Taste

Question 0

What is umami?

Answer 0

Japanese word meaning savoury
Flavour found in meat, fish, seafood and vegetables
Elicited by the amino acids glutamate, aspartate and mono sodium glutamate

Question 1

What are the five main taste qualities that we can detect?

Answer 1

Sweet, salt, sour, bitter, umami

Question 2

What a taste molecules called?

Answer 2

Tastants

Question 3

Name an important characteristic required for tastant molecules?

Answer 3

They must be water soluble/ able to interact with water via hydrogen bonding between molecules of opposite charge

Question 4

Name the four different types of papillae

Answer 4

Filiform - mechanical and somatosensory functions
Cover most of the tongue
Do no detect taste

Fungiform - pressure, temperature and touch
Front part of the tongue

Foliate - serous glands that secrete fluid
Either side of the tongue towards the back

Circumvallate - back of the tongue
Also contain glands like foliage papillae

Question 5

Describe the types of taste cell found in the mouth?

Answer 5

There are three types of taste cell involved in taste transmission:
type I which are thought to detect salt taste;
type II which detect sweet, umami and bitter tastes;
and type III which detect sour taste.
Type III cells have synaptic connections to gustatory afferent neurons.

Question 6

How is salt taste detected and transduced?

Answer 6

Salt	NaCl (or Na+)	
Type I	taste cells
Entry of Na+ ions into cell through epithelial sodium channels (ENaCs) and other types of Na+ channel causes a depolarisation

Question 7

What elicits a sour taste and how is the signal transduced?

Answer 7

Acids which in solution produce H+ ions
Detected by Type III taste cells
Transduction occurs by Entry of H+ ions into cell through H+ channels or entry of acids into cell cause decrease in pH and depolarisation
This depolarisation causes the opening of Ca2+ channels and the subsequent lead of serotonin from vesicles in the cell which stimulate the creation of an action potential in the afferent neuron which synapses with the type III taste cell

Question 8

How is sweetness detected and transduced?

Answer 8

Sweet tastants (various different sizes and structures) e.g. glucose, fructose
Detected by Type II taste cells 
Transduction - Binding of tastant to T1R2–T1R3 receptor causes intracellular events leading to release of intracellular Ca2+ ions and entry of cations, leading to depolarisation and the release of ATP which has a stimulatory effect on the afferent neurons

Question 9

What substance elicits and umami taste and how is it transduced?

Answer 9

Glutamic acid, short peptides, ribonucleotides
Detected by Type II taste cells
Transduction - Binding of tastant to T1R1–T1R3 receptor causes intracellular events as described for sweet taste

Question 10

What are e two substances which are detected as bitter?

Answer 10

N-phenylthiocarbamide (PTC)
6-propylthiouracil (PROP)

People can be defined according to their sensitivity to PROP. They can be PROP tasters or super tasters who are very sensitive to the chemical
The receptor thought to be responsible for binding PROP is T2R1. They have shorter extracellular domains compared to receptors for sweet and umami and do not form dimers
Bitter tastes are detected on type II taste cells

Question 11

How is fat detected and what is the receptor thought to be responsible?

Answer 11

The detection of fat in foods has been previously thought to be due to texture and detected as part of the sensory system
It is now thought hat a membrane protein called CD36 may detect dietary lipids

Question 12

Give a rough breakdown of how the five tastes are detected and transduced?

Answer 12

Figure 4.26 pg. 264 represents the current understanding of transduction in taste cells for the five basic tastes and the type of taste cell in which these mechanisms occur.

Sweet, umami and bitter tastes are mediated by G-protein-coupled receptors of the T1R (sweet, umami) and T2R (bitter) families on type II taste cells. Tastants bind to these receptors and activate phospholipase C which results in intracellular Ca2+ release and influx of cations, causing a depolarisation.

Sour taste is detected by a change in intracellular pH by influx of H+ ions or dissociation of intracellular acids, or by activation of acid-sensitive receptors on type III taste cells.

Salt taste is mediated by activation of channels resulting in influx of Na+ and other cations in type I taste cells.

Question 13

Why does common salt have taste but no smell?

Answer 13

Salt is a water-soluble solid that has a high melting temperature. It is non-volatile: to elicit an olfactory perception, a molecule must be volatile so that it can enter the nasal epithelium. Its water solubility allows it to come into contact with taste cells on the tongue and soft palate.

