Week 3- Gustation

Question 1

What are the two chemical senses?

Answer 1

-Taste + Smell
-Sensation arises due to chemicals from the environment coming into contact with chemoreceptors

Question 2

True or false? The chemical senses developed relatively late compared to other senses…

Answer 2

-False
-The chemical senses are ancient senses i.e. they developed first
-The reason they developed first is because they are integral to survival e.g. making sure we eat the right foods

Question 3

What are the five recognized sub modalities of taste?

Answer 3

Sweet, Salty, Sour, Bitter, Unami

Question 4

What is Unami as a taste?

Answer 4

-Means great taste literally
-Roughly corresponds to a savory taste
-Foods under this category are usually high in amino acids i.e. cured meat

Question 5

Is the taste the same as flavour?

Answer 5

No, taste is pure sensation whereas flavor involves a mix of sensory data such as taste, smell, somatosensory, sound and vision

Question 6

What term means that sensory data is mixed to form a percept?

Answer 6

Multisensory integration

Question 7

In the real world do our senses exist in isolation?

Answer 7

No, this pulls into questions the validity of some experiments- what can they really tell us about how our senses function in the real world?

Question 8

What general areas are invovled in the gustatory system?

Answer 8

-Tongue (taste buds, taste neurons, tastant receptors)
-Cranial nerves (7, 9, 10)
-Brain stem
-Thalamus
-Primary taste cortex (insular-frontal operculum)
-Secondary taste cortex (Orbito frontal cortex)

Question 9

What is the definition of a tastant? What are some features of them?

Answer 9

-Any molecule that elicits taste
-Tend to be water soluble chemicals as this allows them to dissolve in salvia and thus be small enough to reach and activate tastant receptors or ion channels
-Must be in sufficient concentrations in order to elicit a taste sensation

Question 10

What is the tastant to receptor ratio?

Answer 10

1 tastant activates 1 receptor/ion channel type (specificity)

Question 11

What are papillae? What types are invovled in taste?

Answer 11

-Folds in the tongues epithelium
-Types invovled in taste are circumvallate, foliate and fungiform
-Papillae are not the same as taste buds-they house taste buds

Question 12

Where are taste buds found?

Answer 12

In the trenches/ grooves between the papillae of the tongue

Question 13

How many taste buds are there in humans?

Answer 13

5000-10,000

Question 14

In terms of the taste system what is the role of taste buds?

Answer 14

They are the sensory organ containing the taste receptor neurons needed to interact with tastants

Question 15

Is the number of taste buds an individual has fixed?

Answer 15

No, the taste buds are constantly replenishing every 7-10 days.
-In addition, the number of tastebuds changes across the lifespan of an individual (Increases from birth to about 40 and then decreases after that)

Question 16

How are the taste buds structured so that taste receptor neurons come into contact with tastants?

Answer 16

-TRN extend microvilli through the taste pores (essentially holes) into the papillary trench which contains saliva with tastants dissolved in it

Question 17

Is the tongue map legit?

Answer 17

-The image of the tongue map (the idea that different areas of the tongue are invovled in different sub modalities of taste) became popular in textbooks but is based on little evidence

-Experiments show minute differences in threshold detection for different tastes across the tongue but this reflects a difference in sensitivity not a difference in sensation

-It is likely that the concept just got lost in translation as Hanig (1901)was citing previous work by Hoffman (1875) about the topography (Location) of taste buds. A tongue map diagram was never meant to come from this research.

Question 18

What did collings (1974) find in relation to the tongue map?

Answer 18

-Looked at NaCl (salty), sucrose (sweet), quinine and urea (both bitter), and citric acid (sour)
-Found that sensitivity thresholds exist but mainly going by taste not area
-It appears that all tastes exist on all parts of the tongue (i.e. there are receptors for all five traditional tastes anywhere there are taste buds)

Question 19

How many cells does each taste bud have? What 3 types are the cells there?

Answer 19

-40-60 cells per taste bud
-Basal cells, support cells, taste receptor cells

Question 20

What can the basal cells of the taste buds differentiate into?

Answer 20

-Support cells
-Taste receptor cells

Question 21

Where are tastant receptor proteins found? What different types are there?

Answer 21

-On the microvilli of taste receptor neurons
-Have tastant receptors proteins for sugars, amino acids and alkaloids, for acids or salts there are ion channels

Question 22

What are the three types of TRN?

