However, scientists looking closely have uncovered remarkable details about the pieces making up the taste system, and how these pieces fit together [ 1 ]. What do we see when we stick out our tongues? Lots of bumps. Most people think they are taste buds, but it is a little more complicated than that Figure 1.
The bumps we see are called papillae , and they are a special tough part of our skin. The real taste buds are made up of delicate cells nestled like sections of an orange beneath the surface of the papillae, where they are well protected. Only the tips of the taste buds poke through to the surface of the tongue.
The taste buds cannot be seen with the naked eye, but if you could zoom in, you would see that each of our papillae contains thousands of taste buds, all peeking out [ 2 ]. At their very tips, where they poke out from the tongue, each taste bud cell stores tiny proteins called taste receptors Figure 1 [ 3 ]. The role of taste receptor proteins is to detect substances in your mouth, such as food particles. Taste receptors activate when chewed food mixes with saliva, then flows over and around the papillae like a mushy river.
The receptor proteins ignore most of the mix, but when they detect their target food particles they react, notifying their cells that a taste substance has been detected. This process can be imagined as if the receptors are locks and the food particles are keys.
Just as a lock opens only with its matching key, a taste receptor reacts only to its matching type of food particle. When a taste bud cell is notified that a substance such as food has been detected, it goes into action Figure 2. The taste bud puts dozens of proteins inside the cell to work. These proteins cooperate, rapidly shifting electrically charged atoms called ions here and there, to produce a tiny electrical current inside the cell [ 2 ].
This impulse is so tiny you cannot feel it. However, it is detected by the nerves in your tongue, which are specialists at detecting and passing on electric signals. When the nerves in your tongue receive signals from taste bud cells, they pass them on to more nerves and then more, sending the message racing out the back of your mouth, up through a tiny hole in your skull, and into your brain. There, your gustatory cortex the taste center of your brain finishes the job of telling you, which taste you perceive, sweet, salty, bitter, sour, or savory.
For that, we need to look to Edwin G Boring. In the s, this graph was reimagined by Boring, a Harvard psychology professor, in his book Sensation and Perception in the History of Experimental Psychology.
Indeed, results from a number of experiments indicate that all areas of the mouth containing taste buds — including several parts of the tongue, the soft palate on the roof of your mouth and the throat — are sensitive to all taste qualities.
Our understanding of how taste information is carried from the tongue to the brain shows that individual taste qualities are not restricted to a single region of the tongue.
There are two cranial nerves responsible for taste perception in different areas of the tongue: the glossopharyngeal nerve in the back and the chorda tympani branch of the facial nerve in the front. In , surgeon TR Bull found that subjects who had had their chorda tympani cut in medical procedures also reported no loss of taste. And in , Linda Bartoshuk from the University of Florida found that by applying anesthesia to the chorda tympani nerve, not only could subjects still perceive a sweet taste, but they could taste it even more intensely.
Modern molecular biology also argues against the tongue map. Over the past 15 years, researchers have identified many of the receptor proteins found on taste cells in the mouth that are critical for detecting taste molecules. Thus, the area of the brain controlling appetite is linked to the region devoted to pleasure and reward.
When these MCH neurons are stimulated, it leads to an increase in intracellular calcium and release of endocannabinoids. Conversely, when leptin receptors on MCH neurons are activated, voltage-gated calcium channels are blocked, suppressing endocannabinoid release, and this leads to an appetite-suppressing effect of leptin.
Endocannabinoids also likely enhance taste cell responses to sweeteners. Intraperitoneal administration of endocannabinoids led to a dose-dependent increase in CT glossopharyngeal nerve responses to sweeteners in mice [ 79 ].
This was not observed for salty, sour, bitter, or umami compounds or in CB1 knockout mice. The authors further demonstrated by immunohistochemistry that sweet taste cells expressing T1R3 also express CB1 receptors. These findings suggest that endocannabinoids may enhance sweet taste response in sweet taste cells expressing T1R3. Sweet taste receptors and sweet taste molecules are involved in transduction of sweet taste in taste buds. Furthermore, it is clear that sweet taste pathways are present in the gut and in the CNS, including the appetite center in the hypothalamus.
Accumulating data suggest that these pathways act as nutrient sensors in the gut and the brain. They also serve to regulate energy balance, glucose homeostasis, and food intake. Interactions between peripheral and central pathways are carefully regulated with input from peripheral mediators, such as leptin, ghrelin, insulin, GLP-1, and endocannabinoids.
Further elucidation of these pathways may provide invaluable insight into the pathogenesis of common diseases, including obesity and type 2 diabetes mellitus. Owyang and PDK C. National Center for Biotechnology Information , U. Journal List Nutrients v. Published online Jun Allen A. Author information Article notes Copyright and License information Disclaimer. Received Apr 10; Accepted Jun This article has been cited by other articles in PMC.
