CHEMORECEPTION

CONCEPT

Chemoreception is a physiological process whereby organisms respond to chemical stimuli. Humans and most higher animals have two principal classes of chemoreceptors: taste (gustatory receptors), and smell (olfactory receptors). Though our sense of smell assists us in distinguishing among tastes, the gustatory and olfactory receptors are different in many respects—not only in their locations but also in terms of their chemical and neurological makeup. Capabilities of taste and smell vary widely among people, as a function of genetics, age, and even personal habits. Likewise, culture influences attitudes toward taste and smell. As for the animal kingdom, certain creatures are gifted with exceedingly acute senses, particularly where smell is concerned, but for some invertebrates, such as worms, there is really little distinction between taste and smell. Among the most interesting aspects of chemoreception in animals is the use of smell for communication, particularly through the release of special chemicals called pheromones. As to whether pheromones, which function as sex attractants, play a role in human interaction, many scientists remain skeptical.

HOW IT WORKS

The Senses

The term sense, which also may be called sensory reception and sensory perception, refers to the means by which an organism (usually an animal) receives signals regarding physical or chemical changes or both in its environment. Sensory reception and perception entail the translation of these signals, which represent changes in the matter or energy of the environment, into processes within the body and brain. For example, if you eat a cookie, your sense of taste translates the chemical data from that cookie into the sensation of sweetness, a sensation your brain most likely perceives as a pleasing one.

In everyday language, people are accustomed to speaking of five senses possessed by humans and, to a greater or lesser degree, by other animals: sight, touch, smell, hearing, and taste. The reality is rather more complex. For one thing, there are not really just five senses; rather, there are at least five others—and there may be more, depending on just how one defines and classifies the senses. These other senses include the kinesthetic sense, or the discernment of motion; the sensation of temperature, or distinguishing relative heat and cold; the awareness of pressure; the sense of equilibrium or balance; and the perception of pain.

All of these senses involve a response to stimuli, which may be defined as any phenomenon (that is, an observable fact or event, such as an environmental change) that directly influences the activity or growth of a living organism. Each of these senses has a biological component as well as a physical or a chemical one. In the next few paragraphs, we briefly discuss what this means, first by considering the nervous system in general terms and then by looking at the physical and chemical receptors that transmit data through that system.

THE NERVOUS SYSTEM.

All sensation is biological in the sense that an organism not only must be living to experience it but also must have a functioning nervous system. The latter is a network, found in the bodies of all vertebrates (animals with internal skeletons), whose purpose is to receive and interpret stimuli and to transmit impulses. Parts of the nervous system include the brain, spinal cord, nerves, and other components.

To be experienced through the senses, all data must be transmitted to the brain through the nervous system. This happens through the conversion and transmission of physical or chemical information, which takes place in sensory nerves known as receptors. A receptor is a structure in the nervous system that receives specific stimuli and is affected in such a way that it sends particular messages to the brain. The brain interprets these messages as sensations corresponding to the stimuli. Primary receptors are those that directly convert stimuli to electronic signals, which they send to the brain. Neurons, or nerve cells, serve as primary receptors. In addition, there are secondary receptors, which simply transmit signals between neurons. Secondary receptors induce a response in an adjoining neuron, thus sending the signal down the line toward the brain.

PHYSICAL AND CHEMICAL RECEPTORS.

As we have noted, the changes in the outside environment that the brain interprets as sensation are either physical or chemical in origin. By physical, we mean the types of data and phenomena that are studied in the science of physics: matter, energy, and the interactions between them. Likewise, chemical, refers to objects of study in the scientific realm of chemistry: elements, compounds, and mixtures.

In the human body there are sensory nerves devoted to the interpretation of physical or chemical data. Among the receptors of physical data are photoreceptors, which respond to light and therefore play a part in the sense of sight; thermoreceptors, which respond to temperature and are concerned with the sense of heat and cold; and nociceptors, or pain receptors. In addition, there are mechanoreceptors, which respond to the mechanical properties of matter and are involved in the senses of touch, hearing, and equilibrium.

As for chemical data, they are interpreted through chemoreceptors, which govern the senses of smell and touch. For this reason, chemoreceptors sometimes are referred to as the chemical senses. Our focus in the present context, of course, is on chemoreception, the term for the physical process whereby organisms—not just humans, but virtually all animals—respond to chemical stimuli.

