Chemoreception - Real-life applications
Taste Buds
Humans have only about 10,000 taste buds, whereas rabbits have 17,000 and cows some 25,000. This seems more than a little ironic, since humans enjoy by far the most varied diet. Both of the other animals are herbivores, meaning that they do not eat meat, nor are they accustomed to sweets and the many other varieties of taste in the diet of the average well-fed American. If anything, cows, with about 50% more taste buds than rabbits, eat a diet even more plain than that of their furry, fleet-footed fellow mammals.
Though our tasting equipment (that is, the chemoreceptors for taste in our tongues) may be much less sophisticated than that of cows or rabbits, the number of tastes our palate can recognize is as varied as a spectrum of color swatches at a paint store. Despite such variation, there are only a few basic tastes, most notably, the ones that once were thought to constitute primary tastes analogous to the primary colors: sweet, sour, salty, and bitter.
THE FOUR "BASIC TASTES."
When you lick an ice cream cone, you may notice that you are experiencing the sweetness of it primarily at the tip of your tongue. This is not simply because you are licking it with the tip but also because there is a heavier concentration of sweetness receptivity in that area. On the other hand, if you eat a sour gumball, you experience the taste most notably on the sides of your tongue, where receptivity to sour tastes is strongest. Reception of salty tastes takes place near the front of the tongue, just behind the tip. As for bitter tastes, the focal point of receptivity appears to be near the back of the tongue. The latter may be a highly useful adaptive mechanism we have developed along the way, since many poisons are bitter, and the gagging reflex takes place near the back of the mouth. Not all bitter tastes are revolting, however: olives, which many people love, are bitter as well, as is coffee when it has no cream or sugar to alter its flavor.
As we noted earlier, the organization of taste receptors on the tongue is not quite as simple as once was believed. For example, though our receptivity to sweetness seems to take place primarily at the tip, to a lesser extent we taste sweetness and other flavors all over the tongue. Furthermore, such flavors as sweet, sour, bitter, and salty are not the sum total of "taste" as we experience it; not only smell but also texture affects the taste of substances. Genetic and cultural factors also influence a person's unique tastes and may explain why one person loves sweets while another cannot get enough of tangy tastes.
Taste and Other Senses
Taste does not work alone; on the contrary, our sense of the smell, texture, and temperatures of foods affects our overall perception of its flavor and in some cases its desirability. When food is in the mouth, it produces a scent, which enters the nose through the nasopharynx, an opening that links the mouth and the nose. Because we experience smell more directly and our noses are more sensitive to olfactory sensations than our taste buds are to gustatory ones, people often experience the flavor of food first by its smell. This greatly affects our perception of what we eat.
For example, while many people like blue cheese, many others despise it, and this probably has more to do with its smell than with its taste. While its taste is rather tangy, it is not quite as "radical" as the aroma of this cheese, which has been compared to everything from old socks to vomit. Other cheeses, such as gorgonzola and, particularly, Limburger, are even more pungent and therefore have more than their share of detractors—but again, the smell is more extreme than the taste.
On a more pleasant note, anyone who has ever enjoyed a good steak or a hamburger cooked on an open grill will attest to the fact that a great deal of that enjoyment comes from the aromas of cooking. Usually it is the smell of a delectable food item, which we detect long before we taste it, that causes our salivary glands to begin operating, preparing us for the process of consuming and digesting the dish. Additionally, different types of cooking have particular smells and tastes associated with them, which people may find more or less appealing. For example, scrambled eggs cooked over an open campfire are likely (all other things being equal) to be more appealing to most people than eggs cooked in a skillet on an electric stove. But if the campfire is fueled with burning dung instead of wood, most Americans would choose the stove. In sparsely forested parts of the third world, however, animal dung is a principal form of fuel, and without it people might have to eat meals raw.
TEXTURE AND TEMPERATURE.
Texture and temperature may have less impact on taste than does smell, but these are still significant factors. Take the example of three plates of French fries. One is limp and soggy in consistency, and another is so crispy that the French fry crunches like potato chips. The third, however, is just a bit crispy on the outside and just a bit soft on the inside. Most people, though certainly not all, would judge the third plate of French fry the most delectable—purely on the basis of texture.
