Gustation ppt

The Chemical Senses

• Lectures by Dr. James Dillon

  • Lecture 1: Smell (Olfaction)

  • Lecture 2: Taste (Gustation)

• Learning Outcomes:

  • Describe the specialized sense organs involved in gustation at a molecular and cellular level.

  • Describe how gustatory information can be coded.

  • Describe the structures of the brain that process this type of sensory information in the CNS.

The Sense Organ: The Tongue

• The tongue features specialized structures called taste papillae.

• Types of Papillae:

  • Fungiform:

    • Located on the front 2/3 of the tongue

    • Contains approx. 3 taste buds on the apical surface.

  • Circumvallate:

    • Located at the back of the tongue

    • Contains approx. 250 taste buds in deep trenches.

  • Foliate:

    • Organized into parallel ridges with approx. 600 taste buds.

Gustation: Contact Chemosensation

• Taste Bud Positioning:

  • Taste buds are embedded within circumvallate papillae, which emphasize their functionality by concentrating dissolved chemicals.

• Taste buds contain sensory receptor cells for detecting chemicals in food.

Response to Chemicals

• Taste receptors respond to different chemicals on geographic locations of the tongue in this sequence:

  • Fungiform → Foliate → Circumvallate.

• Basic Tastes:

  • Umami: Glutamate (e.g., MSG; associated with a 'meaty' taste).

  • Five Basic Tastes:

  • Bitter: Caffeine, Nicotine, Quinine (0.008 mM),

  • Sour: Acids (H+) (2 mM),

  • Salt: NaCl (10 mM),

  • Sweet: Glucose (20 mM).

Taste Bud Structure

• Taste receptor proteins are located in microvilli found at the tips of taste cells.

• A single taste bud can contain up to 50 specialized epithelial cells (taste cells).

• Cell Structure

  • Microvilli increase surface area and join at the taste pore for concentrating chemicals on the receptor proteins.

  • Regeneration: Basal cells allow for the regeneration of taste cells.

Transduction Mechanism in Taste Cells

• Two key domains:

  • Apical Domain: Contains receptor proteins including ion channels and GPCRs.

  • Basolateral Domain: Houses ion channels and machinery required for neurotransmitter release.

• Neurotransmitters Involved:

  • Serotonin and ATP.

Sensing Mechanism for Salt and Sour

• Salt:

  • Amiloride-sensitive Na+ channel detects sodium ions.

• Sour:

  • H+-sensitive TRP channel detects hydrogen ions.

  • Blockage of K+ channels by H+ ions leads to depolarization and neurotransmitter release.

Taste Receptors and Signaling Pathways

• T1R and T2R Receptors:

  • Sweet and Umami: Mediated by T1R proteins forming heterodimers.

  • Bitter: Mediated by T2R proteins; many subtypes helping to detect a variety of bitter compounds.

• Taste Blindness:

  • Phenylthiocarbamide (PTC) sensing is influenced by T2R receptor gene, affecting approximately 25% of the population.

Central Processing of Gustation

• 1st Order Neurons: Cranial nerves VII/IX/X relay gustatory information to the gustatory nucleus (nucleus of the solitary tract in the medulla).

• 2nd Order Neurons: Gustatory nucleus connects to the ventral posterior medial nucleus of the thalamus.

• 3rd Order Neurons: Projections from the thalamus to cortical structures including the insula and frontal lobes.

Encoding Taste

• Taste information is conveyed using labeled line coding, where specific cells are dedicated to detecting certain taste modalities.

• Gustatory mappings showcase how different tastants activate distinct regions in the insular cortex.

General Summary of Chemosensation

• Gustation and olfaction are both mediated by specific receptors; gustatory receptors have a limited range but higher turnover than olfactory receptors.

• Signaling mechanisms in taste involve both ion channels and GPCRs, while odorant receptors primarily consist of GPCRs.