Taste and Smell
Taste and Smell
TASTE AND SMELL
The sense of taste influences food preferences and food choices. When people describe how food tastes, they are actually talking about food flavor, and not just the basic tastes of sweet, sour, salty and bitter. The range of flavor experiences also includes aroma, texture, and mouth ‘‘feel’’—and, some would say, even the pleasantness of foods.
Much of food flavor is perceived through taste and smell. The four taste qualities are perceived through receptors located on the tongue and elsewhere in the oral cavity. Some scientists count the taste of monosodium glutamate (MSG) as a fifth basic taste quality known as umami. Mouths also detect the texture and temperature of foods, and the chemical burn of chili peppers and other irritants. Other components of flavor (such as the aroma of vanilla or orange) are airborne particles that come directly through the nasal passages, or enter the nose from the back of the mouth through what is known as retronasal transport.
Scientists have always assumed that taste and smell would undergo dramatic declines with age, much as vision and hearing do. However, it now appears that the chemical senses are much more robust in older people than are vision, hearing, and even touch. Generally, age-related deficits in the sense of smell are more dramatic than taste deficits. These losses do not occur suddenly at any given age. Instead, a progressive decline begins by thirty or forty years of age and continues gradually in later life. Because of the very slow progression of sensory losses, an older person may not even be aware that a decline in taste or smell acuity has occurred.
Aging and the olfactory system
Odors are detected through some fifty million olfactory receptor cells. These cells die and are replaced every sixty days. They are located in the mucus membrane located at the top of the nasal cavities, each of which occupies about 1 square inch. Sniffing concentrates the odors, since less than 10 percent of the air entering the nasal cavity reaches the olfactory epithelium. Odor molecules must dissolve into the mucus membrane surrounding the olfactory receptors, which lines a piece of porous bone called the cribiform plate. Olfactory receptors send axons though the cribiform plate to the olfactory bulb, which lies at the base of the brain. Studies of olfactory epithelium indicate that it becomes scarred and abnormal with age. The scar tissue may block the pores in the cribiform plate, thereby preventing the olfactory receptors from sending axons to the olfactory bulb.
Older people need a higher concentration of a given substance in the air to detect a smell than do younger people. This rise in threshold may reduce the ability to react to the presence of harmful chemicals in the environment and thus may pose a danger to older adults. A study comparing the ability to detect ethyl mercaptan, an ingredient in liquid petroleum gas, showed that adults age seventy-four required ten times more of the chemical to detect it than did twenty-year-olds. However, the ability to detect odors varied across older adults, with some performing as well as younger subjects.
Older people find concentrated odors less intense. A 1986 survey conducted by the National Geographic Society involved a scratch-and-sniff test using six different odors. Respondents rated their intensity on a five-point scale. Intensity ratings declined with age, more so for some odors than for others. Studies normally find a broad loss in olfactory ability for a wide range of smells from pleasant to unpleasant. A more comprehensive scratch-and-sniff test using forty different odors, showed that a person’s ability to identify odors by name starts to decline around the age of sixty. Sometimes that may be due to cognitive impairments or memory loss. The ability to detect an odor—without being able to name it—may remain unchanged.
Losing the ability to smell may affect the flavor and the enjoyment of food. One study compared the ability of young and older subjects to detect the herb marjoram in carrot soup. Older adults performed less well than younger subjects, and some were unable to detect marjoram at all. The deficit was related to smell as opposed to taste. When tested using nose clips, young subjects also were unable to detect or identify marjoram in the soup. Middle-aged and older adults were also less able to detect a woody alcohol odor, and sometimes failed to recognize such common odors as baby powder, chocolate, cinnamon, coffee, and mothballs.
Smell and disease
Smell losses with age are most often linked to upper respiratory infections, and may represent accumulated damage over time. Head trauma can cause sudden loss of the ability to smell. This may occur in whiplash-like accidents that cause a tearing of olfactory filaments near the cribiform plate. More rarely, head trauma may damage nerves involved in taste perception.
Alzheimer’s disease is associated with a reduced sense of smell. This may result from a loss of nerve cells in the olfactory bulb that is sometimes observed prior to the onset of clinical dementia. Smell losses may therefore help in the early diagnosis of Alzheimer’s disease. Patients with Parkinson’s disease are also less able to perceive and identify odors.