By the same token, many people are less than enthusiastic about boiled okra, owing to its slimy consistency, whereas fried okra is more appealing to most Americans, since it lacks that gooey texture. Likewise, temperature plays more of a role than one might think. Many people, for example, find cold coffee unappealing, though others have a fondness for iced coffee, at least if it has milk and sweetener. Similarly, many Americans enjoy the combination of cold ice cream and hot pie or cobbler, partly because the contrast of temperatures adds to the overall flavor.
Different People, Different Sensations
Not all people "taste" the same—that is, not all people have the same sense of taste or the same level of acuity for distinguishing different flavors. One person may have 10-1,100 taste buds per square inch (6.45 cm 2 ) on the tongue, indicating a huge range of sensitivities with regard to gustatory data. Research also has shown that women, on average, have more taste buds than men, proving what many a woman has long asserted—that women have "better taste" than men.
Though it does appear that the average female has a more acute sense of gustation than the typical male, taste buds are not the only biological factor involved in recognizing flavors. For example, the amount of saliva a person naturally generates and the amount of salt that appears in one's saliva play a major role in determining an individual's response to salty foods. A person whose mouth generates less saliva is more sensitive to the salt in foods, whereas a person prone to generating a greater quantity of saliva is less likely to taste the salt that has been added to a dish. That person is therefore more likely to add salt.
Ability to smell also varies from person to person, though it appears that a less acute sense of smell may be a sign not merely of fewer olfactory receptors but also of an actual olfactory disorders. Just as some people may be color-blind, it appears that others are "smell-blind." And just as being color-blind can have very serious consequences, for instance by causing a color-blind driver to miss a red light, much the same is true with an olfactory disorder. To a greater extent than one might immediately guess, smell serves a protective function. For example, without a sense of smell, one cannot tell if food is spoiled, unless, of course, it has reached such a state of putrefaction that it shows visible signs. For a person with an ordinary sense of smell, however, rotten food sends a signal through the olfactory receptors, which may cause a gag reflex when smelling food that has spoiled.
AGE AND SENSE OF TASTE.
It is an experience familiar to many people, and it goes something like this. Let us say that in his boyhood, a man enjoyed a particular brand of candy, of which he could never get enough. Left to his own devices, he probably would have eaten so much that he would have become sick. The fact that this never happened had more to do with his parents—and the fact that his allowance money had to go for other things as well—than it did with his own natural sense of restraint. So he dreamed of the day when he became a grown-up, when he could eat whatever he wanted.
Eventually, he forgets this dream amid the many distractions of adolescence, but then one day many years later, as a grown man, he happens to see this particular item of candy in a store, and all his childhood memories come back to him. He buys several pieces, thrilled that he can enjoy in complete freedom what was once a rare treat. He can barely wait to get into his car, open the first piece, put it in his mouth—and then he recoils in disgust, thinking, How did I ever enjoy that? Disappointed, he throws away the rest of the candy.
The candy, of course, has not changed, but the man—and his taste buds—have. As we mature, so do our taste buds, and their numbers increase, leading to greater sophistication of taste. Children tend to like very basic tastes, particularly sweet and sour, and respond much less favorably to the subtlety in more complex dishes. This fact, combined with an increased awareness of health issues as one ages, explains why an adult might relish a broccoli casserole but find cotton candy so sweet as to be repugnant, whereas a child's reaction would probably be just the opposite.
Just as taste buds mature along with the people who own them, they also age. Every 3-10 days, on average, our taste buds regenerate themselves, replacing old ones that have been worn out by foods that are too hot, too cold, or otherwise too taxing to the chemoreceptors in our tongues. But as people grow older, their taste buds replace themselves less frequently, and therefore their sense of taste becomes less finely tuned. An older person may require much more sweetness or spice to taste a particular food.
Chemoreception Impairment and Disorders
Sense of smell also deteriorates with age; as we noted earlier, this can pose dangers, because a person depends on the sense of smell for protection more than one might imagine. For example, in addition to the inability to detect spoiled food, an elderly person would be far less likely to smell smoke if a building were on fire. Older people also are less likely to be cognizant of olfactory data that send messages concerning unpleasant smells of a less critical nature—body odor, for example.
Many of the problems of gustation and olfaction suffered by the elderly are reflected, at a much younger age, in the bodies of smokers. In addition to its many other negative effects, smoking deadens taste buds and desensitizes the olfactory receptors. It is not uncommon to see a heavy smoker salting pizza or some other food that for people with ordinarily functioning taste buds would not seem to require any salt. As for olfactory sensation, a smoker becomes accustomed to the reek of stale smoke and ashes.
