The electrical changes in the taste cells that prompt signals to the brain are based on the varying concentrations of charged atoms, or ions. Taste cells, like neurons, normally have a net negative charge internally and a net positive charge externally. Tastants alter this state of affairs by using various means to increase the concentration of positive ions inside taste cells, eliminating the charge difference [ see box on next two pages ]. Such depolarization causes the taste cells to release tiny packets of chemical signals called neurotransmitters, which prompt neurons connected to the taste cells to relay electrical messages.
But studies of animals and people show there is not always a strict correlation between taste quality and chemical class, particularly for bitter and sweet tastants. Many carbohydrates are sweet, for instance, but some are not. And very disparate types of chemicals can evoke the same sensation: people deem chloroform and the artificial sweeteners aspartame and saccharin sweet even though their chemical structures have nothing in common with sugar. The compounds that elicit salty or sour tastes are less diverse and are typically ions.
The chemicals that produce salty and sour tastes act directly through ion channels, whereas those responsible for sweet and bitter tastes bind to surface receptors that trigger a bucket brigade of signals to the cells' interiors that ultimately results in the opening and closing of ion channels.
In Margolskee and his colleagues Susan K. McLaughlin and Peter J. McKinnon identified a key member of this bucket brigade. They named the molecule gustducin because of its similarity to transducin, a protein in retinal cells that helps to convert, or transduce, the signal of light hitting the retina into an electrical impulse that constitutes vision.
Gustducin and transducin are both so-called G-proteins, which are found stuck to the undersides of many different types of receptors.
The name G-protein derives from the fact that the activity of such proteins is regulated by a chemical called guanosine triphosphate, GTP. When the right tastant molecule binds to a taste cell receptor, like a key in a lock, it prompts the subunits of gustducin to split apart and carry out biochemical reactions that ultimately open and close ion channels and make the cell interior more positively charged.
In Margolskee and Gwendolyn T. Wong and Kimberley S. Gannon used mice they genetically engineered to lack one of gustducin's three subunits to demonstrate that the G-protein is crucial for tasting bitter and sweet compounds. Unlike normal mice, the altered mice did not prefer sweet foods or avoid bitter substances: they did not avidly drink highly sweetened water and instead drank solutions of very bitter compounds as readily as they did plain water.
Moreover, these mice were indifferent to the umami taste of glutamate. The researchers also showed that key nerves in the mice lacking gustducin had a reduced electrical response to sweet, bitter and umami tastants but could still respond to salts and acidic compounds.
In two groups of scientists--one led jointly by Charles S. Buck of Harvard Medical School--identified in mice and humans the actual receptors that bind to bitter tastants and activate gustducin. Zuker and Ryba's group inserted the genes that encode two of these mouse taste receptors, mT2R5 and mT2R8, into cells grown in the laboratory and found that the engineered cells became activated when they were exposed to two bitter compounds.
The researchers noted that in particular strains of mice a specific version of the gene for mT2R5 tended to be handed down along with the ability to sense the bitterness of the antibiotic cycloheximide, a further indication that the genes for the T2R receptors were responsible for detecting bitter substances. During the past few years, Wolfgang Meyerhof and his colleagues at the German Institute of Human Nutrition have determined for several T2R receptors which bitter compounds they respond to.
In several research groups, including that of Margolskee, identified T1R3, a novel type I taste receptor. Margolskee's colleague Marianna Max mapped T1R3 to the Sac saccharin sweet taste locus in the mouse genome. A specific sequence variation of the T1R3 gene was shown to determine the high preference for sugars, saccharin and other sweeteners displayed by so-called taster strains of mice.
Margolskee and his colleague Sami Damak genetically altered taster mice to lack T1R3 to determine that this taste receptor was essential to a mouse's ability to taste sweet compounds. Researchers are also studying receptors that might be responsible for the taste Japanese scientists call umami, which loosely translates into meaty or savory. In Nirupa Chaudhari and Stephen D.
While it is true that the edge of the tongue has more taste buds than the base and is thus more sensitive, the tongue is not divided into different types of taste. The one exception to this rule is the bitter taste, which is located chiefly at the rear of the tongue.
The taste buds are the organs in the tongue that register taste, and are located around what are known as the gustatory papillae. These are the small structures on the upper surface of our tongue. An adult has approximately 2, to 4, papillae on their tongue. Whenever we eat a salty soup or a sweet dessert, the sensory cells in the taste buds are activated and our brain is informed how salty or sweet the food is. Around half of the sensory cells respond to all five basic tastes, while the remainder specialise in a particular taste.
Our sense of taste deteriorates with age, in a development that is easy to explain. Once taste signals are transmitted to the brain, several efferent neural pathways are activated that are important to digestive function.
For example, tasting food is followed rapidly by increased salivation and by low level secretory activity in the stomach. Among humans, there is substantial difference in taste sensitivity. Roughly one in four people is a "supertaster" that is several times more sensitive to bitter and other tastes than those that taste poorly. Such differences are heritable and reflect differences in the number of fungiform papillae and hence taste buds on the tongue.
In addition to signal transduction by taste receptor cells, it is also clear that the sense of smell profoundly affects the sensation of taste. Think about how tastes are blunted and sometimes different when your sense of smell is disrupted due to a cold. The sense of taste is equivalent to excitation of taste receptors, and receptors for a large number of specific chemicals have been identified that contribute to the reception of taste.
Despite this complexity, five types of tastes are commonly recognized by humans:. None of these tastes are elicited by a single chemical. Also, there are thresholds for detection of taste that differ among chemicals that taste the same.
For example, sucrose, 1-propyl-2 aminonitrobenzene and lactose all taste sweet to humans, but the sweet taste is elicited by these chemicals at concentrations of roughly 10 mM, 2 uM and 30 mM respectively - a range of potency of roughly 15,fold. However, many plants with bitter compounds are toxic. Our ancestors evolved to taste bitterness so they could recognize and avoid poison. Not all bitterness is bad, though. Savory taste is caused by amino acids. Some scientists think tasting savoriness helps increase our appetite and control protein digestion.
Umami is the most recently discovered taste. In , a Japanese researcher named Kikunae Ikeda found glutamic acid in kombu, a type of seaweed. This includes monosodium glutamate , or MSG. Umami was accepted as a new taste when scientists found umami receptors in our taste buds. You might associate odor with literally smelling something. But when you eat food, odor particles in your mouth also enter your nose through the nasopharynx.
This is the upper area of your throat behind your nose. Flavor is the result of this odor plus taste. There are many possible flavors, depending on the intensity of each odor and taste.
Your tongue contains thousands of tiny bumps called taste papillae. Each papilla has multiple taste buds with 10 to 50 receptor cells each. You also have taste receptor cells along the roof of your mouth and in the lining of your throat. When you eat, the receptors analyze the chemical compounds in your food.
Next, they send nerve signals to your brain, which creates the perception of taste. It also enables us to associate different tastes with different emotions.
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