What is chemoreception?
The senses of taste and smell, which are closely related, depend on a type of sensory receptor cell known as a chemoreceptor. This receptor detects molecules of various kinds that respond on contact with them by generating nerve impulses. Although the basis for the detection is incompletely understood, chemoreceptor cells are believed to contain proteins in their surface membranes that are able to combine with and recognize various kinds of molecules. Combination with a recognized molecule causes the protein to open an ion channel in the surface membrane. The resulting ion flow creates an electrical change in the membrane that triggers generation of a nerve impulse by the chemoreceptor cell.
Chemoreceptors for taste occur primarily on the upper surface of the tongue. A comparatively few taste receptors are also located on the roof of the mouth, particularly on the soft palate, and in the throat. The taste receptors in these locations are parts of taste buds, which are small, pear-shaped bundles of modified epithelial cells. Molecules from the exterior environment reach the taste receptor cells through a small pore at the top of a taste bud. Altogether, there are about ten thousand taste buds on the tongue and throat. The taste buds of the tongue, which are only 30 to 40 micrometers in diameter and thus microscopic, are embedded in the surfaces of small, moundlike outgrowths called papillae. The papillae give the surface of the tongue its rough or furry texture.
Taste receptor cells occur in taste buds along with other cells that play a purely supportive structural role. Individual taste receptor cells are elongated and have thin, fingerlike extensions at their tips that protrude through the pore of a taste bud. Combination with chemicals from the environment, which must dissolve in the saliva of the mouth to reach the taste buds, probably occurs in the membranes of the fingerlike processes at the tips of the taste receptor cells. The opposite end of the taste receptor cells makes connections with sensory nerves serving the taste buds.
Each taste receptor cell likely has membrane proteins that can combine with a variety of molecules from the environment; however, individual taste cells, depending on their location on the tongue, typically combine more readily with some molecular types than with others. Taste cells with a preponderance of membrane proteins recognizing and combining with organic molecules such as carbohydrates, alcohols, and amino acids are crowded near the tip of the tongue. Combination of these taste receptors with organic molecules gives rise to nerve impulses that are interpreted in the brain as a sweet taste. Just behind the tip of the tongue is a region containing taste receptor cells that combine most readily with inorganic salts; combination with these substances gives rise to nerve impulses that are interpreted in the brain as a salty taste.
Farther to the rear of the tongue, particularly along the sides, are taste receptor cells that combine most readily with the hydrogen ions released by acids; this combination is perceived as a sour taste. The rear of the tongue contains taste receptor cells that combine most readily with a wide variety of organic and inorganic molecules, particularly long-chain organic molecules containing nitrogen and a group of organic substances called alkaloids. All the alkaloids, including molecules such as quinine, caffeine, morphine, and strychnine, give rise to a bitter taste. People tend to reject substances stimulating the bitter taste receptors at the rear of the tongue. This may have a survival value, because many bitter substances, including alkaloids produced by a variety of plants, are highly toxic. Many of the organic molecules with a bitter taste differ from those with a sweet taste by only minor chemical groups. A few substances, such as capsaicin, primarily stimulate pain rather than taste receptors when present in foods. Trigeminal nerves in the nose and mouth also contribute a generalized chemical sensitivity sometimes referred to as chemesthesis. Trigeminal response describes the fizzy tingle from carbonated beverages, the pungency of mustard or horseradish, and the irritant response to hot peppers or raw onions.
The distribution on the tongue of regions of strongest taste does not mean that the taste receptor cells in these areas are limited to detecting only sweet, salty, sour, or bitter substances; all regions of the tongue can detect molecules of each type to at least some extent. Four pairs of nerves innervate the tongue, making the sense of taste difficult to degrade substantially even during disease or the aging process.