Question 14

How might the presence of amiloride affect the taste perceptions of (a) NaCl and (b) KCl?

Answer 14

Amiloride blocks ENaCs which detect low concentrations of NaCl, so you would expect the threshold of NaCl detection to be increased and therefore foods to taste less salty. The perception of KCl would not be affected as KCl is detected by another mechanism which is not sensitive to amiloride.

Question 15

‘Each sensory taste cell responds to one tastant only.’ Comment on this statement.

Answer 15

The statement is incorrect. For example, cells containing bitter taste receptors respond to more than one bitter tastant. Similarly for sweet taste, cells with sweet taste receptors respond to a range of diverse sweet tastants. However, there is evidence that a single taste cell responds to one taste quality only, for example bitter only or sweet only.

Question 16

‘All taste qualities are receptor mediated.’ Is this statement true or false?

Answer 16

The statement is false.
Sweet, bitter and umami tastes are activated by molecules binding to receptor proteins.

Sour taste involves organic and inorganic acids bringing about a change in pH within the cell, although membrane receptors may also be involved.

Salt taste involves the transport of ions through ion channels, which is not receptor mediated.

Question 17

Describe what is meant by enhancement when discussing taste?

Answer 17

Enhancement is the kind of interaction observed when two or more tastants are mixed together at above-detection threshold concentrations, resulting in a particular taste quality being increased in intensity.

Enhancement is relatively rare between tastants of different qualities but tends to be a general observation for tastants of the same quality. An example of enhancement across taste qualities is observed between sodium choride (salt) and the amino acid arginine (which has both bitter and umami qualities): arginine, although it has no salt taste quality of its own, enhances the salty taste of sodium chloride.

Question 18

Describe what is meant by synergy when discussing taste?

Answer 18

Synergy reflects a stronger form of positive interaction, in which the perceived intensity of a taste quality in a mixture of two tastants is more than the sum of the perceived intensities of the two individual tastants. One example involves the combination of MSG and ribonucleotides, which synergise their respective umami tastes (Section 4.5.4); a second involves the synthetic sweeteners aspartame and acesulfame, which synergise sweetness qualities. This phenomenon seems to be limited to tastants of the same taste quality.

Question 19

Describe what is meant by suppression when discussing taste?

Answer 19

Suppression is the reverse of enhancement; that is, when tastants are mixed together, a particular taste quality is decreased.
It is a common phenomenon, especially among tastants of different taste qualities. Lemon squash drink benefits from the sweetness of sucrose suppressing the sourness of the citric acid in the lemons, for example. Suppression can be either symmetrical in which two taste qualities have suppressive effects on each other, or asymmetrical in which one taste quality affects the perceived intensity of another, but the effect is not seen in the other direction.

Question 20

Describe what is meant by masking when discussing taste?

Answer 20

Masking in the context of taste refers to the same effect that you have already encountered for other senses; that is, a complete blocking of one stimulus by another. Therefore, it is a stronger form of negative interaction than suppression and occurs when the perception of one taste quality is blocked by another. One example involves mixtures of sodium chloride and urea in which sodium chloride masks the bitterness of urea.

Question 21

What nerves are involved in the transduction of taste stimulus?

Answer 21

Taste buds are innervated by the VIIth, IXth and Xth cranial nerves.

The anterior tongue is innervated by the chorda tympani, a branch of the facial nerve (VIIth cranial nerve). This is the region of the tongue containing fungiform papillae, thus the chorda tympani relays information from the taste receptor cells of the fungiform papillae.

The posterior region of the tongue – the region of the circumvallate and foliate papillae – is innervated by the glossopharyngeal nerve (IXth cranial nerve).

The vagus nerve (Xth cranial nerve) innervates taste buds on the epiglottis, larynx and soft palate. The tongue (and taste papillae) is also innervated by the trigeminal nerve (Vth cranial nerve) which provides somatosensory information.

Question 22

Describe the gustatory pathway?

Answer 22

Cranial nerves carry taste information from the taste cells to the solitary nucleus in the medulla (nucleus of the solitary tract), where synapses are formed with neurons projecting to the ventroposteromedial (VPM) nucleus of the thalamus as well as to the amygdala and the hypothalamus. These latter two regions link the gustatory system to the limbic system (affecting emotion, memory and behaviour) and control salivation and feeding behaviour.

From the thalamus there are projections to the primary gustatory cortex (Brodmann’s area 43). It is this area, together with the secondary gustatory cortex, that is responsible for the conscious perception of taste.

