taste

Question 1

how many specialised receptor cells within each taste bud

Answer 1

50-150

Question 2

are taste receptor cells neurons?

Answer 2

Behave like neurons (but aren’t)
Transduce chemical signals
Not neurons, neural epithelial cells

Question 3

Type I taste receptor cell and conc. in cell

Answer 3

glia like functions (housekeeping) - supportive (maintain K+ conc., take up neurotransmitters, don’t send signals)
- 50% of cells in a taste bud

Question 4

Type II taste receptor cell and conc. in cell

Answer 4

respond to sweet, bitter - taste receptors (G protein)
- ⅓ rd of cells in a taste bud
- express G-protein coupled receptors (for sweet, bitter, umami) and their downstream effects
- most have only one type of tastant receptor
- don’t make synapses with afferent fibres
- release ATP through pores (flows out at high conc. to activate nerve fibers), which acts on adjacent cells or nerve fibers to produce neural signals

Question 5

Type III taste receptor cell

Answer 5

respond to sour
- 2-20% of cells in a taste bus (lowest conc.)
- don’t express GPCRs but have the machinery to detect sour tastants
- have chemical synapses with synaptic vesicles
- make sinuses with afferent nerve fibers
- can indirectly respond to type II (bitter, umami, sweet)
- true synapses

Question 6

2 Mechanisms of Taste Transduction

Answer 6

Ion channels (salty Na+, sour H+)
G-protein coupled Receptors (bitter, sweet - larger molecules bind to G-protein)

Question 7

Acid ion channel receptor

Answer 7

acid dissociates into acid+base inside cell, increases acidity of intercellular fluid, closes K+ channel -> depolarization

Question 8

Type 1 taste receptor, T1Rs

Answer 8

sweet
- function as heterodimers (require T1R3)
T1R2-T1R3: ligands include sugars (glucose, fructose, sucrose…)

Question 9

Type 2 taste receptor, T2Rs

Answer 9

bitter
-function as monomers or dimers
ligands: chemicals, like those in nitrogen containing alkaloids
T2Rs activate G-protein signalling cascade

Question 10

Taste Transduction Steps (all these pathways are not on the same cell, all tastants are not on all cells - taste buds are composed of diff cell types that respond to diff ligands)

Answer 10

1.Tastant-transducing channels/GPCRs are activated
2. This leads to depolarization directly (ion channels) or indirectly (GPCRs)
3. Voltage-gated Na+ and K+ channels are activated (graded and action potentials); voltage-gated Ca2+ channels open
4. In type III cell, Ca2+ causes neurotransmitter (serotonin) release into the synaptic cleft (ATP and GABA may also be involved)

Question 11

afferent taste neurons are:

Answer 11

pseudounipolar, (with peripheral endings in tongue/mouth, central
ending in medulla - nucleus of solitary tract), cell
bodies lie in (cranial nerve) ganglia outside of
brainstem

Question 12

Taste info is carried by:

Answer 12

VIIth (facial), IXth
(glossopharyngeal), and Xth (vagus) cranial nerves

that emerge from the brainstem

Question 13

Urban Legend - The Tongue Map

Answer 13

MYTH: different parts of the tongue detect different taste primaries

FACT: receptors for all 4 basic tastes (sweet, salty, sour, bitter) are distributed over the entire tongue
Diff tongue places hae diff sensitivities (small differences/thresholds in diff places. If we inc. conc. we can taste more

Question 14

The Tongue Map Redefined
Regional Coding

Answer 14

All tastes can be detected over the entire surface of the tongue

Chemotopic Organization: Different regions have slightly different thresholds for various tastes (glossopharyngeal nerve innervates back of tongue, facia is front of tongue/tips)

Though thresholds are determined by most sensitive nerve fibres, real-world taste intensities are produced by the summation across fibres with varying thresholds..

