U2 L5 Chemical Senses: Gustation and Olfaction

Chemical Senses

  • Chemical senses: gustation (taste) and olfaction (smell).
  • Chemoreceptors respond to chemicals in aqueous solution.
  • Taste: substances dissolved in saliva.
  • Smell: substances dissolved in fluids of the nasal membranes.

Sense of Smell (Olfaction)

  • Organ of smell: olfactory epithelium, covering the superior nasal concha.
  • Olfactory receptor cells: bipolar neurons with radiating olfactory cilia.
  • Olfactory receptors are surrounded and cushioned by supporting cells.
  • Olfactory stem cells lie at the base of the epithelium.

Physiology of Smell

  • Olfactory receptors respond to various odor-causing chemicals.
  • Ligand binding initiates a G protein mechanism, using cAMP as a second messenger.
  • cAMP opens Na+Na^+ and Ca2+Ca^{2+} channels, causing depolarization and triggering an action potential.

Olfactory Pathway

  • Olfactory receptor cells synapse with mitral cells.
  • Glomerular mitral cells process odor signals.
  • Mitral cells send impulses to:
    • Olfactory cortex
    • Hypothalamus, amygdala, and limbic system.

Taste Buds (Gustation)

  • Approximately 10,000 taste buds are mostly found on the tongue.
  • Located in papillae of the tongue mucosa.
  • Types of papillae: filiform, fungiform, and circumvallate.
  • Fungiform and circumvallate papillae contain taste buds.

Structure of a Taste Bud

  • Gourd-shaped taste bud consists of:
    • Basal cells: dynamic stem cells.
    • Gustatory cells: taste cells.

Taste Sensations

  • Five basic taste sensations:
    • Sweet: sugars, saccharin, alcohol, some amino acids.
    • Salty: metal ions.
    • Sour: hydrogen ions (H+H^+).
    • Bitter: alkaloids (quinine, nicotine).
    • Umami: elicited by the amino acid glutamate.

Physiology of Taste

  • For a chemical to be tasted:
    • Must be dissolved in saliva.
    • Must contact gustatory hairs.
  • Binding of the food chemical:
    • Depolarizes the taste cell membrane, releasing neurotransmitter.
    • Initiates a generator potential, eliciting an action potential.

Taste Transduction

  • Stimulus energy of taste is converted into a nerve impulse by:
    • Na+Na^+ influx in salty tastes.
    • H+H^+ in sour tastes (directly entering the cell, opening cation channels, or blocking K+K^+ channels).
    • Gustducin in sweet, umami, and bitter tastes.

Gustatory Pathway

  • Cranial Nerves VII and IX carry impulses from taste buds to the solitary nucleus of the medulla.
  • Impulses then travel to the thalamus; fibers branch to:
    • Gustatory cortex (taste).
    • Hypothalamus and limbic system (appreciation of taste).

Influence of Other Sensations on Taste

  • Taste is 80% smell.
  • Thermoreceptors, mechanoreceptors, and nociceptors also influence tastes.
  • Temperature and texture enhance or detract from taste.