L24 & L25 (Gustatory)

Question 1

Taste (gustation)

Answer 1

sensations caused by substances that dissolve in saliva (physical stimulus) and penetrate taste buds on the tongue

Question 2

Flavor

Answer 2

sensations arising when odorants in the mouth stimulate receptors for smell through the retronasal passage

flavor sensations are reduced when retronasal passage is blocked by illness/allergies or by plugging your nose (stops food odorants from reaching olfactory cleft)

Question 3

4 kinds of taste papillae in tongue

Answer 3

1. circumvallate
2. foliate (less prominent in adults)
3. fungiform
4. filiform (arrowhead shape)

• all papillae (except filiform) contain taste receptor cells in taste buds
• additional taste buds on roof of mouth between hard and soft palates (not in papillae)

Question 4

3 nerve fibers that innervate the tongue

Answer 4

1. chorda tympani (cranial VII): branch of facial nerve; innervates front 2/3 of tongue
2. glossopharyngeal (cranial IX): back 1/3 of tongue
3. vagus (cranial X): extreme back and epiglottis

Question 5

4 primary taste qualities

Answer 5

• sweet, sour, salty, bitter
• 1 quality predominates for some substances (e.g. sodium chloride tastes salty; hydrochloric acid tastes sour; sucrose tastes sweet; quinine tastes bitter)
• umami may be the 5th quality

Question 6

Salty

primary taste quality

Answer 6

• salts (e.g. table salt, NaCl) dissolve into cations (e.g. Na+) and anions (e.g. Cl-)
• perceived saltiness due to the amount of cation
• why salty taste is needed: need large amounts of sodium for nerve and muscle function

Question 7

Sour

primary taste quality

Answer 7

• acidic substances dissolve into hydrogen (H+) ions
• liked by some at low concentrations
• why sour taste is needed: damage both internal and external body tissues at high concentrations

Question 8

Bitter

primary taste quality

Answer 8

• compounds often contain nitrogen
• cannot distinguish between bitter tastes of different bitter compounds
• why bitter taste is needed: many bitter substances are poisonous but some are good for us (e.g. vegetables)

bitter sensitivity intensifies during pregnancy, which increases survival value

Question 9

Sweet

primary taste quality

Answer 9

• evoked by sugars and contain oxygen, carbon, hydrogen
• why sweet taste is needed: glucose (usually comes from sucrose) is our principal energy source
• e.g. artificial sweeteners mimick chemical structure of sugars but some also activate bitter receptors

Question 10

Umami

potential primary taste quality

Answer 10

• started by monosodium glutamate (MSG) advertising campaign
• detection of nutritionally important protein that occurs in gut (not mouth), though not an essential nutrient for humans (bodies produce it naturally)
• glutamate receptors found throughout body though unclear whether these lead to taste sensations
• protein molecules too large to stimulate taste or odorant receptors

Question 11

Transduction of salty and sour tastes

Answer 11

• salty tastants enter taste receptor cell through sodium (Na+) channel
• sour tastants enter cell through hydrogen (H+) channel or as acid (dissociates into H+ inside the cell)

Question 12

Transduction of sweet and umami tastes

Answer 12

• sweet tastants activate a pair (heterodimer) of G-protein-coupled receptors (TAS1R2 and TAS1R3)
• umami tastants activate a similar heterodimer (TAS1R1 and TAS1R3)

small number of sweet receptors may allow only biologically useful sugars to stimulate our sweet taste

Question 13

Transduction of bitter taste

Answer 13

~25 different G-protein-coupled receptors (TAS2Rs)
* some compounds activate a single receptor (e.g. PROP) while others activate many receptors (e.g. quinine)
* some receptors are activated by specific tastants while others are activated by many different tastants

humans may need a diverse array of bitter receptors to detect the diverse structure of potential poisons

Question 14

3 types of taste receptor cells in each taste bud

Answer 14

1. Type I: for housekeeping (not taste sensations)
2. Type II: no synapses but depolarizes then releases neurotransmitter ATP that acts on adjacent receptor cells or nerve fibers
3. Type III: depolarizes then releases serotonin at synapse with taste nerve fiber

type II is activated by sweet, umami, and bitter tastants while type III is activated by sour tastants

Question 15

2 perceptual dimensions for taste

Answer 15

1. intensity
2. quality

higher concentration produces more intense taste

Question 16

Neural code for taste intensity

Answer 16

at higher concentrations, more neurons fire and individual neurons fire faster

Question 17

2 theories for neural code for taste quality

controversial!

Answer 17

1. labeled-line theory: each taste nerve fiber carries a particular taste quality (doctrine of specific nerve energies); similar to somatosensation and hearing
2. pattern-coding theory: taste quality is carried by the firing pattern across many taste nerve fibers; similar to olfaction and color vision

possible that basic taste quality is coded by labeled-lines while subtle taste differences within a quality are coded by across-fiber firing pattern

Question 18

Evidence for labeled-line theory

neural code for taste quality

Answer 18

• some taste nerve fibers appear to be tuned to specific tastes
• sweet tastes can be temporarily knocked out in humans by gymnema sylvestre

Question 19

Evidence for pattern-coding theory

neural code for taste quality

Answer 19

predictions
* similar across-fiber firing patterns for ammonium (NH4Cl) and potassium chloride (KCl) = taste similar
* different pattern for sodium chloride (NaCl) = taste different

findings
* rats trained to avoid potassium chloride (using electric shock) also avoided ammonium but not sodium chloride
* substances that taste similar to humans show similar firing patterns in rat chorda tympani fibers