Question 15

Neural Coding (Gustation)

Answer 15

the way that the identity, concentration, and pleasurable/aversive values of tastants are represented in a
pattern of AP’s relayed to the brain from the taste buds

Question 16

Labeled Line Coding

Answer 16

different receptors and their associated sensory fibers are responsible for transmitting highly specific information

Question 17

Cross-Fibre Coding

Answer 17

different qualities of sensory modality are distinguished by the pattern of nerve discharges across a large population of fibers

Question 18

Evidence for Combinatorial Model

Answer 18

some taste receptor cells are selectively sensitive to particular tastants; others are broadly tuned (regarding graded potentials of diff tastants)

Question 19

Combinatorial Model

Answer 19

Type III (sour) Generalist: taste receptor that responds secondarily to other taste stimuli via cell-to-cell communication within the taste bud

Type II (sweet, bitter, salty) Specialist: taste receptor that expresses only 1 type of receptor

specialist and generalist sensory ganglion neurons converge (cross-fibre, combinatorial coding) onto afferent axons (which are less selective) -> hindbrain neurons

Question 20

Steps of Gustatory Processing in the Brain

Answer 20

1. Taste signals transmitted via cranial nerves to…
2. Nucleus of the solitary tract (NST, first relay site), a collection of neurons in the medulla
3. NST axons project to a nucleus in the thalamus (VPMN, second relay site)
4. Primary gustatory cortex (in frontal lobe within the Sylvian fissure) = insula and surrounding areas

Question 21

Is there a gustotopic map in the cortex?

Answer 21

The brain was imaged using a
technique called intrinsic signal optical Imaging (rat brains were exposed and light reflectance from tissue was recorded, Tastants were applied to oral cavity)

Different tastants revealed different
patterns of activation (Maps were stereotyped across similar animals, overlap between modalities - not very clear result, complicated)

Question 22

taste processing pathway to secondary taste cortex

Answer 22

taste receptors
cranial sensory ganglia
brain stem (NST)
thalamus (VPMN)
primary taste cortex
secondary taste cortex (connects to hypothalamus and amygdala)

Question 23

secondary taste cortex

Answer 23

aka orbitofrontal cortex (OFC)
-processes higher aspects of taste functions (motivational affects of hunger and satiety)
-some neurons are multimodal (integration area)

Question 24

Miracle Fruit and what compound it contains (glycoprotein)

Answer 24

derived from plant Synsepalum dulcificum, tasteless to humans. Naturally grows: tropical West Africa

Contains a taste altering glycoprotein called miraculin (bind to sweet taste receptors, makes sour tasting foods taste sweet)

Question 25

Natural uses of miracle berry and How it is used to shape our dietary practises:

Answer 25

natives chew it to make acidic foods more palatable (ex: stale maize bread, sour wine and beer) help digest unpalatable food, cure for world hunger, helps obesity epidemic

Question 26

The Sweet Receptor

Answer 26

Heterodimer (T1R2 - T1R3); sweet chemicals bind to the “venus flytrap” portion Small molecules bind to T1R2 Large molecules bind to T1R3 Respond to sugars, sweeteners, small proteins Ligand binding activates the receptor -> activates a G-protein signalling pathway -> ultimately increases Ca2+ in the intracellular fluid

Question 27

ICI 2 procedure

Answer 27

use scaling technique to taste foods before and after berry based on “salty, sweet, sour, bitter” tastes, swab tongue to found number of fungiform papillae within 6mm diameter circle at tip of tongue

Question 28

Flavour is stimulated by

Answer 28

retronasal olfaction: foods emit odorants that rise up behind the palate and enter the nose from the rear The complexity of sensations evoked by foods is attributed to the olfactory system

Question 29

4 Basic Tastes

Answer 29

any of the 4 taste qualities that are generally agreed to describe human taste experience, including sweet, salty, sour, bitter We are either born liking or disliking them (hardwired affect)

Question 30

Salt

Answer 30

Na+ is important to maintain nerve and muscle function (consider the action potential) - eg. NaCl

Question 31

Bitter

Answer 31

most diverse, to detect most poisons. Thousands of bitter molecules detected by 25 bitter receptors -> combinatorial coding. We avoid bitter compounds because they often signal poison/toxicity. - eg. quinine, thiamine, PROP

Question 32

Sour

Answer 32

some people like the sourness of acids in low concentrations, at high concentrations acids will damage external and internal body tissues (damage proteins and other molecules) - eg; citric acid, acetic acid (vinegar)

Question 33

Sweet

Answer 33

sugar is the principle energy source for humans, less receptors (compared to bitter) - eg. glucose (useful energy molecules, simple sugar); sucrose (complex sugar)