Distinguishing Gustation and Olfaction

We tend to associate taste and smell, and indeed there is some relation between them, but there are also numerous distinctions. Not everyone with an acute sense of smell, for instance, has as finely honed a sense of taste. Conversely, people with poor senses of smell do not necessarily suffer a corresponding impairment in their taste buds. The neurological structures that have to do with taste and smell, at least in higher animals, are not the same. Whereas gustatory sensation travels via secondary receptors called epithelial cells, which pass messages to adjacent neurons, olfactory signals enter the nervous system through primary receptors.

The principal sensory organs of gustation and olfaction in a human being are, respectively, the taste buds on the tongue and the olfactory receptors located within the olfactory epithelium in the nasal cavity. They are designed to respond to two entirely different types of chemical materials. Taste buds are best at receiving sensory data from water-soluble chemicals, or those that dissolve in water, while the olfactory receptors are most attuned to vapors that are water-insoluble.

WATER AND OIL.

Almost everyone who has ever eaten a hot, spicy dish has tried to "put out the fire" in their mouths by drinking water, only to discover to their dismay that water only seemed to make the problem worse. On the other hand, milk is usually quite effective. The reason is that most spicy-hot substances tend to be oily, and therefore they do not readily form intermolecular bonds with water. On the other hand, milk, though it is largely composed of water, also contains oily fat particles. Thanks to the chemistry of water-and oil-based substances, it is true (as the old saying goes) that "oil and water don't mix."

In water, hydrogen and oxygen have significantly different levels of electronegativity, or the relative ability of an atom to attract valence electrons, which are used in chemical bonding. Therefore, a water molecule tends to be electrically bipolar, with all the negative charges on the end or side where the oxygen atom is located and all the positive charges on the end or side of the hydrogen molecules. In petroleum and most other oily substances, however, the molecules typically include some combination of carbon and hydrogen, which have similar electronegativity values. For that reason, the electric charges are distributed more or less evenly throughout the molecule. As a result, oily substances form very loose intermolecular bonds, and they tend not to bond with water-based substances.

As for taste buds and olfactory membranes, it is fitting that the taste buds would be more receptive to water-based foods and liquids, since most foods (including jalapeño peppers) contain at least some water. Obviously, if we can taste spicy-hot substances, then there must be some receptivity to oil-based foods. Part of this receptivity involves the olfactory membranes, which, as we have noted, are more receptive to oily materials.

Animals and Chemoreception

Just as taste and smell are sharply distinguished in humans (despite the fact that smell aids us in the process of tasting), the same is true of most vertebrates. In the case of invertebrates, such as worms, however, there is much less differentiation between gustatory and olfactory receptors. These animals, in fact, may have only one chemical sense, with only slight differences between what scientists call distance chemoreception, more or less the same as smell in vertebrates, and contact chemoreception, which corresponds to vertebrates' sense of taste.

Such analogies can be made because distance chemoreceptors appear to respond to non-water-soluble substances in the same way that the olfactory receptors in vertebrates do, while contact chemoreceptors are more responsive to water-soluble chemicals. As with many other animals, these senses are linked to numerous behaviors—not just feeding—in invertebrates. Distance chemoreception enables invertebrates to sense the presence of chemicals that pose a danger, signaling the need to move away, while contact chemoreception assists the invertebrate in determining when to mate and when to lay eggs.

THE MANY FUNCTIONS OF CHEMORECEPTION.

Terrestrial, or land-based, animals whose skins secrete mucus (e.g., snails and slugs) as well as aquatic animals have what scientists call the common chemical sense, which makes them sensitive to the presence of foreign chemicals anywhere on the surface of their bodies. Even humans and other animals whose skin does not secrete mucus across its entire surface have an evolutionary remnant of this sense. Thus, the mucous membranes in the eyes, mouth, nose, and genitals respond to chemical irritants.

This is yet another example of the fact that, while chemoreception in animals is associated most readily with tasting and smelling, it is linked to myriad other functions as well. Some insects may use chemoreception to detect the presence of moisture, and many animals apply it for a variety of purposes. Such purposes include selection and courtship of a mate as well as the identification of friends or foes. A dogs' sense of smell tells it everything it needs to know about a new animal or person it encounters; similarly, a cat may identify another cat by sniffing its rectum. Smell also can be used to mark territory, which is why dogs and cats mark their "turf" by urinating. Both chemical senses, particularly smell, are important mediums of communication in the animal kingdom.

Mechanisms of Taste and Smell

Scientists in the nineteenth century believed that human tongues have receptors for four basic tastes: sweet, sour, salty, and bitter. More current research, however, shows that the taste receptors on the tongue are more complicated than was previously thought. Still, it does seem to be the case that specific kinds of taste buds are clustered in certain areas. Taste buds, so named because they look like plant buds when viewed under a microscope, cover the tongue and, to a lesser extent, can be found on the cheek, throat, and roof of the mouth. As we shall see, however, some people have a greater concentration of taste buds than do others. In the mouth, saliva breaks down the chemical components of substances, which travel through the pores in the papillae (small protuberances on the surface of the tongue) to reach the taste buds themselves.