Question 20

6 factors affecting detection threshold

Answer 20

1. temperature
2. area
3. duration
4. location
5. adaptation
6. substance being tasted

Question 21

Effect of temperature on detection threshold

for each taste quality

Answer 21

• salty or bitter: most sensitive (low threshold) at low temperatures
• sweet: most sensitive at high temperatures
• sour: temperature has little effect

Question 22

Effect of area and duration on detection threshold

Answer 22

• area: low threshold for larger areas of tongue stimulation
• duration: lower threshold for longer durations of stimulation (200-1500 ms)

Question 23

Effect of location of detection threshold

Answer 23

• slight variation in aboslute threshold for primary taste qualities at different tongue locations
• primary taste qualities NOT exclusively associated with particular tongue locations

sweet and salty (tip), sour (sides), bitter (base)

but suprathreshold concentrations can be tasted at any location except middle of tongue (no taste buds)

Question 24

Effect of self-adaptation on detection threshold

Answer 24

detection threshold for tastant increases during continued stimulation of tongue with same tastant

e.g. harder to taste salt while it remains on the tongue

Question 25

Effect of cross-adaptation on detection threshold

Answer 25

occurs when threshold for one tastant increases after exposure to a different tastant of the same quality

e.g. adapting to sodium chloride raises threshold and reduces saltiness of other salty substances

Question 26

Modification

special type of cross-adaptation

Answer 26

exposure to one tastant alters the quality of a different tastant

e.g. chemical in toothpaste increase threshold for sweet and reduces threshold for bitterness

Question 27

Effect of the substance being tasted on detection threshold

Answer 27

• people tend to be least sensitive to sweet tastes and most sensitive to bitter tastes
• but there are large individual differences in thresholds, particularly in vanillin and PTC

• PTC has no taste for nontasters with high threshold and tastes bitter for tasters with low threshold
• propylthiouracil (PROP) is a safer bitter chemical to use for studies of tasters vs. nontasters

Question 28

Inherited trait for tasters vs. nontasters

PTC or PROP

Answer 28

• tasters typically have one or both dominant alleles for the gene TAS2R38 that expresses a specific bitter G-protein-coupled receptor
• nontasters have 2 recessive alleles

Question 29

Steven’s magnitude estimation technique

scaling

Answer 29

suprathreshold perceived taste intensity as a function of concentration

perceived intensity for suprathreshold bitter tastes grows more slowly than for other tastes but we are more sensitive to bitter at low concentrations

Question 30

Cross-modality matching

suprathreshold

Answer 30

• match intensity of sensations that come from different sensory modalities (e.g. loudness of sound to brightness of light to intensity of taste)
• perceived bitterness increases from nontasters to medium tasters to supertasters

Question 31

Supertaster

Answer 31

• experiences strong sensations of taste, flavor, texture, and oral burn
• usually have dense fungiform papillae

e.g. PROP supertasters have lots of fungiform papillae while medium tasters have less

Question 32

Health consequences for the 3 categories of tasters

Answer 32

1. supertasters/medium tasters: higher risk of colon cancer (because they avoid eating bitter vegetables)
2. supertasters: lower risk of cardiovascular disease
3. nontasters: more likely to smoke and consume alcohol (because they don’t taste bitterness of tobacco and caffeine)

Question 33

Taste discrimination

Answer 33

poor intensity discrimination ability for all tastes compared to other sensory modalities

JND is 15-25% of standard intensity

Question 34

Evidence for innate taste hedonics

pleasure/displeasure evoked by primary taste qualities

Answer 34

newborn facial expressions for taste qualities
* smile for sweet, pucker for sour, spit for bitter (even in infants with no cortex)
* salty receptors may be immature at birth but produce smile when functional

Question 35

Specific hungers theory

Answer 35

a deficiency of a given nutrient will produce a craving for that nutrient, but only true for sugar and salt

cravings can only cause an animal to eat a needed nutrient when there’s a sensory cue (e.g. taste) associated (e.g. deficient rats don’t seek out B1)

Question 36

Evidence for specific hungers theory

Answer 36

• study allowing newly-weaned infants to choose their food for 6 years (all thrived)
• infants were eating a variety to prevent boredom and all food choices were healthy
• suggests innate mechanism controlling healthy eating

Question 37

What determines food preferences?

Answer 37

hard-wired preferences contributed by taste and learned preferences, which are contributed by retronasal sensations (odors perceived when chewing and swallowing that determine flavor)

Question 38

Link between retronasal and orthonasal olfaction

Answer 38

• unclear BUT may learn to separate likes/dislikes for retronasal and orthonasal smells (e.g. cut grass has pleasant orthonasal but not retronasal smell)
• aversions learned retronasally often transfer to orthonasal olfaction

Question 39

Special case of taste hedonics for chili peppers

Answer 39

• preference is not innate, rather depends on social influences (learned)
• only humans like them possibly due to health benefits (e.g. may release endogenous opiates)
• variability across individuals depending on the number of papillae/pain fibers

Examples
* chili peppers are introduced to Mexican children at age 3 and they choose to eat them by age 5-6
* exposure to capsaicin (what makes peppers hot) can result in chronic desensitization of pain fibers