The Chemical Senses

The Chemical Senses

• Flavor is a complex combination of sensory inputs including smell, taste, texture, and other factors.
• Gustation is referred to as the most intimate sense due to its direct interaction with food and drink.

Basic Taste Qualities

• Bitter:
    - Associated with varied alkaloids and some nitrogen-containing molecules.
    - Many bitter compounds share some structural similarities with sweet compounds but differ in their molecular arrangements.

• Umami:
    - Described as "brothy," "meaty," or "savory".
    - The primary trigger is the salts of glutamic acid; the reaction ext{glutamic acid} + ext{Na}^+
  ightarrow ext{monosodium glutamate (MSG)} illustrates this.

Learning Outcomes

1. Distinction between taste and flavor.
2. Identification and explanation of basic taste qualities.
3. Mechanisms of chemical transduction on the tongue.
4. Overview of the gustatory neural pathways.
5. Factors influencing gustatory thresholds.
6. Perception of "hot" and "cool" tastes.
7. Characteristics and categorization of olfactory stimuli.
8. Two major theories of olfaction.
9. Key structures involved in olfaction and their transduction mechanisms.
10. Olfactory neural pathways.
11. Influences on olfactory threshold and identification.
12. Psychological effects induced by odorants.

Gustation Overview

• Humans find it challenging to identify foods based solely on taste, emphasizing the importance of olfaction (Mozell et al., 1969) in determining whether food is edible, spoiled, or poisonous.
• Each basic taste serves a specific survival function:
    - Salty: Maintains electrolyte balance.
    - Sour: Detects vitamins and potentially harmful acids.
    - Sweet: Aids in energy acquisition for neurons.
    - Bitter: Helps avoid toxins.

Taste Stimuli/Qualities

• Salty:
    - Triggered primarily by inorganic salts, with cations like Na+ from sodium chloride (NaCl) being crucial.

• Sour:
    - Result of inorganic acids that release H+ ions in solution; not all acids produce a sour taste.

• Sweet:
    - Derived from complex organic molecules; notable examples include sugars and aspartame, found in foods like Parmesan cheese, tomatoes, mushrooms, and Marmite® yeast extract.
    - Ikeda (1909) successfully isolated glutamic acid from seaweed which is now associated with umami taste.

Taste Buds and Receptors

• Each taste bud comprises numerous taste cells (receptors), which are subject to wear and are replaced every 7-10 days; however, rates decrease post-age 45.

• Different taste stimuli activate unique fibers, where the firing rate indicates intensity.

• Various types of fibers respond optimally to specific taste stimuli; Chaudhari, Landin, & Roper (2000) identified G-protein coupled receptors specifically for MSG.

• The tongue features four types of bumps, known as papillae, which contain taste buds:
    - Fungiform: Mushroom-shaped; located on the tip and sides of the tongue.
    - Foliate: Contains folds along the sides of the tongue.
    - Circumvallate: Characterized by flat mounds surrounded by trenches at the back of the tongue.

• Taste buds also exist on the soft palate, with a total of approximately 10,000 buds per individual.

• Microvilli from receptor cells protrude out of taste pores.

Taste Transduction Mechanism

• Sodium ions penetrate the membrane of receptor cells, conducting depolarization.
• Sour substances that contain H+ ions can block ion channels.
• Other substances induce chemical changes within the cell by forming bonds with membrane molecules.

Sensory Coding

• Labeled-line Coding (Specificity Coding):
    - Each nerve fiber is designated for a specific taste sensation.
    - A limited number of fiber types can effectively code stimuli.

• Across-fibre Pattern (Population Coding):
    - Taste quality is represented through activity patterns across multiple fibers, as no single receptor type corresponds exclusively to any taste.
    - E.g., sodium saccharine can switch from sweet to bitter as concentration increases.

Gustatory Neural Pathways

• Nerve fibers from various tongue regions project to the brain through several identified nerves:
    - Fibers from the front/sides descend via the chorda tympani nerve.
    - Back of the tongue fibers travel through the glossopharyngeal nerve.
    - Mouth and throat fibers utilize the vagus nerve.
    - Soft palate fiber connections utilize the superficial petrosal (greater petrosal) nerve.

• These pathways connect to the nucleus of the solitary tract in the medulla, which in turn projects to:
    - Primary Gustatory Cortex: Comprises the frontal operculum cortex and anterior insula responsible for conscious taste perception.
    - Secondary Gustatory Cortex: Occupies the orbitofrontal cortex, linked to emotions and reward responses.

• The hypothalamus and amygdala also contribute to aversions and cravings, adding an affective component to taste.

Threshold Taste Concentrations

• Different tastants have established thresholds as follows:
    - Caffeine (Bitter): $0.0007$ molar concentration.
    - Quinine Sulfate (Bitter): $0.000019$ molar concentration.
    - Citric Acid (Sour): $0.0023$ molar concentration.
    - Acetic Acid (Sour): $0.0018$ molar concentration.
    - Sodium Chloride (Salty): $0.01$ molar concentration.
    - Sodium Fluoride (Salty): $0.005$ molar concentration.
    - Sucrose (Sweet): $0.01$ molar concentration.
    - Sodium Saccharine (Sweet): $0.000023$ molar concentration.
    - MSG (Umami): Estimated threshold at $0.00003$ molar concentration.

