Now scientists at the Miller School of Medicine have conducted experiments that elucidate the path of taste, from detection in the sensory cells of the tongue, to the first relay in the brain.
The results have been published in an article, “Breadth of tuning in taste afferent neurons varies with stimulus strength,” in the online journal Nature Communications. The researchers’ study found that when taste buds detect sugar, quinine, salt, etc., rather than transmitting the information via neurons that are specific for single tastes, the body uses combinations of neurons to process the information and evoke the perceptions of sweetness, sourness, saltiness, umami (savory) and bitterness.
“Our findings show that combinations of neurons, activated simultaneously, generate the different qualities of taste,” said Stephen D. Roper, Ph.D., professor of physiology and biophysics, and principal investigator of the project. “This is akin to combinations of keys on a piano that distinguish Bartok from Beethoven, where no single key on its own is unique to either composer.”
The study offers insights into how our senses package many features of stimuli — identity (e.g., sugar vs. saccharin), quality (sweet vs. umami) and intensity (diluted vs. strong) — into seemingly simple digital signals in neurons.
The new findings have implications for understanding both the complexity of the nervous system and debilitating medical conditions that can seriously impact quality of life, such as dysgeusia (abnormal taste perceptions) and ageusia (loss of taste).
“Because normal taste function is essential for swallowing, patients suffering taste loss or taste disturbances, for example after irradiation to combat oral or throat cancers, may lose normal swallowing reflexes and suffer from malnutrition,” Roper said.
Nirupa Chaudhari, Ph.D., also a professor of physiology and biophysics, and co-author of the study, added, “Our work, though not directly aimed at finding therapies for these conditions, provides fundamental knowledge about the neural processing of taste and other sensory signals.”
Taste buds are made up of tight clusters of 50 to 100 epithelial cells. These cells have receptors that bind molecules in food and drinks, sending a signal to the brain along pathways of neurons. According to the study, even those neurons sensitive only to single taste qualities (sweet, for example) at low concentration, become responsive to a broader range of qualities when the strength of the stimulus is increased.
“Our results contradict the previous belief that taste is encoded by a simple ‘labeled line’ organization, wherein the perception of sweet is conveyed via a linear sequence of dedicated neurons that signal sweet and no other quality,” Chaudhari said. “Previous studies advancing a labeled line organization may not have had the resolving power to discern the combinatorial coding that underlies taste.”
Miller School co-authors are first author An Wu, a graduate student in the Neuroscience Program, Gennady Dvoryanchikov, Ph.D., associate scientist, and Elizabeth Pereira, research associate. The future direction of this project is to decode the specific neuronal combinations underlying the processing of each basic taste.
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