On a more temporary basis, many people find their senses of taste and smell impaired by illness. A person with a cold or flu, even at its final stages, usually has enough congestion that the senses of both smell and taste are limited, if not almost nonexistent. In this instance, the lack
TASTE AND SMELL DISORDERS.
In addition to people whose olfactory and gustatory senses are impaired by age, illness, or smoking, between two million and four million Americans suffer from some sort of taste or smell disorder. The inability to taste or smell not only robs an individual of certain sensory pleasures, it also can be dangerous to one's mental health. Some psychiatrists believe that the lack of taste and smell can have a profoundly negative effect on a person's quality of life, leading to depression or other psychological problems.
Whereas impairments of smell and taste brought on by cold, flu, various viral and bacterial infections, and even allergies are usually temporary, some other illness-related taste and smell disorders are more long term. Such is the case with neurological disorders due to brain injury or diseases such as Parkinson's or Alzheimer's (conditions marked by tremors and mental deterioration, respectively). These conditions can cause more permanent damage to the intricate neural networks that process tastes and smells.
Drugs such as lithium, used to treat bipolar disorder (what used to be called manic depression) also may cause taste and smell disorders. This occurs because certain drugs (lithium is just one example among many) inhibit the action of certain enzymes, affect the body's metabolism, and interfere with the neural networks and receptors involved in tasting and smelling. Exposure to such environmental toxins as lead, mercury, insecticides, and solvents (e.g., paint thinner) also can damage taste buds and sensory cells in the nose or brain.
Culture and Chemoreception
One of the favorite delicacies in the Philippines is known as dinuguan, or pork cooked in pork blood. Chances are that a visitor from England or northern Europe, when told the constituents of the dish, would feel right at home; an American, however, most likely would try to think of an excuse to pass up this Filipino delight. To most Americans, it would seem that eating dinuguan, or the many varieties of blood pudding or blood sausage common in England and Scandinavia, is simply "gross"—that it is objectively and unquestionably disgusting. But in the 1960s or even the 1970s, most Americans, even in large cities, would have said that the idea of eating raw fish was revolting. Today, however, America's cities and suburbs bristle with Japanese restaurants that serve sushi, an indication of the fact that cultural tastes can change.
As is discussed in Parasites and Parasitology, it should be noted that improperly cooked pork and fish (especially raw fish) are quite likely to serve as hosts for disease-carrying worms, so one should exercise care before sitting down to a plate of dinuguan or sushi. But then again, many Americans like their steaks on the rare side, and undercooked beef certainly has its share of pathogens (disease-carrying parasites) as well. It seems that it is not health concerns that explain our cultural double standards about certain food items.
Of course, Americans are not the only people with quirky standards regarding tastes and smells; in fact, every culture has its idiosyncrasies. In China it is not considered at all offensive for one's breath to smell heavily of onions; on the other hand, the smell of dairy products, virtually nonexistent in Chinese cuisine, is considered highly offensive. What does all of this prove? Only that taste and smell are not purely biological but also reflect cultural factors.
During the nineteenth century, many European scientists embraced a racist theory concerning the olfactory capabilities of different peoples around the world. According to this highly unscientific "theory," non-Europeans were more primitive than Europeans and therefore closer to animals, which meant that they had a stronger sense of smell. Completing the loop of this circular logic, subscribers to this nonsense maintained that because non-Europeans had a stronger sense of smell, it proved they were more primitive than Europeans!
Humans, Animals, and Smell
By the early twentieth century, physiologists had begun to explore much more scientific ideas concerning olfactory and gustatory abilities in humans—abilities that, needless to say, are not a function of race or ethnicity. Only one assumption of the old-fashioned European scientists was correct: that animals have a stronger sense of smell than do humans. The human nose is capable of detecting odors so faint that their proportion of the surrounding air is in the range of only a few parts per trillion. Many researchers are beginning to wonder whether smell does not play a greater role in human behavior and biology than previously was believed. For example, research has shown that only a few days after the birth of her baby, a human mother can smell the difference between a vest worn by her baby and one worn by another.
Nevertheless, the fact remains that the olfactory abilities of many animals are far beyond those of humans. This is a fact that hardly needs scientific verification, since most of us have observed dogs' reliance on their strong sense of smell. This explains why police use dogs to detect illegal drugs and explosives and to track runaway prisoners or the bodies of murder victims. Dogs are not the only animals gifted with acute senses of smell, which aid them in finding their way to specific targets. Salmon, for example, manage to find their way back to the streams where they were hatched, guided by their sense of smell. (For more about animals' navigational abilities, see Migration and Navigation.)