When specific proteins in food bind to receptors on the taste buds, these receptors send messages to the cerebral cortex, a surface layer on the brain that coordinates sensory information. And though taste buds in certain regions of the tongue have an affinity for particular flavors, as we discuss later, the intricacies of the neural and chemical networks tend to suggest that nothing is clear-cut about this highly complex biological process.

OLFACTION: A DIRECT SENSE.

Before modern times, several of the leading theories of vision maintained that the eye actually interacts with the objects it sees. Now we know that our eyes simply receive light reflected off those objects. By contrast, smelling is a direct experience, because we inhale microscopic portions of substances that have evaporated and make their way into the nasal cavity, where they chemically interact with sense receptors. Cells in the nose detect odors through receptor proteins on the cell surface, which bind to odor-carrying molecules. A specific odorant docks with an olfactory receptor protein in much the same way that a key fits into a lock. This, in turn, excites the nerve cell, causing it to send a signal to the brain.

Although there are many tens of thousands of odor-carrying molecule types in the world, meaning that there are as many different smells, there are only hundreds (or at most about 1,000) different types of olfactory receptors in even the most sensitive animal species. This finding has led scientists to speculate that not every receptor recognizes a unique odorant molecule; rather, similar odorants can bind to the same receptor. Another way to put this, in light of the lock and key analogy, is that a few loose-fitting odorant "keys" of roughly similar structure can enter the same receptor "lock."

Just as we sense smells directly, the olfactory sense is also direct in the way that signals are transmitted to the brain. Vision, by contrast, puts into play several steps: a receptor cell detects light and passes the signal to a nerve cell, which passes it on to another nerve cell in the central nervous system, which then relays it to the visual center of the brain. In olfaction, on the other hand, the olfactory nerve cells perform all these functions.

In most animals these cells take scent messages directly to the nerve cells of the olfactory bulb in the brain. With insects and other invertebrates whose brains are relatively simple, functioning primarily as clearinghouses for sensations, the olfactory nerves send signals to the olfactory ganglia, a mass of nerve tissue that connects nerve cells external to the brain and spinal cord. In higher animals, such as humans, olfactory signals go to the olfactory cortex, a structure in the brain where higher functions, such as memory and emotion, are coordinated with the sense of smell. Hence, as many people have observed, the sense of smell is linked strongly with long-term memory in a way that such senses as sight and touch are not.

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 cm²) 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 of ability to taste serves to illustrate the strong link between gustation and olfaction: the taste buds themselves are working fine, but the lack of smell, resulting from congestion, hinders the brain's ability to process flavor.

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.

KEY TERMS

CHEMORECEPTION:

A physiological process whereby organisms respond to chemical stimuli.

GUSTATORY:

Of, or involving, the sense of taste (gustation ).

INVERTEBRATE:

An animal without an internal skeleton.

NERVOUS SYSTEM:

A network, found in the bodies of all vertebrates, whose purpose is to receive and interpret stimuli and to transmit impulses. Parts of the nervous system include the brain, spinal cord, and nerves.

OLFACTORY:

Of, or involving, the sense of smell (olfaction ).

PHEROMONES:

Chemical substances produced and secreted by animals, which serve as stimuli for one or more behavioral responses on the part of other animals of the same species.

RECEPTOR:

A structure in the nerv ous system that receives specific stimuli and is affected in such a way that it sends particular messages to the brain. These messages are interpreted by the brain as sensations corresponding to the stimuli.

SENSE:

The means by which an organism (usually an animal) receives sig nals regarding physical or chemical changes or both in its environment. Some times called sensory reception or sensory perception.

STIMULUS:

Any phenomenon (i.e., an observable fact or event, such as an environmental change) that directly influences the activity or growth of a living organism.

VERTEBRATE:

An animal with an internal skeleton.

Chemoreception

The detection of chemicals by smell, taste, or other means is generally known as chemoreception. A phenomenon that occurs widely in nature, chemoreception is found in the simple chemotaxis of a motile bacterium toward food as well as the more complex interpretative pathways associated with an animal's ability to smell and taste.

