Chemosensory Systems

Abstract

Chemosensory systems are highly specialised sensory systems of which taste and smell are prototypical examples. Taste, primarily, but olfaction as well, are screening mechanisms for potentially useful/harmful compounds. The variety of compounds we can detect dictates the diversity of signalling mechanisms. Sweet, bitter, umami and olfaction are mediated through an array of specialised G‐protein‐coupled receptors. Sour and salty taste are ion‐channel mediated. Taste is innate and, at least for sweet and bitter, uses the labelled line principle for central projections. While peripheral taste receptors are fast overturning epithelial cells, olfactory receptor cells are neuronal in origin and thus slower to turn over. This explains the difference in their vulnerability.

Key Concepts

  • Taste receptor cells are epithelial and turn over fast.
  • Olfactory receptor cells are neuronal and are prone to damage, a reason taste disorders are rare and olfactory deficiency is frequent https://jamanetwork.com/journals/jamaotolaryngology/article‐abstract/619670.
  • Taste receptors in the tongue have a role to detect the taste chemicals, though bitter and sweet taste receptors (chemoreceptors) act as antibacterial sentinels outside the gustatory system throughout the gastrointestinal and respiratory systems.
  • Olfactory receptor cells are distributed throughout the body as chemoreceptors of volatile compounds.
  • Taste and smell are screening mechanisms and provide a survival benefit.

Keywords: taste; olfaction; bitter; sweet; salty; sour

Figure 1. Approximate location of three chemosensitive areas in the human. The olfactory epithelium is located in the upper regions of the nasal airway. The olfactory receptor cells send axons through the cribriform plate and into the olfactory bulb. The vomeronasal organ (exaggerated in size for the purposes of this illustration) is a tubular structure (less than 1–2 mm) located in the nasal septum. The tongue contains the majority of taste buds in the oral cavity. Spielman et al. . Reproduced with permission of John Wiley & Sons.
Figure 2. The cellular organisation of the olfactory epithelium. The olfactory receptor neurones (the thin light cells of the figure) send cilia to form a single dendritic projection into the mucus‐covered air space of the nasal cavity. The single axon passing from the cell body centrally bundles with axons of other receptor neurones to form cranial nerve I and synapse with other neurones in the olfactory bulb. Olfactory stimuli are recognised by receptors on the cilia. Other cell types within the epithelium include sustentacular (supporting) cells, which have a secretory role, and the basal progenitor cells, from which new olfactory receptor neurones emerge. The Bowman's glands secrete the watery mucus of the nasal cavity. Other tissues include blood vessels and submucosal glands. Spielman et al. . Reproduced with permission of John Wiley & Sons.
Figure 3. The cellular organisation of the taste bud. The taste bud contains approximately 50–100 cells that are grouped together in a garlic‐shaped organelle. A taste bud contains at least three types of cells. The most numerous are glia‐like supporting cells representing about half of all cells. Another third of the cells are specialized to detect sugar, amino acid and bitter taste. The remainder of taste receptor cells are specialized to detect sour. It is uncertain which taste cells transmit salty taste. All taste cells originally develop from the basal cells. Some taste cells receive innervation from sensory nerve fibres at synapses. Stimuli are recognised by receptors of cells in the taste pore (TP) region, and this recognition is transduced into an electrical signal causing the innervating nerve fibre to alter its firing rate. The taste bud is divided into an apical and a basolateral portion by the tight junctions (TJ) that restrict the penetration of most taste stimuli into the basolateral portions of the bud. EC, epithelial cell. Spielman et al. . Reproduced with permission of John Wiley & Sons.
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References

Behrens M and Meyerhof W (2013) Bitter taste receptor research comes of age: from characterization to modulation of TAS2Rs. Seminars in Cell & Developmental Biology 24: 215–221. DOI: 10.1016/j.semcdb.2012.08.006.

Buck LB and Axel R (1991) A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 85: 175–187 https://www.ncbi.nlm.nih.gov/pubmed/1840504.

Chandrashekar J, Kuhn C, Oka Y, et al. (2010) The cells and peripheral representation of sodium taste in mice. Nature 464: 297–301. DOI: 10.1038/nature08783.

Cockayne DA, Dunn PM, Zhong Y, et al. (2005) P2X2 knockout mice and P2X2/P2X3 double knockout mice reveal a role for the P2X2 receptor subunit in mediating multiple sensory effects of ATP. The Journal of Physiology 567 (Pt 2): 621–639 https://www.ncbi.nlm.nih.gov/pubmed/15961431.

Kang N and Koo J (2012) Olfactory receptors in non‐chemosensory tissues. BMB Reports 45 (11): 612–622. DOI: 10.5483/bmbrep.2012.45.11.232.

Lee H, Macpherson LJ, Parada CA, et al. (2017) Rewiring the taste system. Nature 548 (7667): 330–333. DOI: 10.1038/nature23299.

Lee RJ and Cohen NA (2015) Taste receptors in innate immunity. Cellular and Molecular Life Sciences 72 (2): 217–236. DOI: 10.1007/s00018‐014‐1736‐7 https://www.ncbi.nlm.nih.gov/pubmed/25323130.

