Special Senses I: Taste and Smell

Taste and Smell

Chemical Sense - Taste (Gustation)

• Gustation is the sensation of taste resulting from the action of chemicals on taste buds.
• Lingual papillae are structures on the tongue that contain taste buds; these include:
  • Filiform papillae: Do not contain taste buds; important for texture.
  • Foliate papillae: Do not contain taste buds.
  • Fungiform papillae: Located at the tips and sides of the tongue.
  • Vallate (circumvallate) papillae: Located at the rear of the tongue; contain approximately 1/2 of the taste buds.

Taste Bud Structure

• Taste buds are predominantly located on the sides of tongue papillae, but also found on the cheeks, soft palate, pharynx, and epiglottis.
• Key structures include:
  • Taste pore: An opening on the taste bud.
  • Taste cells: Receptor cells within the taste bud.
    • Apical microvilli (taste hairs) serve as the receptor surface.
    • Taste cells synapse with sensory nerve fibers at their base.
  • Supporting cells: Cells within the taste bud that provide structural and functional support to the taste cells.

Physiology of Taste

• Molecules must dissolve in saliva to be tasted.
• 5 primary taste sensations spread throughout the tongue:
  • Sweet: Concentrated on the tip.
  • Salty: Perceived on the lateral margins.
  • Sour: Perceived on the lateral margins.
  • Bitter: Perceived on the posterior part of the tongue.
  • Umami: Taste of amino acids (e.g., MSG).
• Taste perception is influenced by food texture, aroma, temperature, and appearance.
• Hot pepper stimulates free nerve endings, resulting in a pain sensation.

Taste mechanisms via depolarization:

• High concentrations of cations depolarize taste cells.
  • Salty: Na^+ depolarizes the cell.
  • Sour: H^+ depolarizes the cell.

Taste mechanisms via G-protein-coupled receptor systems:

• Sweet: Glucose (or similar molecules) binds to a G-protein-coupled receptor.
• Bitter: Different types of bitterness receptors detect different compounds.
• Umami: Amino acids bind to G-protein-coupled receptors.

Mechanism of Action for G-protein-coupled receptor systems

1. Water-soluble hormones bind to membrane receptors.
2. This binding activates a G protein.
3. The activated G protein activates adenylyl cyclase.
4. Adenylyl cyclase catalyzes the conversion of ATP to cAMP, the secondary messenger.
5. cAMP activates protein kinases.
6. Protein kinases phosphorylate proteins in the cytoplasm, activating them and altering cell activity.

Projection Pathways for Taste

• Innervation of taste buds:
  • Facial nerve (VII): Innervates the anterior 2/3 of the tongue.
  • Glossopharyngeal nerve (IX): Innervates the posterior 1/3 of the tongue.
  • Vagus nerve (X): Innervates the palate, pharynx, and epiglottis.
• The taste signal travels to the medulla (first stop), then to the thalamus (next stop).
• Primary gustatory cortex location: postcentral gyrus.

Chemical Sense - Smell (Olfaction)

• Olfactory epithelium contains receptor cells for olfaction.
• Olfactory cells are specialized neurons with cilia (olfactory hairs).
  • Receptor proteins on the cilia bind odor molecules in the mucus layer.
  • Each olfactory cell bears one type of receptor protein.
  • Axons pass through the cribriform plate.

Physiology of Smell

• Odor molecules bind to receptors on olfactory hairs.
  • Hydrophilic molecules diffuse through the mucus.
  • Hydrophobic molecules are transported by odorant-binding proteins.
• Binding activates a G protein and cAMP system.
• Ion channels for Na^+ or Ca^{2+} open, creating a receptor potential.
• Action potential travels to the brain.
• Receptors adapt quickly due to synaptic inhibition in olfactory bulbs.

Olfactory Pathway

• Olfactory cells synapse in the olfactory bulb on mitral and tufted cell dendrites.
• Synapses occur in spherical clusters called glomeruli.
• Each glomerulus is dedicated to a single odor.

Olfactory Pathway Output

• Output from the olfactory bulb forms olfactory tracts.
• First stop: Primary olfactory cortex on the inferior temporal lobe (bypasses the thalamus).
• Then to: thalamus, insula, frontal cortex, hippocampus, amygdala, hypothalamus.
• Brain regions are involved in:
  • Identifying odors.
  • Integrating taste and smell into flavor.
  • Linking odors to memories, and emotional and visceral reactions.