Answer 22

-Type 1: detect low salt (Note: type 1 technically is a glial cell so some controversy here)
-Type 2: detect high salt, sweet, bitter, Unami, kokumi
-Type 3: detects sour

Question 23

What happens after the tastant depolarizes the TRN?

Answer 23

-Triggers release of neurotransmitter onto sensory nerve fibers
-This generates an electrical signal (AP) in the nerve fibers =signal transduction

Question 24

What are some aspects of flavour that don’t fit under the traditional 5 ‘taste’ model?

Answer 24

-Feel of food and drink (hot, cool, rich, thick)
-Chemesthesis
-Pungency of chili peppers, ginger, horseradish (spiciness works via capsaicin receptors on free nerve endings- is it pain or taste?)
-Kokumi= a enhancer of both flavour and texture rather than taste

Question 25

Specifically what cranial nerves are invovled in taste?

Answer 25

-Nerves VII (facial nerve), IX (glossopharyngeal), X (vagus) contain axons of the TRN
-The facial neve is innervates the front of the tongue, the glossopharyngeal= the sides, and the vagus the rear

Question 26

What are the two alternative hypotheses’ for how taste information is encoded?

Answer 26

-Labelled line hypothesis: individual TRN encode a signal taste quality (e.g. sweet) and synapse with afferent fibres that are also dedicated to that quality
-Combinatorial model: information is transmitted by combinatory activity in multiple fibres. Overall combination of fibres activated encodes the taste quality (like population coding)

Question 27

What are the two options in the combinatorial model?

Answer 27

Two options:
1. TRNs have multiple receptors, so send mixed messages down one sensory fibre
2. TRN of one kind has 1 kind of receptor, but multiple TRNs innervate a single sensory fibre

Question 28

“some sensory fibres respond best to one tastant”
What hypothesis of encoding does this statement align with?

Answer 28

Labelled line

Question 29

What is some evidence in support of the combinatorial model?

Answer 29

-Taste buds contain both narrowly tined cells and more broadly responsive ones (type 2 vs type 3 etc.)
-Sensory fibres can be broadly tuned or narrowly responsive (i.e. responsive to one or multiple tastants)

Question 30

What is the most likely model for the encoding of taste?

Answer 30

-Have specialized (type 2) cells= devoted to either sweet, bitter or salty tastes (sub populations within the group are devoted to different tastes) : aligns with labelled line!

-Also have, generalized (type 3) cells. Type 3 cells primarily are responsive to sour tastants which is why this terminology is confusing. The cells are generalist because they are activated indirectly for sweet, bitter and salty tastes in addition to the direct role they play in sour tastes. This happens because the type 2 cells when active go onto activate the type 3 cells.

-Specialised and generalists sensory neurons
-Specialised and generalist neurons in the brain stem (hind brain)

Question 31

What two things need to happen to elicit a sour taste from the generalist type 3 taste receptor cells?

Answer 31

-Direct activation via sour tastant
-Relative absence of other tastes: sweet, bitter, salty

Question 32

Is their evidence of rate coding in TRNS or sensory afferents of the gustatory system?

Answer 32

No, there is not (intensity coding happening further up?)

Question 33

Where do the cranial nerves project to in the taste transduction mechanism? What happens after?

Answer 33

-The brain stem nuclei
-Brain stem to thalamus
-From thalamus to gustatory (insular) cortex and orbitofrontal cortex
-From the thalamus projections also happen to the hypothalamus and amygdala

Question 34

What is in the insula + what is it’s function? What is the organisation of the insula like?

Answer 34

-Insula= small region of cortex deep within the lateral sulcus
-It is the primary cortical processing area for taste
-In humans separate areas insular are activated by various tastants supporting a labelled line hypothesis within the cortex
-However, other areas are activated by multiple tastants (so supporting combinational)

Question 35

Where does the gustatory cortex send inhibitory projection to? What is the function of this?

Answer 35

-The mouth
-This blocks pain perception which is important for survival: you have to eat regardless of whether it hurts or not

Question 36

Where is the orbitofrontal cortex how is it invovled in taste?

Answer 36

-Part of the frontal lobe lying above bone (orbit) of the eye socket
-Receives projections from insular cortex
-Involved in processing of temperature, touch, smell and taste (and other stuff…pain, love…)
-Is thought of as a potential integration are whereby sensory modalities combine with emotion to create an overall percept
-Sometimes know as the preference cortex or valence cortex as like with smell it is invovled in making decisions about what tastes good and bad

Question 37

Damage to the OFC cortex causes….