Reduced odor perception through the back of the mouth has been observed in older subjects, even though odor perception through the nose is normal. This may be due to dentures that cover the roof of the mouth. Dentures may block the retronasal transport of odors from the mouth to the olfactory receptors, producing a decrease in the perceived flavor of food.
Aging and taste
Older people lose the ability to detect very low concentrations of bitter and salty substances. In contrast, the perception of sweet and sour is robust even in extreme old age. Not all bitter compounds are affected equally, however.
Age-related taste deficits are most pronounced when testing is localized to specific areas on the tongue. Instead of whole mouth tasting, the stimulus is applied locally to spots on the tongue by using a special apparatus or by applying the tastant with a cotton swab. Scientists believe that whole mouth perception may compensate for some of the regional losses of taste function with age.
Regional losses in taste might be expected, given the anatomy of the taste system. Tastes are detected in the mouth by specialized receptor cells located in the upper part of taste buds and near the taste pore. Taste buds are located not only on the tongue but also in the throat and on the roof of the mouth (soft palate). Taste buds are made up of thirty to fifty individual cells, which are organized into an oblong sphere, much like the segments of an orange. The entire taste bud is regenerated every two weeks. Individual taste cells live for only ten to twelve days, and new cells below them evolve to replace them as they die.
Taste buds in the front of the tongue are located in tiny, mushroom-shaped pieces of tissue known as fungiform papillae. These appear as small, round, pink bumps on the tongue surface. In humans each fungiform papilla contains a number of taste buds, each one opening to the outside through a tiny taste pore. Some people may have twenty-five or more taste buds per papilla. There are also regional differences in taste bud density. Fungiform papillae on the tip of the tongue have more taste buds than do those in the middle region of the tongue. A branch of the facial nerve innervates the fungiform papillae, and carries information about taste on the front of the tongue to the brain.
Papillae in the rear of the tongue, known as circumvallate papillae, are arranged in an inverted V shape and are level with the surface of the tongue. Information about taste in the rear of the tongue is carried to the brain by the glossopharyngeal nerve.
More taste buds are located in tissue folds on the sides of the tongue, just in front of the circumvallate papillae. These foliate papillae are mostly innervated by the glossopharyngeal nerve, but some appear to be innervated by the same nerve as papillae at the front of the tongue. Taste buds in the soft palate are innervated by a branch of the facial nerve, and taste buds in the throat are innervated by the glossopharyngeal nerve and the vagus nerve.
Because taste buds are found all over the oral cavity, the perceived taste of food appears to come from the entire mouth and not from isolated patches on the tongue, throat, and roof of the mouth. It is the sense of touch that serves to localize taste perception, such that the taste is perceived to be coming from the area that is stimulated by touch. Smelling chocolate odor while chewing on tasteless gum is interpreted by the brain as eating chocolate.
Taste and disease
It has always been assumed that the number of taste buds declines with age but that does not appear to be the case. There is no evidence that taste cells are no longer regenerated or that the structure of taste buds changes in healthy older adults. However, both disease and medication use may affect this process. For example, drugs that interfere with cell division or growth, such as cancer chemotherapy agents, can disrupt both taste and smell.
Some, if not all, of the observed taste losses with age may be caused by a cumulative history of disease or by the chronic use of medications. Nerve damage during the extraction of wisdom teeth has been found to reduce taste pore density (and presumably taste bud density) in the front of the tongue. Surgery to repair the nerve increases taste pore density and partially restores taste function.
Ear infections can also cause nerve damage. A history of middle ear infections has been associated with taste abnormalities. Damage from bacterial or viral infection may result in an enhanced perception of bitter taste or in taste phantoms. Taste phantoms are a taste sensation in the absence of a stimulus. Scientists believe that taste nerves mutually inhibit each other. A lessening of nerve input from one area of the tongue may cause another area to take over and thus maintain the level of taste sensation. This may explain why older people often do not perceive a loss in taste, even when damage to taste nerves is readily apparent to a medical professional.
