Taste

Taste (Gustation): Sensations evoked by aqueous solutions contacting receptors on tongue & palate.
Retronasal olfactory sensation
- Odorant released during chewing/swallowing travels “up and behind” the palate ➜ contacts olfactory mucosa.
- Perceived as coming from the mouth even though stimulus–receptor contact is in the nose.
Flavor = True taste (sweet, salty, sour, bitter) + retronasal olfaction.
Figure 15.1 contrasts pathways:
- Orthonasal (through nostrils) vs. Retronasal (from oral cavity) olfaction.
When taste is lost but smell is intact
- Clinical case: Patient could smell lasagna but reported “no flavor.”
- Lab analog: Anaesthetize the chorda tympani (branch of cranial nerve VII) with lidocaine ➜ odor is detected but “flavor” disappears.
Neuroimaging evidence
- Brain treats odors differently when they arrive orthonasally vs. retronasally.
Cross-modal modulation in industry
- Added sugar ↑ perceived fruitiness (olfactory) in juices.
- Heirloom-tomato studies: Volatile compounds ↑ perceived sweetness without added sugar ➜ potential low-calorie sweet-flavor enhancement.

Taste buds
- House gustatory receptor cells; embedded in papillae; convert chemical energy ➜ neural signals carried by cranial nerves.
Four papilla types
1. Filiform – tiny, numerous, textured; $\varnothing$ taste buds.
2. Fungiform – mushroom-shaped (≈ 1 mm Ø), concentrated at tip/edges; ≈ 6 taste buds each.
3. Foliate – lateral folds near rear; contain many taste buds.
4. Circumvallate – large mound-in-trench; form inverted V at posterior tongue; 3–5 per side.
Microvilli
- Membranous extensions of receptor cells protruding into taste pore; binding sites for tastants (not “hairs”).
Tastants
- Category 1: Small ions (e.g., $\text{Na}^+,\,\text{H}^+$ ) ➜ salty, sour via ion channels.
- Category 2: Larger molecules activate GPCRs ➜ sweet, bitter.
Tongue-map myth
- Origin: Hänig (1901) measured thresholds; Boring (1942) redrew as “sensitivity” ➜ exaggerated differences.
- Reality: All four basic tastes detected across entire tongue surface.
Central pathway (Figure 15.3/15.7)
- Taste buds → cranial nerves VII (chorda tympani), IX, X → nucleus of solitary tract (medulla) → thalamus → insula (primary taste cortex) → orbitofrontal cortex (OFC).
- OFC integrates taste with temperature, touch, smell.
Inhibition
- Descending cortical signals suppress oral pain to preserve feeding even with mouth injuries.

Salts are ionic compounds: $\text{cation}^+ + \text{anion}^-$ (e.g., $\text{Na}^+\text{Cl}^-$ ).
Perception & liking are plastic
- Low-Na diet ↑ salt sensitivity.
- Early chloride deficiency or gestational exposure modifies later preferences.

Quinine = prototype.
Broadly tuned receptors—little ability to differentiate bitters.
Many bitter molecules are toxic ➜ aversive signal.
Modulation
- Hormonal: Sensitivity ↑ in pregnancy.
- Learned “switch-off” facilitates vegetable acceptance.
Figure 15.9 lists diverse compounds & their TAS2R receptor affinities.

Evoked by carbohydrates crucial for metabolism.
- $\text{Glucose}$ (primary energy), $\text{Fructose}$ (sweeter), $\text{Sucrose}=\text{Glucose}+\text{Fructose}$ .
Single heterodimeric receptor TAS1R2 + TAS1R3 (Figure 15.10).
- Different ligands bind distinct sites; artificial sweeteners exploit this.

Triggered by $\text{monosodium glutamate (MSG)}$ ; reflects amino-acid content.
Safety: Large acute doses ➜ “MSG symptom complex” (numbness, headache, flushing, etc.); small culinary doses generally safe.

Fatty acids generate tactile & possibly taste signals; rats (and likely humans) express lingual fatty-acid receptors.
Post-ingestive conditioning: Gut fat digestion conditions preference for fat-containing flavors.

PTC / PROP polymorphism (Fox 1931; receptor gene ID 2003)
- Genotype $\text{TAS2R38}$
- $\text{PAV/PAV}$ or $\text{PAV/AVI}$ ➜ tasters.
- $\text{AVI/AVI}$ ➜ nontasters.
Supertasters
- Taster genotype + high fungiform density ➜ extremely intense taste; see Figure 15.4.
Cross-modality matching (Figure 1.9)
- Nontaster bitterness ≈ whisper loudness.
- Medium ≈ smell of bacon / mild headache pain.
- Supertaster ≈ sun brightness / worst pain.
Health links
- Supertasters may avoid bitter vegetables ➜ lower phytonutrient intake; but also perceive fats as bitter ➜ lower fat consumption → ↓ CVD risk.
- PROP bitterness intensity correlated with colon polyp prevalence in men.

Omnivore’s dilemma: Balancing varied options with nutritional needs.
- Smell identifies objects; taste signals nutrient vs. antinutrient.
Innate survival coding
- Bitter → possible poison; Sour → potential tissue-damaging acidity; Sweet & Salty → nutrients ( $\text{Na}^+$ , sugars).
Infant evidence
- Sweet → smiles & sucking; Sour → lip pursing; Bitter → gaping/spitting/vomiting attempts.
Specific-hungers theory
- Sodium or sugar deficits provoke targeted cravings; not supported for most vitamins/minerals.
Evaluative conditioning
- Hedonic valence of consequences transfers to associated flavors ➜ learned likes/dislikes.
Food preference study (422 men)
- Preference ratings for high-fat foods correlated with BMI & waist circumference (r significant), whereas self-reported frequency of eating high-fat foods did not ➜ preferences predict adiposity.

Coding theories
- Labeled lines: Each afferent fiber dedicated to one quality; hearing is example.
- Pattern coding: Qualities from distributed activity patterns; vision & olfaction examples.
- Debate persists for taste; evidence exists for both.
Adaptation & Cross-adaptation
- Prolonged stimulus ↓ subsequent sensitivity (e.g., own saliva $\text{Na}^+$ adapts salt taste).
- Cross-adaptation: One tastant alters perception of another (sweet drink then sour solution tastes extra sour).
Retronasal vs. Orthonasal pleasure learning
- Hedonic responses can be route-specific: e.g., fresh-cut grass pleasant orthonasally but not as flavor.
- Yet aversions learned via retronasal usually transfer to orthonasal (e.g., fish sickness).
Chili pepper (Capsaicin)
- Preference acquisition culturally & socially mediated; humans uniquely enjoy.
- Capsaicin activates pain receptors ⇒ burning; repeated exposure ⇒ receptor desensitization.
- Individual differences tied to papilla density.
- Practical tip: If too hot, wait; subsequent bites feel milder due to desensitization.