Question 34

Eating for survival

Answer 34

Taste provides info about what we should ingest (or not) Each of the 4 basic tastes is responsible for a certain nutrient (sodium, sugar) or antinutrient (acid, poison)

Question 35

Affect of food

Answer 35

Foods can be pleasing or displeasing (affect) Affective experience and reflexive responses are innate (eg: infants show stereotypes facial expression for sweet, sour and bitter) infants with anencephaly (lack cerebral cortex) still exhibit same facial expressions

Question 36

Beyond 4 Basic Tastes Umami

Answer 36

evokes sensation of savoriness (meaty), manufacturers of MSG (monosodium glutamate) claimed it signalled protein, proteins are large molecules that are broken down mostly in the gut

Question 37

Beyond 4 Basic Tastes Fat

Answer 37

fatty acids attached to a support structure, some fatty acids are released in the mouth (most are broken down in the gut)

Question 38

How do we decide what is a basic taste?

Answer 38

Are receptors for the chemicals found in the mouth? Does activating those receptors lead to distinct sensations? Is affect hardwired?

Question 39

Umami and Fatty Acids as Basic Tastes… Support for and Against

Answer 39

Support for: There exist receptors in the mouth that appear to regulate the palatability of protein and fat (ex: umami receptor is a heterodimer made up of TAS1R and TAS1R3) Support Against: Proteins and fats are big molecules - > mostly broken down by the gut during digestion There are receptors for amino acids and fats throughout the gut that don’t contribute to taste Umami and fat are not universally liked/disliked (no hardwired effect) Conscious sensations about fats are evoked by the somatosensory system (eg. oily, creamy), not a taste but a feeling

Question 40

How does miraculin work

Answer 40

T1R2-T1R3 expressing cells (heterodimer) were exposed to acidic (

Question 41

Supertaster

Answer 41

an individual whose perception of taste sensations is the most intense Depends on a variety of factors, including density of fungiform papillae and genetics ** not all supertasters are PTC/PROP tasters Supertasters also experience the most intense sensations of oral burn (ex: chili peppers), oral touch (eg. creamy texture from fats - cause of nerves that innervate oral cavity), and flavour

Question 42

There is large variation among individuals in terms of number of fungiform papillae

Answer 42

5-60 per mm diameter region

Question 43

Specific Hungers Theory and evidence

Answer 43

missing nutrients are craved Clinical case: boy was always craving salt, salt heavy diet. Turns out he had tumour in pituitary gland and was lacking salt in body. Craving and eating this salt was keeping him alive because when he got admitted to hospital and doctors changed his diet, he died. Infant food preferences: gave infants the option of a variety of healthy foods with different nutrient concentrations. They found infants ate a variety of foods + healthy diet when given the choice. Rat diet experiment: rats were first fed a diet deficient in B1 causing them to get sick. Then they gave them option to eat novel diet with lots of B1 ;the rats switched diets.

Question 44

Debunked specific hunger theory

Answer 44

Infant food preferences: babies prefer variety. All foods provided were healthy, as long as they chose anything that would be healthy. People prefer variety and get sick of the same thing. Rat diet experiment: rats avoid sickness. Conditioned taste aversion, did not want to be sick and eat food that makes them sick.

Question 45

Food Preferences Depend on Retronasal Olfaction

Answer 45

We only sense a few nutrients (ex: sugar, salt) Olfactory affect is critical when nutrients aren’t conveniently labeled by taste (odour image in the olfactory bulb can be linked to positive or negative affect depending on the consequences of what we eat) We learn to like or dislike food because of he consequences of consuming them (positive and negative consequences - ex: calories or nausea, respectively, lead to conditioned taste preferences and aversions)

Question 46

Updated theory: food preferences are learned!