The burning mouth syndrome
Some age-related taste abnormalities lead to clinical complaints. Patients with burning mouth syndrome report a persistent burning sensation, usually localized to the front of the tongue, as well as distorted and persistent bad tastes. This condition occurs most commonly in postmenopausal women. Patients with burning mouth syndrome are less sensitive to threshold concentrations of sweet and sour, and tend to rate high concentrations of sugar and salt as less intense than do healthy controls. Burning mouth syndrome is one of the very few conditions that affect taste response to sweet. Both taste and pain pathways may contribute to burning mouth syndrome. One suggested cause has been dry mouth associated with hormonal changes at menopause.
Taste perception is dramatically influenced by salivary function. Taste cells require the presence of gustin, a compound in saliva, in order to develop normally. One component of gustin is zinc. Therefore low dietary levels of zinc, disruption of salivation, or drugs that bind zinc and prevent its use by the body may contribute to taste loss. Patients with disruption in zinc metabolism often experience loss of both taste and smell. Circumvallate papillae of such patients show a disruption of taste buds and taste pores, with obvious cell death. Both taste and smell have been restored in some patients by oral zinc supplementation.
Taste and medication use
Medications may cause a pronounced taste loss or taste distortion. Many drugs prescribed for older adults, such as angiotensin converting enzyme (ACE) inhibitors, used in the treatment of high blood pressure, can bind to zinc. An unpleasant taste in the mouth is one of the main factors limiting use of the ACE-inhibitor captopril. Penicillamine, sometimes used to treat rheumatoid arthritis, also binds metals and interferes with taste perception. Diuretics, which reduce blood pressure by increasing urine output, have occasionally been associated with a loss of taste. Among the known side effects of antiglaucoma agents are changes in taste. Nicotine patches may interfere with taste perception, perhaps because they reduce salivary flow. Radiation treatment for oral cancer frequently directly damages salivary glands and thus can reduce taste function.
Diet and nutrition
There is no dramatic decline in taste or smell function in healthy aging. Rather, a subtle and gradual decline in sensitivity begins around middle age and continues in later life. Smell losses hinder the ability to identify familiar odors and reduce the perceived intensity of flavors in food. Taste losses, if present, are often localized and whole mouth taste sensation is often not affected.
The impact of such losses on diet, nutrition, and health has not always been clear. Some researchers have argued that irreversible changes in taste and smell lead directly to altered food preferences, reduced food consumption, and poor nutrition. However, very few studies have controlled for the subjects’ health status. As noted above, sensory deficits may result not from advanced age but from ill health. Moreover, very few studies have examined taste or smell function, dietary intakes, and nutritional status in the same persons.
For example, it is unclear whether reduced perception of saltiness leads to increased salt intakes among older adults. One study found that older subjects needed twice as much salt to detect its presence in tomato juice as did young subjects. More than half of middle-aged subjects and older subjects (age sixty-seven to eighty-nine) failed to detect the presence of salt in soup. However, there is no clear relationship between the detection of saltiness and preference for salt. Some studies on salt taste preferences reported that older subjects preferred saltier mashed potatoes and chicken broth than did young people, while others found no age-related increase in preference for salt in soup or in tomato juice. These studies did not speak to the key question of whether changes in salt taste perception affect intake of saltier foods on a regular basis.
Findings that older subjects sometimes prefer higher concentrations of sugar and salt in both water and other beverages were interpreted to mean that they would select sweeter and saltier foods. However, food consumption was not actually measured. Very few studies measured salt taste perception, salt taste preference and actual salt consumption among the same respondents. In one such study, young and older adults did not differ in their saltiness intensity ratings for chicken broth. Moreover, older adults generally preferred lower salt concentrations in chicken broth. The hedonic response to salt in soup was not related to daily sodium intakes as assessed by fifteen days of diet records. Sodium intakes (expressed as mg per 1,000 kcal) did not increase with age.
The ability to perceive bitter taste also declines with age. Age was also the strongest predictor of food preferences. Older women expressed increased liking (or reduced dislike) for cruciferous vegetables and bitter salad greens. The reduced response to bitter may increase the acceptance of some bitter foods by women.
