?

Question 28

Central processing of gustation

Answer 28

Question 29

Taste information is relayed via what?

Answer 29

Taste information is relayed, via the thalamus, to cortical structures (different to olfaction!)

Question 30

What does orbitofrontal cortex give and what is it thought to be involved in?

Answer 30

* Orbitofrontal cortex gives us our perception of food * Orbitofrontal cortex is thought to be involved in signaling satiety

Question 31

Central processing of gustation

Answer 31

* Two- way transfer of information between the nucleus of the solitary tract and the hypothalamus and amygdala * Hypothalamus – homeostatic responses \> feeding behaviour * Amygdala – limbic system. Affective response to food – pleasurable or repulsive experience of food, food seeking behaviours

Question 32

**General summary: chemosensation**

Answer 32

* **Gustation and olfaction are mediated by specific gustatory and olfactory receptors. Both receptors have a high turn-over.** * **Signal transduction in gustatory receptors involves both ion channels and GPCRs, whilst odorant receptors are a large family of GPCRs** * **Gustatory receptors respond to a more limited range of stimuli than olfactory receptors and typically have a much lower sensitivity**