Answer 37

Impairment in choice preference tasks (impairs hedonic assessment)

Question 38

The role of nature + taste preferences

Answer 38

-The major 5 tastes largely elicit innate preferences
-Broadly speaking, Unami and sweet= good, sour & bitter= bad
-This is shown especially in children which supports the idea that it is innate preference and not due to learning

Question 39

In relation to preferences how is chemo sensation (i.e. taste and smell) different to other senses?

Answer 39

-Smells/ tastes often grouped as good or bad- tend to be universal preferences
-For other senses there is not as much of a drive to group things as good or bad and if there is a judgement is is subjective i.e. not colour is universal known as a ‘bad’ colour

Question 40

Genetics + taste preferences

Answer 40

-There is a strong genetic component to taste
-Many species have evolved to not express receptor proteins for certain tastes e.g. cats don’t have sweet taste receptors
-There is also individual variation in terms of genetics e.g. the coriander hypothesis (this genetic variation affects the gene for an olfactory receptor)

Question 41

What is Phenylthiocarbamide (PTC) and how does it show the role of genetics in taste preference?

Answer 41

-PTC can taste very bitter or not at all depending on polymorphisms of TAS2R38 gene
-This genetic variation affects the gene for bitter tastant receptor

Question 42

What are some ways that genetic variation can affect the taste system (i.e. what structures could a mutation theoretically affect)?

Answer 42

-Genetics can influence the number of papillae, TRNs, free nerve endings
-And dictate types of taste receptors

Question 43

Physiological factors + taste preference

Answer 43

-Exposure through amniotic fluid or breast milk confers greater taste acceptance in infant post weaning (Schaal et al., 2000)
-Stage of development – sensitivity to bitterness & sourness increases in childhood, sensitivity to sweetness decreases (Vennerød et al., 2018)
-Mineral deficiency in childhood drives preference
for salty food in later life
-Hormones: pregnancy affects bitterness sensitivity

Question 44

What does nurture mean in terms of taste perference?

Answer 44

-Nurture= learned taste preference
-We seek novelty, but crave familiarity or conformity (for safety)
- +/- reinforcement
-Conditioned taste aversion (eat something, get sick, develop STRONG aversion to that food)
-Psychosocial influences, esp. in adolescence (what we eat is influenced by those around around us)
-Cultural practice – embryonic or early life
exposure contributes to food acceptance and
preferences later (Mennella et al., 2001; 2005)

Question 45

What are the two functions of taste?

Answer 45

-Survival value
-Hedonic feeding

Question 46

How do we use taste for survival in relation to the five basic tastes?

Answer 46

Taste is a system for detecting nutrients and antinutrients

-Bitter: Might signal poisons (many bitter plants are poisonous)

-Sour: Acidic solutions might harm the body or indicate spoilt, unsafe food

-Sweet & Salty: Our bodies need sodium and sugar to survive

-Umami: Signals protein content

Question 47

What is the specific hungers theory (Curt Richter, 1940s)? Has it been shown in humans?

Answer 47

-Posits that deficiency of a given nutrient drives hunger for that nutrient
-Many animals (rats, insects) can self-select foods to
address micronutrient deficiencies (e.g. B vitamins)
-Only demonstrated for sweet or salty foods in humans –we’re hungry eaters, but not healthy eaters

Question 48

Do we always avoid sour food? What relationship exists for sour taste preference?

Answer 48

-No, nearly every culture on earth adds microbes to food to make them more sour
-The relationship is an inverted U- there is an limit to the sour taste we like after this point too acidic/ damaging

Question 49

Why might a species have lost a certain taste modality what is an example?

Answer 49

-Pandas have lost the ability to taste Unami
-This is likely driven out of resource constraint. If meat was scarce it makes sense to lose then receptors associated with Unami so that you stop wasting time and energy searching for a resource that is just not there.

Question 50

Why is advantageous to have hedonic feeding?

Answer 50

-It reinforces eating behaviour when you get a positive response from eating food
-Food= vital to survival so this behaviour needed to be preserved

Question 51

What does taste cause in the brain (hedonic feeding)?