Chemicals detected by chemoreception are small molecules that can readily diffuse through a fluid. Airborne odor molecules must be small enough to become volatile (less than 400 molecular mass), and taste molecules are generally soluble in aqueous solutions. Detection of a specific molecule occurs on the outer surface of a receptor cell at a plasma membrane protein known as chemoreceptor. Generally, a chemoreceptor will detect just a single chemical or a small range of structurally similar chemicals. Detection elicits a series of biochemical changes inside the receptor cell. The response of the receptor cell may signal yet other cells in multicellular organisms such as animals or cause a response unique to the sensing cell.

Chemotaxis

Chemoreception that leads to movement of a cell or organism is known as chemotaxis. For example, a simple motile bacterium responds by swimming toward food molecules released by a decomposing organism. In this example of chemoattraction, the bacterium follows an increasing concentration of food molecules to their source. Conversely, the same bacterium might swim away from repellant chemicals released from an unfavorable environment.

In other cases a cell's movement may simply be a change in its shape. For example, in the mating of the single-celled yeast, the nonmotile yeast cells of opposite mating types respond to their mutual detection of complementary pheromones by elongating toward each other. Chemotaxis also plays an important role in chemically directed movement of cells during mammalian development and in human immune cells as they migrate toward invading bacteria near the site of an infected wound.

Smell

The ability to smell odor molecules is known as olfaction. Scientists have estimated that humans can sense over ten thousand different types of smells and that their detection can influence mood, memory, emotions, mate choices, and the immune and endocrine systems.

Olfaction begins with the extremely sensitive detection of odor molecules by one or more of the 12 million receptor cells that line the nasal cavity. Covering this olfactory epithelium is a mucous secretion that may hold some of the more hydrophobic odors to be detected. The chemoreceptors of the olfactory receptor cells are located on nonmotile cilia projecting into the mucous layer. At the other end of these receptor cells are axons that project up through the skull and terminate in the two olfactory bulbs near the front and base of the cerebral hemispheres. From there, olfactory nerve tracts project to the limbic system (an ancient region of the brain concerned with motivation, emotion, and certain kinds of memory) and to the thalamus and then to the frontal cortex for recognition of the odor.

Taste

Gustation is the sense of taste. The tongue is the major organ associated with gustation, and it is the tongue's taste buds that sense the majority of chemicals dissolved in saliva. During gustation receptor cells in taste buds and elsewhere respond and make synaptic contact with cranial nerves VII (serving the anterior two-thirds of the tongue), IX (serving the posterior one-third of the tongue), or X (serving parts other than the tongue, for example, the epiglottis and pharynx ). From there cranial nerve fibers project to the thalamus and then the primary gustatory cortex in the lower parietal lobe of the cerebrum. It is at this final location that the taste is recognized. Commonly recognized tastes are salty, sweet, bitter, sour, and umami (a meaty taste). The flavor of food and drink, often associated solely with taste, is actually a combination of the gustatory and olfactory processes.

see also Brain; Immune Response; Signaling and Signal Transduction

Michael L. Gleason

Bibliography

Finger, Thomas E., Wayne L. Silver, and Diego Restrepo. The Neurobiology of Taste and Smell, 2nd ed. New York: John Wiley & Sons, 2000.

Gustation: The Sense of Taste. North Carolina State University Online. <http://www.csa.com/crw/web/web-taste.html>.

Olfaction: The Sense of Smell. North Carolina State University Online. <http://www.csa.com/crw/web/web-smol.html>.

Smith, David V., and Robert F. Margolskee. "Making Sense of Taste." Scientific American 284, no. 3 (March 2001): 32–39.

Chemoreception

Chemoreception is the biological recognition of chemical stimuli, by which living organisms collect information about the chemistry of their internal and external environments. Chemoreception has three sequential stages: detection, amplification, and signaling.

In detection, a molecule typically binds to a chemoreceptor protein on the surface of a cell, changing the shape of the chemoreceptor. All chemoreceptors therefore have some degree of specificity, in that they only bind to specific molecules or specific classes of molecules.

In amplification, the cell uses energy to transform the shape change of the chemoreceptor into biochemical or electrical signals within the cell. In many cases, amplification is mediated by formation of cAMP (cyclic adenosine monophosphate), which increases the cell’s permeability to sodium ions and alters the electrical potential of the cell membrane.

In signaling, the amplified signal is transformed into a physiological or behavioral response. In higher animals the nervous system does the signaling, while in single-celled organisms signaling is intracellular, which may be expressed as chemotaxis (a directional movement in response to a chemical stimulus).

