JD - Gustation

Question 1

What is present on the surface of the tongue?

Answer 1

The surface of the tongue has specialised invaginations called taste papillae

Question 2

Where are taste buds located on the tongue? (4)

Answer 2

• Circumvallate (50% of taste buds)
• Fungiform papilla (25% of taste buds)
• Foliate papilla (25% of taste buds)
• Filliform papilla does not have taste buds

Question 3

Position of taste buds in circumvallate papilla

Answer 3

deep troughs

Question 4

What occurs when food is ingested? (2)

Answer 4

• 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 receptor cells

Question 5

What are the 5 basic tastes?

Answer 5

1. Bitter (G-protein)
2. Sour (ion channel)
3. Sweet (G-protein)
4. Salt (ion channel)
5. Meaty taste- umami (G-protein)

There is a regional difference in the threshold of responsiveness

Question 6

What are 2 features of the taste bud?

Answer 6

• A single taste bud can contain up to 50 specialised epithelial cells = taste cells
• The tips of the cells have microvilli – increase surface area, come together at the taste pore – and concentrate chemicals onto receptor proteins

Question 7

What acts to regenerate taste buds?

Answer 7

Basal cells

Question 8

What 2 key domains are present in taste cells adn what do they contain?

Answer 8

Apical microvilli
contains receptor proteins

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

Basolateral surface
contains ion channels and cellular machinery required to release neurotransmitter (serotonin and ATP)

Question 9

Salt and Sour (4) sensing via ion channels

Answer 9

Salt- Amiloride-sensitive Na+ channel

Acids (sour) - H+ sensitive TRP channel

1. H+ ions from sour food can also block the K+ channel
2. Prevents K+ leak
3. Causes depolarisation and opening of Ca2+ channel
4. Influx of Ca2+ causes neurotransmitter release

Question 10

What are features of T1Rs?

Answer 10

Found in sweet and umami

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

Question 11

What is taste blindness? (4)

Answer 11

• 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 12

What is a feature of T2R subtypes?

Answer 12

Many T2R subtypes recognise different combinations of ligands

• Sequence diversity enables the detection of many different bitter compounds

Question 13

Sweet: Signalling Pathway (6)

Answer 13

1. T1R2/T1R3 activated
2. Activates PLC β2
3. Activates downstream IP3
4. Increases concentration of endoplasmic reticulum Ca2+
5. ER releases Ca2+
6. Ca2+ activate TRPM5 and results in influx of more Ca2+

Leads to indirect depolarisation of taste cell

Question 14

Umami: Signalling Pathway (6)

Answer 14

1. T1R1/T1R3 activated
2. Activates PLC β2
3. Activates downstream IP3
4. Increases concentration of endoplasmic reticulum Ca2+
5. ER releases Ca2+
6. Ca2+ activate TRPM5 and results in influx of more Ca2+

Question 15

What are 4 features of the Umami signalling pathway?

Answer 15

• Expressed in mouse taste cells
• Produces functional responses to glutamate in CHO cells
• Rats respond to the mGluR4 agonist L-AP4 in the same way as glutamate and they taste similar
• Inhibits cAMP signaling

Question 16

Bitter: Signalling Pathway

Answer 16

Specific G-protein in taste cells: α -gustducin

Same pathway however utilises PP2 in downstream signalling pathways

Question 17

Central processing of gustation (3)

Answer 17

• Cranial nerve VII: facial nerve and Chorda Tympani (from tongue and palate)
• Cranial nerve IX: lingual branch of the glossopharyngeal nerve (from back of tongue)
• Cranial nerve X: superior laryngeal branch of the vagus nerve ( from epiglottis, oesophagus)

Project to the nucleus of the solitary tract (gustatory nucleus)

Question 18

What is the topographical organisaton in the gustatory nucleus?

Answer 18

Cranial nerve VII – rostral
Cranial nerve IX – mid-region
Cranial nerve X – caudal

Question 19

How is the nucleus of the solitary tract involved in the integration of sensory information about taste and visceal information? (3)

Answer 19

1. Projections from the rostral part of the solitary nucleus project to the thalamus, where they terminate in the medial half of the Ventral Posterior Medial Nucleus
2. The VPM projects to the anterior insula in the temporal lobe (called: insular taste cortex) & the operculum
3. A secondary taste centre located in the orbitofrontal cortex (located in the frontal lobe)

Question 20

How is taste conveyed to the CNS?

Answer 20

Using labelled line coding

• In the T2R rescue mice, PLCB2 has been ‘knocked in’ to PLCB2 knockout genetic background and selectively rescued back in T2R cells

Question 21

What gives our perception of food?

Answer 21

Orbitofrontal cortex

Question 22

Describe the 3 orders of neurons involved in gustation

Answer 22

1st order: (VII/IX/X) —> Gustatory Nucleus

2nd order: Gustatory Nucleus —> Ventral Posterior Medial nucleus of the thalamus

3rd order: VPM Thalamus —> Gustatory cortex (insula and frontal)