Answer 51

-Release of dopamine
-DA signals salience or pleasure
-Either way this helps to reinforce feeding behaviour

Question 52

In the modern world does our eating behaviour lean towards hedonistic or homeostatic?

Answer 52

-Is becoming more hedonistic (driven by pleasure rather than need)
-This is reflected in the rising numbers of overweight individuals

Question 53

How are companies exasperating the problem of hedonistic eating behaviours?

Answer 53

-Companies rise the amount of salt and sugar in food as well as certain chemicals which produce intense tastes (MSG, kokumi substances etc.)
-Food tech can be used to alter nutritive value signal of food to reinforce feeding behaviour
-Certain chemicals can mask/ alter the tastes of tastants (alter the activity of ion channels or receptors on TRN either to block or augment activity)

Question 54

What experimental variables can influence taste detection experiments?

Answer 54

-Sub modality:
absolute threshold and JND of tastes is modality specific and interestingly not all studies find the same taste thresholds.

This is described by stevens power law as when you increase stimulus magnitude perceived intensity goes up differently depending on the sub modality.

Even within a sub modality individuals can have different sensitives i.e. an individuals sensitivity to fructose has to correlation with their sensitivity to sucrose (peng et al., 2016)

-Temperature

Question 55

What participant variables can influence taste detection experiments?

Answer 55

-context:
adaptation- need to instruct participants to not eat or drink anything before they come in so adaptation is standardized.

Expectation- i.e. knowing the identity of a tastant can improve detection performance

Familiarity of food, location

-sex:
effect of sex is mixed with often no difference but some studies show females better than males at detecting tastes. This could potentially just be due to time of testing though as gender differences seem more apparent in sated than in hungry participants (Laeng et al,1993)

-age: detection declines with age especially for salt and bitter. Very consistent findings. Correlates with age-dependant loss of TRNs

-genetics

Question 56

McBurney et al (1967)

Answer 56

-Adaptation to salt : expose participants to high or low NaCl
-Perceived intensity goes ↓ as expected, but also the JND increases! i.e. salt exposure blunts response to salt in 2 ways
-These effects can be longlasting and have a profoundeffect on diet

Question 57

What is cross-adaptation?

Answer 57

-When taste of one food affects another
E.g. NaCl in saliva blunts perceived intensity of salt (adaptation)
E.g. A sour beverage tastes too sour after eating something sweet (crossadaptation)

Question 58

How good are in general at discriminating tastants? Do taste discrimination tests have ecological validity?

Answer 58

-We can often detect a tastant but have difficulty naming it
-All the factors effecting taste detection also effect taste discrimination
-In reality real world tasting is a multisensory experience so testing discrimination in a laboratory setting is not the best for ecological validity

Question 59

What is Ageusia and what can be the causes?

Answer 59

Ageusia – absence of taste perception (partial/complete, transient/permanent)

• Causes:
• congenital oral trauma
• nerve trauma/infection CNS damage (isolated ageusia due to CNS lesions very rare)
• chemotherapy (transient) as chemotherapy targets stem cells like the basal cells in the taste buds
• Commonly reported effects - loss of interest in food (but < anosmia), forgetting to eat, ????? (it’s a rare disorder)
• Taste buds highly susceptible to COVID-19 infection. Emerging evidence of ageusia as a “long-COVID” symptom (Biadsee et al., 2021)

Question 60

What is hypoguesia and what can be the causes?

Answer 60

• Hypogeusia – diminished taste perception
• Causes:
• xerostomia (dry mouth)
• commonly associated with nerve toxins and oral health
  medications (some in particular as they induce xerostomia: e.g.
  anti-inflammatories)
• As mentioned, part of natural aging – natural drop in bud#, saliva, energy needs
• hypogeusia can be confounded by hyposmia

Question 61

What is Dysgeusia and what can be the cause?

Answer 61

Dysgeusia – altered taste perception
* Causes:
- usually follows anosmia
- link to bacterial, viral or fungal infections of gums
- chronic acid reflux, neurological disorders, metabolic disorders, use of dental prosthesis, pregnancy

Question 62

Do people with dysgeusia typically report ‘good’ distorted taste

Answer 62

• Typically very unpleasant. People rarely report a “good” distorted taste….
• cacogeusia – foul taste, sometimes even without tastants present (phantom percept)
• rotten tastes
• metallic tastes (often applies to ALL tastants)
• sweet/salty tastes no longer taste sweet or salty (e.g. sour/bitter)