CHEMICAL SENSES 1

Chemical senses, which include taste and smell, are the most ancestral and common of the senses.
- Present in both simple single-cell organisms, such as bacteria.

Finding Food Sources: Detecting and locating food in the environment.
Judging Nutritional Value and Safety of Foods: Evaluating food for dietary needs and potential toxicity.
Avoiding Predators and Hazardous Environments: Sensing dangers to enhance survival.
Social Communication and Mating: Using chemical signals, such as pheromones, in social interactions and mating behaviors.
Monitoring Internal Physiological State: Providing internal feedback related to health and nutrition.

Taste: Refers to the sensations relayed by taste receptor cells located in the oral cavity.
Foods activate different unique combinations of only five basic tastes.
Flavor: The perception of flavor arises from a combination of taste (gustation) and smell (olfaction), creating a multisensory experience.
- Various factors influence flavor perception including visual cues, auditory sensations (like crunch), and somatosensory feedback (texture, pain, and temperature).

The five basic tastes are:
- Sweet: Non-ionic; recognized as a nutrient indicator.
- Sour: Ionic; typically indicates acidity and potential spoilage.
- Salty: Ionic; associated with necessary minerals.
- Bitter: Non-ionic; often signals the presence of toxic substances.
- Umami: Non-ionic; associated with the amino acid glutamate and often found in foods like MSG.
Innate vs. Acquired Tastes: Basic tastes are innate, while more complex tastes can be acquired and modified through experience.

A sensitive and versatile taste system is essential for organisms that exploit a wide variety of food sources.
Categorization of Tastes:
- Nutrients: Generally attractive tastes such as sweet and salty.
- Anti-nutrients: Repulsive tastes like sour and bitter, which often indicate potentially harmful substances.
Sensitivity to bitter substances is particularly high, with detection thresholds in the nanomolar range, as many bitter-tasting compounds can be poisonous.

Certain taste perceptions are directly linked to chemical properties of tastants:
- Salty: Associated with salts (nutrients).
- Sour: Associated with acids (anti-nutrients).
Examples of How Other Tastes Are Perceived:
- Sweetness: Can be triggered by a diverse range of chemicals:
- Sugars
- Certain proteins (e.g., thaumatin)
- Sugar substitutes such as:
  - Saccharin (benzoic sulfinide)
  - Aspartame (aspartic acid/phenylalanine dipeptide)
  - Sucralose ("Splenda," a chlorinated sucrose).

Primary Function of the Tongue: Taste perception.
Taste Buds (TB's):
- Grouped within three of the four accessory structures called papillae:
- Vallate
- Foliate
- Fungiform
- Majority of the tongue demonstrates sensitivity to all tastes, although there are subtle regional differences in taste sensitivity.

Taste buds are specialized epithelial structures containing taste cells.
Quantity of Taste Buds: Humans have between 2000 – 5000 taste buds, with roughly 50 – 100 taste cells per taste bud, in addition to basal stem cells.
Taste cells are short sensory receptors that undergo turnover approximately every 10 days, differentiating from basal stem cells within the taste bud.

Apical Pole: Features microvilli that extend through the taste pore into the oral cavity, providing a large surface area to maximize contact with dissolved tastants.
Basolateral Pole: Contains typical organelles of epithelial cells and synapses with primary gustatory afferents, which are the first-order taste neurons that project to the brain via the central gustatory pathway.

Direct Transduction Mechanisms:
- Some tastants are ionic, passing through ion channels to generate a membrane potential.
Indirect Transduction Mechanisms:
- Non-ionic tastants selectively bind to G protein-coupled membrane receptors, activating complex intracellular signaling pathways.
Taste cells can utilize both direct and indirect mechanisms for tastant detection.

Salty Taste Transduction:
- Sodium ions (Na+) enter through amiloride-sensitive Na+ channels, directly causing depolarization of the membrane.
Sour Taste Transduction:
- Hydrogen ions (H+) permeate amiloride-sensitive Na+ channels and also block potassium (K+) channels, leading to membrane depolarization.
Bitter, Sweet, and Umami Transduction:
- G protein-coupled receptors activate signaling cascades (e.g., G protein/PLC/IP3), raising internal calcium levels which lead to the opening of unique Na+ channels, further enhancing depolarization and inducing neurotransmitter release.

Receptor potential magnitude is proportional to both tastant type and concentration.
Taste cells show selectivity, with 90% responding to multiple tastes; thus, gustatory afferents are multi-input receptors, which suggest a complex preference for specific tastants across taste buds.

Gustatory afferent neurons exit the mouth as part of three cranial nerves:
- 7th Cranial Nerve (Facial)
- 9th Cranial Nerve (Glossopharyngeal)
- 10th Cranial Nerve (Vagus)
These nerves project to the solitary nucleus in the medulla, then relay through the ventral posterior nucleus of the thalamus to reach primary and secondary gustatory cortex areas (ipsilateral).

Many central gustatory neurons respond strongly to particular tastants but may also react to other tastants, indicating complex coding schemes.
Imaging studies reveal distinct spatial patterns of taste representation in the gustatory cortex, which supports both labeled line and population coding theories for taste perception.

Orbitofrontal Cortex: The location where chemical signals merge to form perceptions of flavor.
- This cortex is connected with medullary motor nuclei that control various feeding behaviors:
- Swallowing
- Chewing
- Gagging and vomiting
- Salivation
- Respiration
Hypothalamus and Amygdala: These regions are crucial for assessing the motivational and hedonic value of food, driving behaviors related to:
- Hunger
- Palatability