Most vertebrates other than humans have many more olfactory nerve cells in a proportionately larger olfactory epithelium, and this probably gives them much more sensitivity to odors. In addition, most land vertebrates have a specialized scent organ in the roof of their mouths called the vomeronasal organ, which gives them far more sensitivity to odors than humans have.
PHEROMONES.
Some animals are known particularly for the odors they excrete, especially when it is an animal such as a skunk or stinkbug that puts off a repellent odor as a defense mechanism. But animals also send out much more subtle smells known as pheromones. Chemical substances produced and secreted by animals, pheromones serve as stimuli for behavioral responses on the part of other animals of the same species. Pheromones are common among insects as well as many vertebrates, but they are nonexistent among bird species.
Among so-called "social insects" such as bees and ants, pheromones play a particularly strong role. The queen honeybee gives off what is called the queen substance, a pheromone that acts to prevent the development of ovaries among workers, which are biologically unproductive females. Pheromones are vital for communication among social insects, which have little or no sense of sight. Ants, bees, and wasps send out smells to alarm others of danger, and ants may create a path of pheromones to guide others to a food source.
The function for which pheromones are most widely known, however, is as a sex attractant. Male musk deer are noted for their excretion of musk, which, as the result of to its pleasant and powerful smell, is often an ingredient in perfume manufacture. Though musk is a sex attractant, it is not a pheromone, which is a much less obvious scent and which, as we have noted, likely has an effect only on animals of the same species.
Experiments have shown that male mice who lack a gene for a pheromone receptor are likely to attempt to mate with males, simply because they cannot tell the difference. With humans, of course, it is easy to tell the difference between males and females on sight, but do humans also respond to pheromones? The fact that these chemicals theoretically could induce mating behavior led cologne and perfume makers long ago to embrace the idea of human pheromones, whose presence in a manufactured scent obviously would be a boon to many a single man or woman. Despite the enthusiastic claims of perfume manufacturers, however, many scientists have yet to be convinced that pheromones play a significant role for humans.
Regarding the fact that the human anatomy includes the vestige of a vomeronasal organ, the olfactory researcher Charles Wysocki told Lee Bowman in an article published on the National Library of Medicine Web site, "It's like the appendix—it's there, but it doesn't seem to do anything." As for scent makers' promises that pheromones will help users attract partners, Wysocki said, "Sure the claims are out there. … 'All you have to do is put this on and you'll score.' But there's nothing in the published biomedical literature [to indicate] that we have any kind of pheromone that draws a partner." Research does suggest that people give off chemical messages that correspond to certain moods, but it is a long way from this to the idea of a spray-on aphrodisiac.
WHERE TO LEARN MORE
Ackerman, Diane. A Natural History of the Senses. New York: Vintage Books, 1991.
Bowman, Lee. "A Nose for Romance?" U.S. National Library of Medicine/National Institutes of Health (Web site). <http://www.nlm.nih.gov/medlineplus/news/fullstory_6125.html> .
Chemical of the Week—Chemoreception: The Chemistry of Odors. Science Is Fun/University of Wisconsin-Madison (Web site). <http://scifun.chem.wisc.edu/CHEMWEEK/Odors/chemorec.html> .
Chemoreception Links. Leffingwell & Associates (Web site). <http://www.leffingwell.com/links5.htm> .
The ChemoReception Web (Web site). <http://www.csa.com/crw/home.html> .
Evans, David H. The Physiology of Fishes. Boca Raton, FL: CRC Press, 1998.
Finger, Thomas E., Wayne L. Silver, and Diego Restrepo. The Neurobiology of Taste and Smell. New York: Wiley-Liss, 2000.
Monell Chemical Senses Center (Web site). <http://www.monell.org/> .
Pybus, David, and Charles Sell. The Chemistry of Fragrances. Cambridge, England: Royal Society of Chemistry, 1999.
Rivlin, Robert, and Gravelle, Karen. Deciphering the Senses: The Expanding World of Human Perception. New York: Simon and Schuster, 1984.
Whitfield, Philip, and D. M. Stoddart. Hearing, Taste and Smell: Pathways of Perception. Tarrytown, NY: Torstar Books, 1984.
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