The Chemical Senses

The Chemical Senses
Chapter 16: Functions of Smell and Taste

1. Overview of the Chemical Senses

  • Taste (gustation) and smell (olfaction) are considered gatekeeper senses.

  • Survival Logic: They help organisms distinguish between nutrients (essential for survival) and toxins (harmful to survival).

  • Affective Dimension: These senses are unique in their strong connection to emotions and memory, providing immediate positive or negative responses to stimuli.

2. Neurogenesis and Receptor Lifespan

  • Because receptors are exposed to the external environment (chemicals, heat, bacteria), they undergo continuous renewal, a process called neurogenesis.

  • Olfactory Receptors: Renewed approximately every 5 to 7 weeks.

  • Taste Receptors: Renewed approximately every 1 to 2 weeks.

3. The Gustatory System (Taste)

  • Five Basic Taste Dimensions:

    • Sweet: Signals high caloric content and carbohydrates.

    • Sour: Signals acidity, potentially identifying spoiled food.

    • Salty: Signals the presence of sodium, an essential electrolyte.

    • Bitter: Often signals toxicity; humans have a high sensitivity to bitterness for protection.

    • Umami: A "savory" or meaty taste, stimulated by glutamate (often found in MSG).

  • Anatomy of Taste:

    • Papillae: The tongue surface consists of four distinct structures:

    1. Filiform: Cone-shaped; found across the entire surface; primarily used for texture as they contain no taste buds.

    2. Fungiform: Mushroom-shaped; located at the tip and sides of the tongue.

    3. Foliate: A series of folds located on the back sides of the tongue.

    4. Circumvallate: Large mounds shaped like flat trenches located at the very back.

    • Taste Buds: There are approximately 10,000 taste buds in the human mouth, primarily on papillae. Each bud contains 50 to 100 taste cells that transmit signals to the brain.

4. The Olfactory System (Smell)

  • Classification of Species:

    • Macrosmatic: Animals (like dogs or rodents) where olfaction is highly developed and critical for survival (finding food, mating, territory).

    • Microsmatic: Animals (like humans) where smell is less developed but still impacts behavior, flavor perception, and warning signals.

  • Chemical Communication:

    • Pheromones: Molecules released by an individual that trigger specific physiological or behavioral responses in others of the same species (e.g., menstrual synchrony in humans).

  • Olfactory Physiology:

    • Olfactory Mucosa: A specialized area at the top of the nasal cavity containing Olfactory Sensory Neurons (OSNs).

    • Cilia: Tiny hair-like structures on the ends of OSNs that contain the actual protein receptors for odors.

    • Olfactory Bulb: Signals from the OSNs travel here, organizing specifically into structures called glomeruli before moving to the brain.

5. Flavour Perception: Sensory Interaction

  • Definition: Flavour is the perceptual fusion of taste, smell, and somatosensory sensations (texture, temperature, and spiciness).

  • The Retronasal Route: While eating, aromatic compounds reach the olfactory mucosa via the pharynx (the back of the throat). This interaction is why food tastes "muted" when the nose is blocked.

  • Orbitofrontal Cortex (OFC): This is a bimodal region of the brain where taste and smell signals converge. The OFC is also influenced by cognitive factors, such as expectations, price, and satiety (the