Traditionally, the wide range of different flavors that humans can differentiate, which easily amounts to thousands, was considered to be the result of subtle combinations of four primary flavors: sweet, salty, sour, and bitter. More recently, umami was identified as a distinct taste. Often described as “savory,” umami is present in foods such as meat, cheese, and mushrooms as well as in the additive monosodium glutamate (MSG). There are indications that the picture may be even more complex than this. Other taste categories that have been proposed include metallic and astringent. An individual may be “taste-blind” for certain very specific, single molecules, such as the chemical phenylthiocarbamide (PTC). The ability to taste this substance, which has a bitter flavor, is hereditary; some persons can taste PTC, and some cannot. The pattern of inheritance suggests that a membrane protein able to combine with PTC is present in some persons and not in others. Persons taste-blind for PTC do not have the specific membrane protein and cannot respond to the presence of the chemical even though other bitter flavors can be detected. It is possible that there are a wide variety of specific membrane proteins like the one responsible for detecting PTC distributed in the surface membranes of the taste receptor cells of the tongue. On the other hand, there are also “supertasters” of PTC who are hypersensitive to the taste.
The chemoreceptors responsible for the other chemical sense, the sense of smell, are located within the head at the roof of the nasal cavity. The receptor cells detecting odors, called olfactory cells, are distributed among supportive cells in a double patch of tissue totaling about 5 square centimeters in area. Although limited to this area, the olfactory region contains between ten and one hundred million olfactory cells in the average person. Unlike taste receptor cells, olfactory receptor cells are nerve cells, often called olfactory sensory neurons.
Each olfactory cell bears between ten and twenty fine, fibrous extensions, or cilia, that protrude into a layer of mucus that covers the olfactory area. The membranes of the extensions contain the protein molecules that recognize and combine with chemicals to trigger a nerve impulse by an olfactory cell. To reach the fibrous extensions, molecules detected as odors must dissolve in the mucous solution covering the olfactory region.
The olfactory sensory neurons have a long arm, or dendrite, that extends to the surface of the nasal passage and ends in a knoblike swelling. From these knobs protrude ten to thirty fine hairs, or cilia. The cilia reach through a layer of mucus. The olfactory sensory neurons have another arm, or axon, which extends through the bony shelf of the cranium into the region of the brain called the olfactory bulb. Short axons terminate in globular structures called glomeruli. Additional neurons in the glomeruli send axons through the olfactory tract into the central nervous system.
Efforts to identify primary odors equivalent to sweet, salty, sour, bitter, and umami flavors have been largely unsuccessful. Studies of the genetic coding for the olfactory receptor proteins suggest that as many as one hundred to one thousand primary olfactory sensations exist. Similar to taste-blindness for PTC, discrete odor blindness has been identified for more than fifty substances.
The chemoreceptors responsible for the senses of taste and smell typically adapt rapidly to continued stimulation by the same molecules. In adaptation, a receptor cell generates nerve impulses most rapidly when first stimulated; with continued stimulation at the same intensity, the frequency of nerve impulses drops steadily until a baseline of a relatively few impulses per second is reached. Adaptation for the senses of taste and smell also involves complex interactions in the brain, because discernment of tastes and smells continues to diminish even after chemoreceptors reach their baselines.
For the sense of taste, adaptation is reflected in the fact that the first bite of food, for example, has the most intensely perceived taste. As stimulation by the same food continues, the intensity of the taste and a person’s perception of the flavor steadily decrease. If a second food is tasted, the initial intensity of its taste is high, but again intensity drops off with continued stimulation. If the first food is retasted, however, its flavor will again seem stronger. This effect occurs because adaptation of the receptors detecting the initial taste lessens during the period during which the second food is tasted. If sufficient time passes before the first food is retasted, the flavor will appear to be almost as strong as its first taste. For this reason, one gains greater appreciation of a meal if foods are alternated rather than eaten and finished separately.
Taste receptor cells have a life expectancy of about ten days. As they degenerate, they are constantly replaced by new taste cells that continually differentiate from tissue at the sides of taste buds. As humans reach middle age, the rate of replacement drops off, so that the total number of taste receptor cells declines steadily after the age of about forty-five. This may account for the fact that, as people get older, nothing ever seems to taste as good as it did in childhood. Smoking also decreases the sensitivity of taste receptor cells and thereby decreases a person’s appreciation and appetite for foods.