Abstract Sweet taste receptors are composed of a heterodimer of taste 1 receptor member 2 T1R2 and taste 1 receptor member 3 T1R3. Keywords: sweet taste receptors, glucose sensing, nutrient sensing, leptin, hypothalamus.
Chemosensory Cells in the Tongue Humans can distinguish between five basic tastes, including sweet, salty, umami, bitter, and sour. Open in a separate window. Figure 1. Sweet Taste Signaling Sweet taste receptors can be activated by a wide range of chemically different compounds, including sugars glucose, fructose, sucrose, maltose , artificial sweeteners e.
Figure 2. Chemosensory Cells in the GI Tract Although taste receptors were initially discovered in taste buds, a growing number of studies have demonstrated that sweet taste receptors are expressed throughout the body, including the nasal epithelium, respiratory system, pancreatic islet cells, and even in sperm and testes [ 12 , 13 , 14 ]. Table 1 Enteroendocrine cells of the mammalian gastrointestinal tract. L Cells Sweet taste receptors are expressed by L cells in the distal small intestine.
K Cells K cells in the proximal intestine secrete glucagon-like insulinotropic peptide GIP in the presence of glucose. Enterochromaffin Cells Enterochromaffin EC cells are distributed throughout the GI tract; the EC cells secrete serotonin 5-HT to mediate changes in motility and secretion as well as in transduction of visceral stimuli [ 31 ].
Neuroanatomy of Sweet Taste Upon activation of sweet taste receptors, neural afferents of cranial nerves send gustatory information to the rostral division of the nucleus tractus solitarius rNTS of the medulla Figure 3 [ 44 ]. Figure 3. Figure 4. Central Actions of Gut Hormones 7. Effect of Leptin Leptin is an anorexigenic hormone that is primarily produced by adipocytes.
Leptin and Sweet Taste in Humans Plasma leptin levels show a diurnal variation in humans, with levels peaking around midnight and lowest around noon to mid-afternoon [ 70 ].
Effect of Endocannabinoids Cannabinoids, such as Cannabis sativa marijuana , have long been known to have an appetite-stimulating effect. Sweet Enhancing Effect of Endocannabinoids Endocannabinoids also likely enhance taste cell responses to sweeteners. Conclusions Sweet taste receptors and sweet taste molecules are involved in transduction of sweet taste in taste buds.
Conflicts of Interest The authors declare no conflict of interest. References 1. Laugerette F. CD36 involvement in orosensory detection of dietary lipids, spontaneous fat preference, and digestive secretions. Janssen S. Nutrient sensing in the gut: New roads to therapeutics? Trends Endocrinol. Finger T. Neuroscience: ATP signalling is crucial for communication from taste buds to gustatory nerves. Yang R. Immunocytochemical analysis of P2X2 in rat circumvallate taste buds.
BMC Neurosci. Chaudhari N. The cell biology of taste. Cell Biol. Nelson G. Mammalian sweet taste receptors. Human receptors for sweet and umami taste. Jiang P. Depoortere I. Taste receptors of the gut: Emerging roles in health and disease. Dotson C. Peptide regulators of peripheral taste function.
Cell Dev. Kohno D. Sweet taste receptor in the hypothalamus: A potential new player in glucose sensing in the hypothalamus. Lee R. Bitter and sweet taste receptors in the respiratory epithelium in health and disease. Meyer D. Furness J. The gut as a sensory organ. Raybould H. Nutrient Tasting and Signaling Mechanisms in the Gut. Sensing of lipid by the intestinal mucosa. Liver Physiol. Owyang C. New insights into neurohormonal regulation of pancreatic secretion.
The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. In the surface of the tongue are raised bumps, called papilla, that contain the taste buds. There are three types of papilla, based on their appearance: vallate, foliate, and fungiform.
The number of taste buds within papillae varies, with each bud containing several specialized taste cells gustatory receptor cells for the transduction of taste stimuli. These receptor cells release neurotransmitters when certain chemicals in ingested substances such as food are carried to their surface in saliva. Neurotransmitter from the gustatory cells can activate the sensory neurons in the facial and glossopharyngeal cranial nerves. As previously mentioned, five different taste sensations are currently recognized.
This depolarizes the cells, leading them to release neurotransmitter. For example, orange juice, which contains citric acid, will taste sour because it has a pH value of about 3. Of course, it is often sweetened so that the sour taste is masked. As the concentration of the hydrogen ions increases because of ingesting acidic compounds, the depolarization of specific taste cells increases. The other three tastes; sweet, bitter and umami are transduced through G-protein coupled cell surface receptors instead of the direct diffusion of ions like we discussed with salty and sour.
The sweet taste is the sensitivity of taste cells to the presence of glucose dissolved in the saliva.
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