Sense of Taste
The sense of taste is mediated by groups of cells called taste buds which sample oral concentrations of a large number of small molecules and report a sensation of taste to centers in the brainstem. In most animals, including humans, taste buds are most prevalent on small pegs of epithelium on the tongue called papillae. The taste buds themselves are too small to see without a microscope, but papillae are readily observed by close inspection of the tongue's surface. To make them even easier to see, put a couple of drops of blue food coloring on the tongue of a loved one, and you'll see a bunch of little light colored bumps - mostly fungiform papillae - stand out on a blue background.
In addition to signal transduction by taste buds, 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. These include receptors for such chemicals as sodium, potassium, chloride, glutamate and adenosine. Despite this complexity, five types of tastes are commonly recognized:
The umami taste is that of monosodium glutamate and has recently been recognized as a unique taste, as it cannot be elicited by any combination of the other four taste types. Glutamate is present in a variety of protein-rich foods, and particularly abundant in aged cheese.
Perception of taste also appears to be influenced by thermal stimulation of the tongue. In some people, warming the front of the tongue produces a clear sweet sensation, while cooling leads to a salty or sour sensation.
It should be noted that these tastes are based on human sensations and some comparative physiologists caution that each animal probably lives in its own "taste world". For animals, it may be more appropriate to discuss tastes as being pleasant, unpleasant or indifferent.
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 amino-4-nitrobenzene 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 15,000-fold. Substances sensed as bitter typically have very low thresholds.
Taste Bud Anatomy
Taste buds are composed of groups of about 40 columnar epithelial cells bundled together along their long axes. Taste cells within a bud are arranged such that their tips form a small taste pore, and through this pore extend microvilli from the taste cells. The microvilli of the taste cells bear taste receptors and it appears that most taste buds contain cells that bear receptors for 2 or 3 of the basic tastes.
Interwoven among the taste cells in a taste bud is a network of dendrites of sensory nerves called "taste nerves." When taste cells are stimulated by binding of chemicals to their receptors, they depolarize and this depolarization is transmitted to the taste nerve fibers resulting in an action potential that is ultimately transmitted to the brain. One interesting aspect of this nerve transmission is that it rapidly adapts - after the initial stimulus, a strong discharge is seen in the taste nerve fibers but within a few seconds, that response diminishes to a steady-state level of much lower amplitude.
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.
Considerable attention has been devoted to understanding the benefits to survival and wellbeing that accrue from having a sense a taste. Some have speculated that an ability to taste bitterness may protect animals from ingesting certain natural poisons. There is no doubt that animals, including humans, develop taste preferences. That is, they will choose certain types of food in preference to others.
Interestingly, taste preference often changes in conjunction with body needs. Similarly animals often develop food aversions, particularly if they become ill soon after eating a certain food, even though that food was not the cause of the illness - surely you have experienced this yourself. Food preferences and aversions involve the sense of taste, but these phenomena are almost certainly mediated through the central nervous system rather that directly through taste cells.
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Source: Republished with permission by Richard Bowen - Hypertexts for Biomedical Sciences