The age-associated decline in the sense of smell is also thought to have nutritional consequences. Some scientists believe that olfactory deficits reduce the pleasantness of foods and are the direct cause of reduced food intake and malnutrition in the elderly. A study of smell in eighty older women (sixty-five to ninety-three years of age) showed that half had severe olfactory dysfunction. The dysfunction did not affect appetite, and was unrelated to total energy intake, body weight, or the body mass index (BMI). Body mass index, weight divided by the square of height (1kg/m2), is a measure of body mass. However, women with smell losses reported a lower interest in food-related activities (e.g., enjoying cooking, eating a wide variety of foods); lower preference for foods with sour/bitter taste, such as citrus fruits; higher intake of sweets; and higher intake of fats. Smell losses may lead people to select foods that are sweet or rich in fat, such that the taste and texture will contribute to sensory appeal.
Amplifying foods with noncaloric flavors is thought to be a promising approach in promoting energy intake by older persons and reversing age-related anorexia. In one such study, 75 percent of elderly subjects preferred foods that had been amplified or enhanced with added flavor. In some cases, carrots were amplified with carrot flavor; in other cases, peas were enhanced with bacon flavor or cauliflower with a cheese flavor. The results showed that flavor enhancement, as opposed to a simple flavor amplification, was the more effective method. The most effective enhancers of food intake among older persons were synthetic meat flavors: bacon, roast beef, and ham.
Data from nutritional surveys suggest that dietary variety, defined as the number of different foods consumed each day, often declines with age. This effect was not observed among healthy older people of means, but is reported to be common among institutionalized persons. Researchers believe that the mechanism of sensory-specific satiety encourages the consumption of a varied diet. Sensory specific satiety reduces preference for the just-consumed foods and foods much like them. In contrast, preferences for new foods are maintained at higher levels. In laboratory studies, sensory-specific satiety diminished with age and was lowest in persons over sixty-five. Though the reason for this reduction was unclear, it was unlikely to be caused by any deficits in sensory function.
Consumer studies show that food choices are very largely determined by how foods taste. However, economic factors and health concerns also play a role. Demographic and psychosocial factors have a major impact on the quality of the diet of older adults. When it comes to food choices, deficits in taste and smell, if present, can be compensated for by prior learning and experience.
Adam Drewnowski Susan E. Coldwell Barbara J. Rolls
See also Hearing; Malnutrition; Vision.
Doty, R. L.; Shaman, P.; Applebaum, S. L.; Giberson, R.; Siksorski, L.; and Rosenberg, L. ‘‘Smell Identification Ability: Changes with Age.’’ Science 226 (1984): 1441–1443.
Drewnowski, A.; Henderson, S. A.; Driscoll, A.; and Rolls, B. J. ‘‘Salt Taste Perceptions and Preferences are Unrelated to Sodium Consumption in Healthy Older Adults.’’ Journal of the American Dietetic Association 96 (1996): 471–474.
Drewnowski, A.; Henderson, S. A.; Driscoll, A.; and Rolls, B. J. ‘‘The Dietary Variety Score: Assessing Diet Quality in Healthy Young and Older Adults.’’ Journal of the American Dietetic Association 97 (1997): 266–271.
Drewnowski, A., and Warren-Mears, V. ‘‘Chemical Senses and Food Choices in Aging.’’ In Handbook of Nutrition in the Aged. Edited by R. R. Watson. Boca Raton, Fla.: CRC Press, 2000. Pages 237–250.
Duffy, V. B.; Backstrand, J. R.; and Ferris, A. M. ‘‘Olfactory Dysfunction and Related Nutritional Risk in Free-Living, Elderly Women.’’ Journal of the American Dietetic Association 95 (1995): 879–884.
Rolls, B. J. ‘‘Appetite, Hunger and Satiety in the Elderly.’’ Critical Reviews in Food Science and Nutrition 33 (1993): 39–44.
Rolls, B. J. ‘‘Do Chemosensory Changes Influence Food Intake in the Elderly?’’ Physiology and Behavior 66 (1999):193–197.
Rolls, B. J., and Drewnowski, A. ‘‘Diet and Nutrition.’’ In Encyclopedia of Gerontology. Edited by J. E. Birren. San Diego: Academic Press, Inc. 1996. Pages 429–440.
Schiffman, S. S., and Warwick, Z. S. ‘‘Effect of Flavor Enhancement of Foods for the Elderly on Nutritional Status: Food Intake, Biochemical Indices, and Anthropometric Measures.’’ Physiology and Behavior 53 (1993): 395–402.
taste and smell
The taste/smell system fulfils two separate physiological roles. Not only does it help us to identify ‘good’ food, containing essential nutrients (salts, carbohydrates, proteins and fats), but it also provides a warning of the unsuitability of harmful and potentially toxic substances by detecting them before they are ingested.
