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28 Gustation

Learning Objectives

After reading this section, you should be able to-

  • Explain the process by which tastants activate gustatory receptors.
  • Trace the path of gustation from gustatory receptors through specific cranial nerves to various parts of the brain.
  • Describe the primary taste sensations.

Gustation relies on five primary modalities—salty (Na⁺), sour (H⁺), sweet (sugars & sweeteners), bitter (diverse alkaloids), and umami (L-glutamate)—detected by specialized receptor cells clustered in taste buds on four tongue papillae (circumvallate, foliate, fungiform, filiform). These bipolar gustatory cells release neurotransmitter onto afferent fibers of cranial nerves VII (anterior tongue), IX (posterior two-thirds), or X (extreme posterior), conveying chemical taste information to the brain.

The left panel shows the image of a tongue with callouts that show magnified views of different parts of the tongue. The top right panel shows a micrograph of the circumvallate papilla, and the bottom right panel shows the structure of a taste bud.
Figure 28.1 – The Tongue: The tongue is covered with small bumps, called papillae, which contain taste buds that are sensitive to chemicals in ingested food or drink. Different types of papillae are found in different regions of the tongue. The taste buds contain specialized gustatory receptor cells that respond to chemical stimuli dissolved in the saliva. These receptor cells activate sensory neurons that are part of the facial and glossopharyngeal nerves. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Tasting of Flavors

  • Salty: Na⁺ diffuses through epithelial Na⁺ channels → depolarization → neurotransmitter release

  • Sour: H⁺ enters via proton channels → depolarization → neurotransmitter release

  • GPCR‐Mediated Tastes (Sweet, Bitter, Umami): Tastant binding activates the G-protein gustducin. The Gα subunit of gustducin stimulates phospholipase C-β₂, generating IP₃ and DAG. IP₃ releases Ca²⁺ from intracellular stores, and DAG activates protein kinase C—together amplifying the depolarizing receptor potential → neurotransmitter release

    • Sweet: Glucose and sweeteners bind sweet GPCRs

    • Bitter: Alkaloid and other bitter compounds bind diverse bitter GPCR subtypes (posterior-tongue concentrated for toxin detection)

    • Umami: L-glutamate binds umami GPCRs, signaling savory, protein-rich flavors

Central pathway

  • CN VII (facial): anterior 1/3 of tongue

  • CN IX (glossopharyngeal): posterior 2/3 of tongue

  • CN X (vagus): extreme posterior tongue/pharynx

  • → all converge on solitary nucleus (medulla)

  • → second-order fibers to ventral posteromedial (VPM) thalamus

  • → projections to gustatory cortex (insula & frontal operculum)

Central Processing of Taste Information

The sensory pathway for gustation travels along the facial,  glossopharyngeal and vagus cranial nerves, which synapse with neurons of the solitary nucleus in the brain stem. Axons from the solitary nucleus then project to the ventral posterior nucleus of the thalamus. Finally, axons from the ventral posterior nucleus project to the gustatory cortex of the cerebral cortex, where taste is processed and consciously perceived.
 
Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 15.
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