L24 & L25 (Gustatory) Flashcards
Taste (gustation)
sensations caused by substances that dissolve in saliva (physical stimulus) and penetrate taste buds on the tongue
Flavor
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)
4 kinds of taste papillae in tongue
- circumvallate
- foliate (less prominent in adults)
- fungiform
- 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)
3 nerve fibers that innervate the tongue
- chorda tympani (cranial VII): branch of facial nerve; innervates front 2/3 of tongue
- glossopharyngeal (cranial IX): back 1/3 of tongue
- vagus (cranial X): extreme back and epiglottis
4 primary taste qualities
- 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
Salty
primary taste quality
- 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
Sour
primary taste quality
- 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
Bitter
primary taste quality
- 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
Sweet
primary taste quality
- 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
Umami
potential primary taste quality
- 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
Transduction of salty and sour tastes
- 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)
Transduction of sweet and umami tastes
- 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
Transduction of bitter taste
~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
3 types of taste receptor cells in each taste bud
- Type I: for housekeeping (not taste sensations)
- Type II: no synapses but depolarizes then releases neurotransmitter ATP that acts on adjacent receptor cells or nerve fibers
- 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
2 perceptual dimensions for taste
- intensity
- quality
higher concentration produces more intense taste
Neural code for taste intensity
at higher concentrations, more neurons fire and individual neurons fire faster
2 theories for neural code for taste quality
controversial!
- labeled-line theory: each taste nerve fiber carries a particular taste quality (doctrine of specific nerve energies); similar to somatosensation and hearing
- 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
Evidence for labeled-line theory
neural code for taste quality
- some taste nerve fibers appear to be tuned to specific tastes
- sweet tastes can be temporarily knocked out in humans by gymnema sylvestre
Evidence for pattern-coding theory
neural code for taste quality
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
6 factors affecting detection threshold
- temperature
- area
- duration
- location
- adaptation
- substance being tasted
Effect of temperature on detection threshold
for each taste quality
- salty or bitter: most sensitive (low threshold) at low temperatures
- sweet: most sensitive at high temperatures
- sour: temperature has little effect
Effect of area and duration on detection threshold
- area: low threshold for larger areas of tongue stimulation
- duration: lower threshold for longer durations of stimulation (200-1500 ms)
Effect of location of detection threshold
- slight variation in aboslute threshold for primary taste qualities at different tongue locations
- primary taste qualities NOT exclusively associated with particular tongue locations
but suprathreshold concentrations can be tasted at any location except middle of tongue (no taste buds)
Effect of self-adaptation on detection threshold
detection threshold for tastant increases during continued stimulation of tongue with same tastant
Effect of cross-adaptation on detection threshold
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
Modification
special type of cross-adaptation
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
Effect of the substance being tasted on detection threshold
- 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
Inherited trait for tasters vs. nontasters
PTC or PROP
- 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
Steven’s magnitude estimation technique
scaling
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
Cross-modality matching
suprathreshold
- 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
Supertaster
- 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
Health consequences for the 3 categories of tasters
- supertasters/medium tasters: higher risk of colon cancer (because they avoid eating bitter vegetables)
- supertasters: lower risk of cardiovascular disease
- nontasters: more likely to smoke and consume alcohol (because they don’t taste bitterness of tobacco and caffeine)
Taste discrimination
poor intensity discrimination ability for all tastes compared to other sensory modalities
JND is 15-25% of standard intensity
Evidence for innate taste hedonics
pleasure/displeasure evoked by primary taste qualities
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
Specific hungers theory
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)
Evidence for specific hungers theory
- 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
What determines food preferences?
hard-wired preferences contributed by taste and learned preferences, which are contributed by retronasal sensations (odors perceived when chewing and swallowing that determine flavor)
Link between retronasal and orthonasal olfaction
- 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
Special case of taste hedonics for chili peppers
- 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