Gustatory receptorsThe receptors involved in gustation are found in specialized ‘end-organs’ called taste buds, embedded in the epithelium that covers the surface of the tongue, soft palate, pharynx, larynx and epiglottis. However, they are not uniformly distributed in these regions. The taste buds on the tongue are associated with characteristic ‘papillae’ (from the Latin for pimples), whereas those in the other regions are found on the smooth epithelial surface. In humans, the number of taste buds varies considerably from person to person, with the majority having 2000 to 5000, distributed over the various regions. However, the number can be as low as 500 and as high as 20 000 in some individuals.
The papillae in different regions of the tongue have distinctive shapes and characteristic numbers of taste buds associated with them. Scattered over the main body of the tongue are approximately 200, small, mushroom-shaped (fungiform) papillae, which have, on average, only three taste buds each. Larger (foliate) papillae are found at the back and sides of the tongue. They consist of up to nine folds of epithelium and have as many as 600 taste buds each. Eight to 12 larger mushroom shaped (circumvallate) papillae, each surrounded by a circular trough, lie at the back of the tongue in a V-shaped formation; these have on average 250 taste buds each. Scattered taste buds are also found in the epithelium of the soft palate, pharynx, larynx and epiglottis.
Each taste bud is contacted, at its base, by the terminals of sensory nerve fibres. These taste fibres belong to three different cranial nerves, connected to the brain. The nerve supply for most of the taste buds on the soft palate and towards the front of the tongue come from a division of the VIIth (facial) cranial nerve, called the chorda tympani, because its route to the brainstem passes close to the tympanic membrane in the ear. The IXth (glossopharyngeal) and Xth (vagus) nerves innervate taste buds in the back of the mouth and the pharynx respectively.
Each taste bud contains 50–150 neuroepithelial receptor cells arranged, like segments of an orange, to form a compact, pear-shaped structure, about 70 μm high and 40 μm in diameter. There is a small 2–10 μm opening in the epithelial surface called the taste pore, which allows direct contact between chemicals dissolved in the saliva and the tips of the receptor cells. These exposed parts of the receptor cells are made up of many long corrugated folds in the membrane called microvilli, which provide a greater surface area for contact with the saliva. It is difficult to taste food with a dry mouth. Saliva is essential for normal taste because it acts as both a solvent for the chemicals as well as a transport medium for those chemicals to reach the receptors. A layer of saliva extends into the taste pores and constantly bathes the receptors. The dissolved chemicals diffuse through this thin layer of saliva to reach the microvilli. Reflex secretion of saliva from the salivary glands under the tongue and in the cheeks is stimulated by chewing, taste and smell, to varying degrees. And, as Ivan Pavlov demonstrated in his classical experiments on dogs, the simple form of unconscious learning known as conditioning couples the reflex secretion of saliva to the familiar signs of an impending meal – the sound of a dinner bell, the clatter of crockery, the sight of the food.
The taste bud complex is a dynamic system in which the receptor cells are rapidly turning over. The life-span of an individual receptor cell is about 10 days: cells are continually being born (through the division of epithelial stem cells within the bud), maturing, performing their gustatory function and eventually dying. Even though the receptor cell does not itself have an axon or fibre, the base of the cell has specialized regions that look like the terminals of nerve fibres. The cytoplasm in these regions is packed with tiny spherical vesicles, filled with a chemical transmitter substance, which is released when the potential inside the receptor cell becomes more positive (depolarization). In close association with these regions are the endings of the sensory nerve fibres, making an assembly like a synapse. Each taste bud is innervated by more than one nerve fibre and each single nerve fibre can connect to a number of receptor cells, taste buds and even papillae. This suggests a high degree of convergence of input from taste buds on to the sensory nerve fibres. Because of the rapid turnover of receptor cells, the connections between cells and nerve fibres is constantly changing. The nerves are continuously sprouting new processes, forming new synapses with young cells and retracting synaptic connections with dying cells. At any one time less than a third of the cells in the taste bud are innervated.