Factors Impacting Gustatory Threshold

• Taste does not localize strictly at the tongue, debunking the "tongue map" misconception based on a misleading graph published in 1942.
• Sensitivity to tastes is highest between the temperatures of 22-40 °C.
• Individual differences can affect sensitivity:
    - Example case by Fox (1931) involved phenylthiocarbamide (PTC), leading to varying responses of tasteless (28%), bitter (66%), and other tastes (6%).
    - Further studies (Bartoshuk et al., 1994; 1998) with PROP revealed 25% supertasters (sensitive to bitterness due to papillae profusion), 50% medium tasters, and 25% nontasters.
    - Salty taste is typically harder to yield as saliva contains salt itself.

Cross-Adaptation and Multiple Coding Mechanisms

• Cross-adaptation refers to the phenomenon where adaptation to one substance diminishes sensitivity to another, indicating that specific tastes like sweet and bitter may have multiple coding mechanisms.
• Examples include chemical sensations:
    - Capsaicin relates to heat sensation from chili peppers.
    - Menthol imparts a cooling sensation not primarily through evaporation.
    - The stinging sensation from carbonation may also evoke gustatory responses.

Chemesthesis

• Defined as sensations stemming from the activation of pain, touch, or thermal receptors by chemical constituents in food.

• Capsaicin:
    - Located primarily in the chili pepper membrane; it triggers a heat signal via Ca2+ influx.
    - Commonly utilized in topical analgesics like Zostrix® to relieve arthritis.

• Menthol:
    - Extracted from peppermint and acts as a potent counter-irritant, eliciting a cool sensation through TRPM8 (cold receptor) activation.

Olfaction Overview

• Odor stimuli are classified as odorants, which are volatile molecules.
• Humans can distinguish over 1 trillion odors (Bushdid et al., 2014).
• Classification of smell qualities can include basic categories such as pungent, succulent, acid, and astringent based on evaporation rates.

Theories and Categories of Odors

• Aromas:
    - Top notes evaporate swiftly, followed by middle then base notes.

• Henning’s Odor Prism and Stereochemical Theory (Amoore, 1970):
    - Classifies odorants based on molecular shapes.

• Primary odor molecules include characteristics like:
    - Camphoraceous: Spherical, e.g., mothballs.
    - Pungent: Electrically charged, e.g., vinegar.

Odor Recognition and the Olfactory System

• Odorants bind to olfactory receptors (ORs) located on the cilia of olfactory receptor neurons (ORNs).
• This binding opens ion channels, resulting in depolarization.
• Olivechnic Boundary:
    - Human existence has approximately 10 million ORNs, which are functional for 5-7 weeks.
• Nobel Prize winners Buck and Axel (2004) highlighted that odors are encoded combinatorially, reflecting how different odors combine using ORs.

Olfactory Pathways

• The axons of ORNs and the dendrites in olfactory bulb create clusters, termed glomeruli.
• Pathways lead to the primary olfactory cortex (piriform cortex) and the limbic system, essential for emotion and memory.

Threshold for Smell

• Notable odor thresholds include:
    - Carbon Tetrachloride (Sweet): $4.533$ mg/L air.
    - Amyl Acetate (Banana Oil): $0.039$ mg/L air.
    - Hydrogen Sulfide (Rotten Eggs): $0.00018$ mg/L air.
    - Citral (Lemonlike): $0.000003$ mg/L air.
    - Ethyl Mercaptan (Decayed Cabbage): $0.00000066$ mg/L air.
    - Camphor: $0.000000113$ mg/L air.
    - Trinitro-tertiary-butyl Xylene (Musk): $0.000000075$ mg/L air.

Factors Affecting Odor Perception

• Influences like age, smoking habits, and even the menstrual cycle can cause a variance in smell perception, potentially as vast as 20 times in sensitivity d'Veries & Stuiver (1961).
• Olfactory impairments, particularly with COVID-19, involve damage to sustentacular cells, resulting in significant loss of taste and smell; olfactory training has shown potential recovery effects.

Psychological and Behavioral Effects of Odor

• Aromachology:
    - Investigates the effects of odors on mood and behavior, despite considerable anecdotal evidence lacking extensive scientific backing.
• The “Proust effect”: Smells evoking autobiographical memories, linking emotional resonance with olfactory stimuli.

Pheromones and Their Impact

• Pheromones are chemical signals that convey information among members of the same species, influencing behaviors including sexual attraction.
• Specific examples:
    - Bombykol from silkworm moths attracts males from long distances.
    - Androstenol and androstadienone are human scent-related compounds linked to mood effects and behavioral responses.

Sexual Arousal Studies

• Research by Hirsch et al. demonstrated scents' impact on sexual arousal, though results may be attributable to conditioning or nostalgia rather than pheromonal action.
• Despite potentially stimulating effects, replication of studies remains critical for establishing factual validity.