Malik B, Elkaddi N, Turkistani J, Spielman AI and Ozdener MH (2019) Mammalian Taste Cells Express Functional Olfactory Receptors. Chemical Senses 44 (5): 289–301.

Peng Y, Gillis‐Smith S, Jin H, et al. (2015) Sweet and bitter taste in the brain of awake behaving animals. Nature 527: 512–515. DOI: 10.1038/nature15763.

Roper SD and Chaudhari N (2017) Taste buds: cells, signals and synapse. Nature Reviews. Neuroscience 18 (8): 485–497. DOI: 10.1038/nrn.2017.68.

Spielman AI, Yan W and Brand JG (2007) Chemosensory systems. Encyclopedia of Life Sciences. McMillan Publishers: Hampshire,U.K.

Spielman AI and Brand JG (2016) Is Tasting Innate. Oral Diseases 22 (4): 251–252. DOI: 10.1111/odi.12460.

Spielman AI and Brand JG (2018) Wiring taste receptor cells to the central gustatory system. Oral Diseases 1: 2. DOI: 10.1111/odi.12833.

Zhang Y, Hoon MA, Chandrashekar J, et al. (2003) Coding of sweet, bitter, and umami tastes: different receptor cells sharing similar signaling pathways. Cell 112 (3): 293–301 https://www.ncbi.nlm.nih.gov/pubmed/12581520.

Zhang Y, Hao J, Zhang W, et al. (2019) Sour sensing from the tongue to the brain. Cell. DOI: j,cell.2019.08.31.

Zhao GQ, Zhang Y, Hoon MA, et al. (2003) The receptors for mammalian sweet and Umami taste. Cell 115: 255–266 https://www.ncbi.nlm.nih.gov/pubmed/14636554.

Further Reading

Behrens M and Meyerhof W (2013) Bitter taste receptor research comes of age: from characterization to modulation of TAS2Rs. Seminars in Cell & Developmental Biology 24: 215–221. DOI: 10.1016/j.semcdb.2012.08.006.

Buck LB and Axel R (1991) A novel multigene family may encode odorant receptors: A molecular basis for odor recognition. Cell 85: 175–187 https://www.ncbi.nlm.nih.gov/pubmed/1840504.

Chandrashekar J, Kuhn C, Oka Y, et al. (2010) The cells and peripheral representation of sodium taste in mice. Nature 464: 297–301. DOI: 10.1038/nature08783.

Cockayne DA, Dunn PM, Zhong Y, et al. (2005) P2X2 knockout mice and P2X2/P2X3 double knockout mice reveal a role for the P2X2 receptor subunit in mediating multiple sensory effects of ATP. The Journal of Physiology 567 (Pt 2): 621–639 https://www.ncbi.nlm.nih.gov/pubmed/15961431.

Deems DA, Doty RL, Settle RG, et al. (1991) Smell and Taste Disorders, a study of 750 patients from the University of Pennsylvania Smell and Taste Center. Archives of Otolaryngology – Head & Neck Surgery 117: 519–528 https://jamanetwork.com/journals/jamaotolaryngology/article‐abstract/619670.

Lee RJ and Cohen NA (2015) Taste receptors in innate immunity. Cellular and Molecular Life Sciences 72 (2): 217–236. DOI: 10.1007/s00018‐014‐1736‐7 https://www.ncbi.nlm.nih.gov/pubmed/25323130.

Lee H, Macpherson LJ, Parada CA, et al. (2017) Rewiring the taste system. Nature 548 (7667): 330–333. DOI: 10.1038/nature23299.

Huang A, Chen X, Hoon MA, et al. (2006) The cells and logic for mammalian sour taste detection. Nature 442: 934–938. DOI: 10.1038/nature05084.

Kang N and Koo J (2012) Olfactory receptors in non‐chemosensory tissues. BMB Reports 45 (11): 612–622. DOI: 10.5483/bmbrep.2012.45.11.232.

Peng Y, Gillis‐Smith S, Jin H, et al. (2015) Sweet and bitter taste in the brain of awake behaving animals. Nature 527: 512–515. DOI: 10.1038/nature15763.

Roper SD and Chaudhari N (2017) Taste buds: cells, signals and synapse. Nature Reviews. Neuroscience 18 (8): 485–497. DOI: 10.1038/nrn.2017.68.

Spielman AI and Brand JG (2016) Is tasting innate. Oral Diseases 22 (4): 251–252. DOI: 10.1111/odi.12460.

Spielman AI and Brand JG (2018) Wiring taste receptor cells to the central gustatory system. Oral Diseases 1: 2. DOI: 10.1111/odi.12833.

Zhang Y, Hoon MA, Chandrashekar J, et al. (2003) Coding of sweet, bitter, and umami tastes: different receptor cells sharing similar signaling pathways. Cell 112 (3): 293–301 https://www.ncbi.nlm.nih.gov/pubmed/12581520.

Zhao GQ, Zhang Y, Hoon MA, et al. (2003) The receptors for mammalian sweet and Umami taste. Cell 115: 255–266 https://www.ncbi.nlm.nih.gov/pubmed/14636554.

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How to Cite close
Spielman, Andrew I, Ozdener, Mehmet H, and Brand, Joseph G(Nov 2019) Chemosensory Systems. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000038.pub3]