Answer 46

innate preferences (eg.sugar, salt) + learned olfactory effect = food likes and dislikes reasons that explain eating behavior: social environment, sadness/mood, motivation this explain gustatory effect, not eating behavior and food choices (we eat, crave and enjoy mcdonalds even though we feel gross after)

Question 47

2 perceptual aspects of taste:

Answer 47

intensity quality (sweet, salty, bitter, sour)

Question 48

Electrogustometry (clinical)

Answer 48

deliver small electric current through an electrode or metal disk to a specific point on the tongue or oral cavity Stimulus application is highly discrete in time and space (specific location on tongue, determine length of time stimulus is applied) Fast Can measure taste detection thresholds Useful for locating lesions of taste pathways Problems: may not accurately represent different tastants - best activates ion receptors like salt/acid, not sweet/bitter. Not great at detecting some clinical conditions that alter taste perceptions

Question 49

Chemogustometry

Answer 49

actual tastant molecules are applied to tongue, person needs to detect what/when taste is there Regional (use cue tip/filter paper with tastant) Whole-mouth (take sip of each cup of diff flavours - 4 water and 4 sugar for example - and differentiate them) Problems: adaptation, not applying to discrete locations which can be useful when looking for nerve damage, can't control how long person keeps taste on tongue, not space and time discrete

Question 50

Ageusia

Answer 50

total loss of taste Injury to gustatory nerves Medications (ex: cancer, depression): some radiation/anticancer drugs kill cells, medication can also secrete into saliva making food taste different

Question 51

Hypogeusia

Answer 51

reduction in taste sensitivity Dry mouth Smoking Illness (eg.flu, diabetes)

Question 52

Dysgeusia

Answer 52

taste perceptions are distorted (can detect food but it taste different than it should) Eg: sweet things taste salty Eg: metallic taste for cancer patients, medication can also secrete into saliva making food taste different

Question 53

Taste Detection Thresholds

Answer 53

Bitter(most sensitive) < sour < salty < sweet threshold

Question 54

in terms of discriminability taste is...

Answer 54

the least sensitive sensory function in terms of discriminability (largest Weber fractions - need to add a lot to notice difference of one conc. Vs another)

Question 55

Taste Thresholds are affected by:

Answer 55

Other tastants in a mixture (masking) ex: add salt to reduce bitter Temperature (optimal at 22-25 degrees celsius) lowest threholds at room temp Location on the tongue Age older ppl have inc.threshold due to less taste buds Stimulation area, full mouthful will be easier to taste than just one region Genetics

Question 56

Taste Adaptation:

Answer 56

prior or ongoing stimulation reduces the perceived intensity or sensitivity to a tastant (time required depends on concentration of tastant). First bite tastes best

Question 57

Cross-Adaptation

Answer 57

perceived intensity of a compound decreases because of adaptation to a different compound of the same taste quality Ex: lemonade tastes more sour after eating a sweet dessert, or if you eat a lot of sweet and then eat a diff sweet thing - the second will taste less sweet Note: the 4 classical taste qualities don't cross-adapt (consistent with labeled- line coding: specific receptors for each taste quality)

Question 58

Mixture Suppression (masking):

Answer 58

when one taste quality suppresses another (ex: the sugar in tonic water makes it taste less bitter)

Question 59

The bitter Receptor and how many do humans express

Answer 59

Type 2 taste receptors (T2Rs or TAS2Rs) May function as monomers or dimers Ligands: chemicals, especially those in the nitrogen containing alkaloids (eg. quinine) Humans express at least 25 different bitter receptor proteins (some respond to specific compounds, others are generalists)

Question 60

Phenylthiocarbamide (PTC) and propylthiouracil (PROP)

Answer 60

organic powders which, when dissolved in water, taste bitter to some individuals (they both bind to bitter receptors encoded by the TAS2R38 gene) Taster (of PTC/PROP): an individual that perceives PTC/PROP as bitter-tasting. Born with 1-2 dominant versions of the TAS2R38 gene Nontaster (of PTC/PROP): an individual that is unable to taste PTC/PROP (or very high threshold). Born with 2 recessive versions of the TAS2R38 genes

Question 61

Taste and Health

Answer 61

1. Some bitter foods are actually good for us (can we “turn off” bitter sensations to make people eat their vegetables?) 2. Women are more sensitive to bitterness during pregnancy -> adaptive (sensitivity diminishes after menopause) 3. Artificial Sweeteners (non-sugar molecules) are attractive to dieters because their sweet taste comes with essentially no calories. (aspartame - but do these artificial sweeteners actually help?) 4. Taste receptor cells are lost with age → seniors lose appetite 5. Supertasters are more finicky eaters because bitter sensations are more intense (Reduced vegetable intake increases risk for colon cancer. BUT, are less likely to smoke and consume alcohol)