Olfactory cells also adapt rapidly to the continued presence of the same molecules; they slow or stop generating nerve impulses if the concentration of the odoriferous substances is maintained. This response is also reflected in common experience. When engaged in an odor-generating activity such as cooking or interior painting, a person is strongly aware of the odors generated by the activity only initially. After exposure for more than a few minutes, the person’s perception of the odor lessens and eventually disappears almost completely. If the person leaves the odoriferous room for a few minutes, however, allowing the olfactory receptors and brain centers to lose their adaptation, the person is usually surprised at the strength of the odor if he or she returns to the room.
The region at the top of the nasal cavity containing the olfactory cells lies outside the main stream of air entering the lungs through the mouth and nose. As a result, the molecules dissolving in the mucous layer covering the olfactory cells are carried to this region only by side eddies of the airflow through the nose. Flow to the olfactory region is greatly improved by sniffing, a response used by air-breathing vertebrates as a way to increase the turbulence in the nasal passages and thereby to intensify odors from the environment. Head colds interfere with people’s sense of smell through congestion and blockage of the nasal cavity, which impedes airflow to the olfactory region.
Although humans are not nearly as sensitive to odors as are many other animals, their ability to detect some substances by smell is still remarkable, particularly in the case of smells generated by putrefaction. Some of the mercaptans, for example, which are generated in decaying flesh, can be detected in concentrations in air as small as 0.0000000002 milligram per milliliter. One of these substances, methyl mercaptan, is mixed in low concentration in natural gas. The presence of this mercaptan allows people to detect natural gas, which otherwise would be odorless.
The idea that taste and smell receptors operate by recognizing specific molecular types is an old one, dating back to the first century BCE, when Titus Lucretius Carus proposed that the sense of smell depends on recognition of atomic shapes. Definitive experimental demonstration of this mechanism for the sense of smell, however, was not obtained until 1991, when Linda Buck and Richard Axel finally isolated members of a large family of membrane proteins that can actually do what Lucretius proposed: They bind with specific molecular types and trigger responses by olfactory cells. Axel and Buck have obtained indications that there are hundreds of different proteins in the family responsible for molecular recognition in the sense of smell.
One of the many interesting features of the family is that, as with the sense of taste, many people inherit a deficiency in one or more of the membrane proteins and are congenitally unable to detect a particular odor. There are in fact many thousands of different odors to which persons may be insensitive, which directly supports the idea that the family of membrane proteins responsible for detecting individual molecular types is very large indeed. Another interesting feature of the mechanism is that there are many odors for which people must be “educated.” People cannot recognize them on first encounter but later learn to discern them. This indicates that membrane proteins recognizing previously unknown molecules may be induced; that is, they may be newly synthesized and placed in olfactory cell membranes in response to encountering a new chemical in the environment. People can also smell, and often taste, artificial substances never before encountered by humans or indeed any other animal. Thus, the chemoreceptors have membrane proteins capable of recognizing molecules not encountered in animal evolution.
Both taste and smell receptors are linked through nerve connections to regions of the brain stem that control visceral responses, as well as to the areas of the cerebral cortex registering conscious sensations. As a result, different odors and tastes may give rise to a host of involuntary responses, such as salivation, appetite, thirst, pleasure, excitement, sexual arousal, nausea, or even vomiting, as well as to consciously perceived sensations. The odor of a once-enjoyed food may make someone ill in the future if the person became sick after eating the food; previously unobjectionable or even pleasant odors and tastes may become unpleasant and nauseating to women during pregnancy. The odor of other foods, such as some of the ranker cheeses, may be repulsive at first experience but later become appetizing as a person learns to enjoy them. The degree to which many substances are perceived as pleasant or unpleasant is also related to their concentration. Many substances perceived as pleasantly sweet in low concentration, for example, taste bitter and unpleasant at higher concentrations.
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