Taste

Question 1

Where are the taste receptor cells located

Answer 1

Clustered together in taste buds, which are located in papillae embedded in the epithelium of the tongue

Question 2

How can the taste papillae be classified

Answer 2

Into circumvallate, foliate, and fungiform papillae

Question 3

What do receptors in the taste buds on the tongue synapse with

Answer 3

Afferent fibres projecting in the chorda tympani and glossopharyngeal nerves

Question 4

What do taste buds on the epiglottis synapse with

Answer 4

Superior laryngeal nerve

Question 5

What are the 4 classical taste qualities

What is the fifth

Could there be a sixth

Answer 5

Bitter
Sweet
Sour
Salt

Umami (the taste of glutamate)

Some suggestion of a sixth quality corresponding to starch

Question 6

Which taste qualities involve specific receptor mechanisms (3)

What do they use

Answer 6

Bitter
Sweet
Umami

Second messenger cascade

Question 7

How are sour and salt responses mediated

Answer 7

More directly via modulation of ion channels in the taste cell surface membrane

Question 8

How can taste cells be subdivided (3)

Answer 8

Receptor cells
Presynaptic cells
Glial like cells

Question 9

What do receptor cells for taste respond to

Answer 9

They have a narrow specificity expressing receptors for bitter sweet or umami

Question 10

What do taste presynaptic cell respond to

Answer 10

Respond directly to acid stimulation

Question 11

What do glial like cells respond to

Answer 11

They carry out potassium and transmitter homoeostasis and may participate in salt transduction

Question 12

What underlie the umami and sweet responses

Answer 12

Heterodimeric GPCRs

Umami: T1R1 and T1R3

Sweet: T1R2 and T1R3

Question 13

What underlies bitter responses

Answer 13

Sensed by a family of ~30 T2R receptors which dimerise - accounts for diversity of bitter compounds

Question 14

Why do we have so many bitter receptors

Answer 14

Bitter indicates food is not right so a wide range allows better sensitivity

Question 15

What happens to the GPCRs of taste once ligand has bound

Answer 15

β and γ subunits stimulate production of IP3 —> Ca2+ release + TRP5M cation channel opening

Increased [Ca2+] and depolarisation leads to transmitter release from receptor cells via gap junction hemichannels

Question 16

What does sour transduction involve

Answer 16

Intracellular acidification by weak acid

Mechanism not understood but involves K+ channel which is blocked by intracellular protons, leading to depolarisation, opening of Ca2+ channels and vesicular transmitter release from presynaptic cells

Question 17

What does salt transduction involve

Answer 17

Na+ entry through epithelial sodium leak channels which depolarised glial like cells

Protons can also enter these channels

Question 18

How do taste afferents in the cranial nerves VII, IX and X process

Answer 18

Synapse in solitary nuclear complex in the medulla within gustatory nucleus l

In the primate an uncrossed pathway projects to the ventral posterior medial nucleus of the thalamus and then on to gustatory neocortex in the anterior Insula and frontal operculum

Question 19

How do projections pass from the primary taste cortex (2)

Answer 19

• To the secondary taste area in the orbitofrontal cortex, where units modulate the discharge according to the pleasantness of the taste of the food,
• and to the amygdala, which plays a role in the affective component of taste response

Question 20

Which taste projections modulate feeding behaviour

Answer 20

Projections to the lateral hypothalamus

Question 21

Describe the thalamocortical pathway in some primate species such as rodents

Answer 21

Pathway runs via a pontine relay in the parabrachial nucleus where changing physiological conditions may modify feeding behaviour

Question 22

True or false

Individual taste fibres respond to a range of stimuli but tend to prefer just one

Answer 22

True

Recordings from afferents in the chorda tympani show this

Question 23

Individual taste fibres respond to a range of stimuli but tend to prefer just one. What does this indicate

How can this discrepancy be resolved

Answer 23

across fibre code, in which taste is identified by the pattern of afferent activity invoked in different fibres
So taste is based on the pattern evoked

By noting that afferent fibres receive broadly tuned input, not only from the receptor cells via ATP but also from the presynaptic cells via serotonin release. Each afferent fibres can therefore respond to a range of taste qualities

Question 24

How do receptor cells release ATP

Answer 24

Gap junction hemi channels

Question 25

Can each afferent fibre respond to arrange of taste qualities

Answer 25

Yes