One interesting point to note about the gustatory pathway is that it is ipsilateral; that is, there is no crossing over of projections from the left hand-side of the tongue to the right-hand side of the brain, and vice versa. This is in contrast to the other sensory systems discussed in this module.

As its name implies, the primary gustatory cortex is the first cortical area where a stimulus is represented and therefore the first stage of taste perception. However, taste recognition is believed to occur in the secondary gustatory cortex.

Question 23

Name the areas of the brain involved with primary and secondary gustation centres?

Answer 23

The anterior insula and the anterior frontal operculum form the primary gustatory cortex; the secondary gustatory cortex is the orbitofrontal cortex.

Question 24

The sweetness of sucrose is perceived as more intense in the presence of low concentrations of sodium chloride but less intense at higher concentrations of sodium chloride. Describe the taste interactions present in these two situations.

Answer 24

At the lower sodium chloride concentration, sweetness enhancement is observed (synergy is unlikely to occur across two taste qualities). At the higher concentration, sweetness suppression is present.

Question 25

It is often observed that people who have had surgery in which the chorda tympani to each side of the tongue is severed retain their perception of, and sensitivity to, all taste modalities. From what you understand of the gustatory system, explain why this is so.

Answer 25

The chorda tympani branches of the VIIth cranial nerve innervate the anterior two-thirds of the tongue. If these are cut then any taste sensation will be due to taste cells from the posterior one-third of the tongue and also from the soft palate. There is no evidence for a ‘topographic’ organisation of taste detection in the oral cavity; in other words, it is thought that all regions of the tongue and soft palate contain cells that respond to all taste qualities, so this would be consistent with the sense of taste across all modalities being maintained.

Question 26

What area of the brain is involved in the convergence of smell and taste?

Answer 26

It has been proposed that the main sites of convergence for the taste and smell pathways are the orbitofrontal cortex and the amygdala

Question 27

When you have a cold, why can’t you ‘taste’ your food as well as you do normally?

Answer 27

It is not taste that is being affected here but the sense of smell and how it interacts with taste to produce the perception of flavour. Secretions produced by colds block the nasal passages and restrict the access of odorants to the olfactory epithelium. Consequently, the population of orbitofrontal cortex neurons that respond to smell, or smell and taste combined, has reduced inputs, and the perception of flavour is dramatically reduced.

Question 28

In a psychophysical sweetness test, a strawberry-smelling odorant that is not tasted is perceived as being sweet. How can this be?

Answer 28

A strawberry odour and a sweet taste are congruent sensations. The odorant presumably activates neurons in the orbitofrontal cortex that normally receive the congruent inputs from both the olfactory and gustatory systems, the latter associated with a sweet taste. So, when these neurons are activated, the pathway by which the odour enhances a sweet taste is activated and a sweet taste perception is elicited.

Question 29

In a psychophysical taste test using scales of 0–100, a tasteless caramel odour was assigned a ‘sweetness’ rating of 78 and a ‘sourness’ rating of 10. What effect do you think this odour might have on perceived sweetness when combined with sucrose, and on the perceived sourness when combined with citric acid?

Answer 29

The caramel odour is congruent with a sweet taste but not a sour taste. Consequently, the odour would be expected to enhance the sweetness perceptions of sucrose and will either have no effect on the sourness of citric acid or it could diminish the sourness via a cross-modal interaction.

In reality, caramel odour does indeed enhance sweetness, and it also suppresses sourness.

Question 30

Terms used in smell and taste disorders….

Answer 30

Condition Description

Anosmia complete lack of sense of smell

Ageusia complete lack of sense of taste

Congenital anosmia complete lack of sense of smell from birth

Hyposmia (or microsmia)/hypogeusia - partial loss of sense of smell or taste

Hyperosmia/hypergeusia - increased sense of smell or taste

Dysosmia/dysgeusia - altered perception of smell or taste, usually unpleasant

Parosmia/parageusia - unpleasant smell/taste triggered by another smell or taste

Phantosmia/phantageusia - unpleasant smell or taste arising spontaneously

Question 31

Name the common causes of smell disorders?

Answer 31

normal ageing being the most common cause of the loss of smell.
Exposure to toxins includes smoking, alcohol abuse and some occupational toxins.
Side effects of medical treatments include the effects of some drugs and treatments such as radiation to the head and neck.
Psychiatric disorders
Head injury