Taste: Cellular Basis

Abstract

Taste buds are the peripheral end organs of the gustatory system. Several thousand of these sensory organs are distributed throughout the oral cavity. Each taste bud consists of up to 100 cells that detect food chemicals and transmit this information to sensory afferent fibres that connect taste buds to the hindbrain. Individual taste buds detect many compounds, including those that elicit sweet, salty, bitter, sour, umami and perhaps other perceptions. There is no taste map on the tongue. Sensory receptor cells in taste buds transduce taste stimuli using G protein‐coupled taste receptors and ion channels. Gustatory stimulation causes the taste bud cells to secrete neurotransmitters, including ATP and serotonin, that excite sensory afferent fibres as well as mediate cell–cell (paracrine) interactions within a taste bud. Taste information travels from the hindbrain to cortical centres where the signals are integrated with olfaction to generate perception of flavours.

Key Concepts

  • Taste buds respond to chemical compounds found in foods and beverages.
  • Taste buds consist of many cells that are dedicated to sensing multiple tastes, including sweet, sour, salty, bitter and umami.
  • There is no taste map on the tongue.
  • Taste cells comprise a renewing cell population with taste cells having lifespans of 8–12 days.
  • There are 4 major types of cells in taste buds, each having a distinct function.
  • Gustatory stimulation causes taste bud cells to secrete ATP and serotonin.
  • Taste‐evoked ATP release is highly unusual: it is nonvesicular and involves secretion through large‐pore ion channels.
  • Cells within a taste bud communicate with each other during taste stimulation via paracrine synaptic interactions.
  • Gustatory signals transmitted to the brain cortex converge there with olfactory input to generate flavours.

Keywords: gustation; sweet; salty; sour; bitter; umami; olfaction; flavour; sensory afferent

Figure 1. Henning's 1916 taste tetrahedron. Tastes are represented as points situated between the four primary qualities: sweet, salty, sour and bitter. For example, the taste of grapefruit (shaded circular spot) might be represented as the combination of sour, bitter and sweet.
Figure 2. Schematic representation of a taste bud embedded in the stratified squamous epithelium of the tongue. A single taste bud contains 50–100 cells, which include receptor cells, stem cells and glial‐like sustentacular cells.
Figure 3. Summary of taste transduction mechanisms. Abbreviations: DAG, diacylglycerol; ENaC, epithelial sodium channel; Gβγ, G proteins β and γ; IMP, inosine monophosphate; IP3, inositol trisphosphate; KATP, ATP‐sensitive(blockable) potassium channels; Kir2.1, inward‐rectifying potassium channel member 2.1; mGluR1,4, metabotropic glutamate receptors, members 1 and 4; MSG, monosodium glutamate; PKA, protein kinase A; PLCβ2, phospholipase C class, member β2; TAS1R1,2,3 and TAS2Rs, G protein‐coupled taste receptors; TRPM4/5, Transient receptor potential cation channel subfamily M members 4 and 5.
Figure 4. Sour (acid) taste is transduced by two mechanisms. (a) Protons from the dissociated acid solution (yellow) penetrate the apical, exposed tips of Type III taste receptor cells in the taste pore via OTOP 1 channels. This generates an inward (depolarising) current (IH+) as well as leads to (intracellular) protons blocking Kir2.1 channels. Protons do not diffuse across the junctional complex at the taste pore and are limited to acting on the exposed, apical membrane of Type III cells. (b) In contrast, organic acid molecules (e.g. acetic or citric acids, orange) readily cross the junctional complex into the taste bud and penetrate cell membranes to dissociate and acidify the cytosol. Intracellular protons block Kir2.1, as above. Organic acids also dissociate extracellularly and generate protons and provide a depolarising current via OTOP1 channels, as shown. The principal difference is that organic acid molecules readily cross the junctional complex and cell membranes, thereby acidifying the cytosol and inhibiting Kir2.1 channels.
Figure 5. Summary of the taste axis, from peripheral sensory organs (taste buds) to the cerebral cortex. Sensory information from taste buds ascends to the primary gustatory cortex and association cortices. Not shown are descending (efferent) pathways believed to exist at several levels in the taste axis. Also, not shown is olfactory input to the primary gustatory cortex (insula and frontal operculum).
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Further Reading

Behrens M and Meyerhof W (2016) G protein‐coupled taste receptors. In: Zufall F and Munger SD (eds) Chemosensory Transduction, pp 227–244. Academic Press: London.

Chaudhari N and Roper SD (2010) The cell biology of taste. The Journal of Cell Biology 190 (3): 285–296.

Gutierrez R, Fonseca E and Simon SA (2020) The neuroscience of sugars in taste, gut‐reward, feeding circuits, and obesity. Cellular and Molecular Life Sciences: 1–34.

Kinnamon SC and Finger TE (2019) Recent advances in taste transduction and signaling. F1000Research 8: 2117.

Liman ER, Zhang YV and Montell C (2014) Peripheral coding of taste. Neuron 81 (5): 9841000.

Ohla K, Yoshida R, Roper SD, et al. (2019) Recognizing taste: coding patterns along the neural axis in mammals. Chemical Senses 44 (4): 237–247.

Roper SD and Chaudhari N (2017) Taste buds: cells, signals and synapses. Nature Reviews. Neuroscience 18 (8): 485–497.

Roper SD and Chaudhari N (2017) Taste buds: cells, signals and synapses. Nature Reviews. Neuroscience 18 (8): 485–497.

Roper S (2020) The microphysiology of taste buds. In: Fritzsch B (ed.) The Senses, A Comprehensive Reference, 2nd edn. Academic Press.

Shigemura N and Ninomiya Y (2016) Recent advances in molecular mechanisms of taste signaling and modifying. International Review of Cell and Molecular Biology 323: 71–106.

Small DM and Green BG (2012) A proposed model of a flavor modality. In: Murray MM and Wallace MT (eds) The Neural Bases of Multisensory Processes. CRC Press: Boca Raton (FL).

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Roper, Stephen D(Sep 2020) Taste: Cellular Basis. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0029202]