Detection, amplification, and signaling are often connected by feedback pathways. Feedback pathways allow adjustment of the sensitivity of the chemoreceptive system to different concentration ranges of the elicitor molecule. Thus, the sensitivity decreases as the background concentration of the molecule increases; the sensitivity increases as the background concentration of the molecule decreases.

Chemoreceptive systems detect chemical changes within an organism (interoreception) or outside an

organism (exteroreception). The most familiar examples of exteroreception in humans are the senses of taste and smell.

Humans have chemoreceptor cells for taste in taste buds, most of which are on the upper surfaces of the tongue. Each human has about 10, 000 taste buds and each taste bud consists of about 50 cells. An individual taste bud is specialized for detection of a sweet, sour, salty, or bitter taste. The sense of smell is important in discriminating among more subtle differences in taste.

Human chemoreceptors in the nasal cavity can discriminate thousands of different odors. One theory of odor perception in humans proposes that each chemoreceptive cell is connected to a single neuron and that an odorant molecule binds to many different chemoreceptors with different affinities. Thus, the neural signals from many different neurons can be integrated in many different ways to yield a rich panoply of odor sensations.

Many chemoreception systems also collect information about the internal environment of multicellular organisms. For example, the carotid body in the carotid artery of humans has chemoreceptive cells that respond to changes in the pH and oxygen levels in the blood. As the amount of dissolved oxygen in the blood decreases, chemoreceptive cells in the carotid body emit an electrical discharge, which stimulates specific neurons in the hindbrain respiratory centers to increase the rate of breathing. The hypothalamus in the human brain has chemoreceptive cells that respond to changes in blood glucose levels. When blood glucose levels fall, the chemoreceptive system causes a person to feel hungry; when blood glucose levels rise, this chemoreceptive system causes a person to feel satiated. The endocrine and nervous systems also have many other chemoreceptive cells that signal different organs within the body to change their activity.

Chemoreception

Chemoreception is the biological recognition of chemical stimuli, by which living organisms collect information about the chemistry of their internal and external environments. Chemoreception has three sequential stages: detection, amplification, and signaling.

In detection, a molecule typically binds to a chemoreceptor protein on the surface of a cell , changing the shape of the chemoreceptor. All chemoreceptors therefore have some degree of specificity, in that they only bind to specific molecules or specific classes of molecules.

In amplification, the cell uses energy to transform the shape change of the chemoreceptor into biochemical or electrical signals within the cell. In many cases, amplification is mediated by formation of cAMP (cyclic adenosine monophosphate), which increases the cell's permeability to sodium ions and alters the electrical potential of the cell membrane .

In signaling, the amplified signal is transformed into a physiological or behavioral response. In higher animals the nervous system does the signaling, while in single-celled organisms signaling is intracellular, which may be manifested as chemotaxis, a directional movement in response to a chemical stimulus .

Detection, amplification, and signaling are often connected by feedback pathways. Feedback pathways allow adjustment of the sensitivity of the chemoreceptive system to different concentration ranges of the elicitor molecule. Thus, the sensitivity decreases as the background concentration of the molecule increases; the sensitivity increases as the background concentration of the molecule decreases.

Chemoreceptive systems detect chemical changes within an organism (interoreception) or outside an organism (exteroreception). The most familiar examples of exteroreception in humans are the senses of taste and smell .

Humans have chemoreceptor cells for taste in taste buds, most of which are on the upper surfaces of the tongue. Each human has about 10,000 taste buds and each taste bud consists of about 50 cells. An individual taste bud is specialized for detection of a sweet, sour, salty, or bitter taste. The sense of smell is important in discriminating among more subtle differences in taste.

Human chemoreceptors in the nasal cavity can discriminate thousands of different odors. One theory of odor perception in humans proposes that each chemoreceptive cell is connected to a single neuron and that an odorant molecule binds to many different chemoreceptors with different affinities. Thus, the neural signals from many different neurons can be integrated in many different ways to yield a rich panoply of odor sensations.

Many chemoreception systems also collect information about the internal environment of multicellular organisms. For example, the carotid body in the carotid artery of humans has chemoreceptive cells which respond to changes in the pH and oxygen levels in the blood . As the amount of dissolved oxygen in the blood decreases, chemoreceptive cells in the carotid body emit an electrical discharge, which stimulates specific neurons in the hind brain respiratory centers to increase the rate of breathing. The hypothalamus in the human brain has chemoreceptive cells which respond to changes in blood glucose levels. When blood glucose levels fall, the chemoreceptive system causes a person to feel hungry; when blood glucose levels rise, this chemoreceptive system causes a person to feel satiated. The endocrine and nervous systems also have many other chemoreceptive cells which signal different organs within the body to change their activity.