An intact nerve supply is necessary for the normal function of taste buds. If the nerves are damaged the taste buds degenerate and slough off, and following regeneration of the nerves, the taste buds reappear.
Since the time of Aristotle (384–322 bc) there have been attempts to categorize taste into primary or basic tastes. Although many hundreds of different chemicals can stimulate activity in taste receptor cells, the four basic taste qualities of salt, sour, sweet and bitter have stood the test of time. However, there is still controversy as to whether combination of these four primary tastes adequately describes all gustatory experiences. Metallic and astringent tastes have, in the past, been suggested as primaries, and more recently Japanese researchers have proposed that the characteristic taste of monosodium glutamate (used as a taste enhancer by the food industry) is also a basic taste, with its own receptive mechanism. They have called it “umami” meaning “delicious taste”
. Because of the dual role of gustatory receptor cells, detecting both nutrients and toxins, they must be able to respond, either individually or collectively, to a wide variety of chemicals. These chemicals range from simple ions such as sodium (salt) and hydrogen (sour) to the more complex compounds that give the sensations of sweet (e.g. sugar) and bitter (e.g. quinine). The mechanisms by which the chemical stimuli are translated into electrical events in the receptor cell (transduction) are numerous, varied and complex. The essential process depends on specific interactions between taste substances and specialized protein receptor molecules embedded in the membrane of the receptor cell, which trigger a series of chemical reactions, leading to a change in the flow of ions through pores in the membrane, and hence a change in the electrical potential inside the cell. However, there does not appear to be a unique mechanism for each of the basic tastes: each seems to use several different mechanisms. There may even be similarities of mechanism for different basic tastes. The wayin which we can perceive many subtle tastes and distinguish between different compounds of the same basic taste category might be explained by the multiplicity and specificity of these mechanisms.
The evidence for a particular receptor mechanism is best for sweet sensation. First, certain drugs have specific effects on the detection of sweetness. For instance, after eating a West African fruit called miracle fruit, even quite acidic substances (such as lemon juice), which would normally be sour, taste extraordinarily sweet. Miracle fruit contains a substance that is thought to attach to the protein receptor molecules that detect sweet-tasting substances. A subsequent increase in acidity in the saliva is thought to alter the binding of this substance with the sweet receptor protein such that it stimulates the receptor, like a genuine sweet substance. In contrast, gymnemic acid, found in an Indian plant, Gymnema sylvestre, blocks the sweet receptor in some manner, and abolishes the sensation of sweetness for half an hour or so. Very recently, a gene called T1r2 has been identified in mice, which is selectively switched on in taste bud receptor cells. It turns out that strains of mice that lack sweet taste (they don't prefer sweet food to non-sweet) have a mutation of this gene. There is a very similar gene in human beings.
Researchers have recorded with tiny electrodes from individual nerve fibres innervating the taste buds, in anaesthetized animals. One might have expected that each fibre would respond, with a burst of impulses, when a solution of just one of the primary taste substances was dripped on to the appropriate taste bud or buds. Such selectivity of response is, in fact, very rare. Most nerve fibres respond to two or more of the basic taste stimuli, the magnitude of the response varying from one taste substance to another. In other words, the activity of such a fibre does not provide unambiguous information to the brain about the nature of the stimulus. At some point the brain must perform a comparison between the activity in several different nerve fibres in order to decide what the taste actually is.
The signals from the taste buds are relayed, via a chain of nerve cells and fibres, at various cell stations in the brainstem and thalamus, up to the cerebral cortex. Some experiments in monkeys suggest that nerve cells at higher levels in the taste pathway respond more selectively, with a larger proportion of them essentially responding to only one basic taste. At the first relay in the brainstem almost no neurons respond to one taste, yet in the taste area of the cortex, about 75% of neurons respond to a single taste.
The ‘common chemical sense’ is the sensation caused by the stimulation of free nerve endings by potentially harmful chemicals. The evidence suggests that the free nerve endings are ‘polymodal’ nociceptors (receptive nerve endings that respond to mechanical, thermal and noxious stimulation). Amongst the chemicals that are known to stimulate these receptors, besides noxious, damaging chemicals, are alcohol, menthol, peppermint and capsaicin (chilli pepper).
Olfactory receptorsThe human olfactory organ, the olfactory epithelium or mucosa, is a sheet of cells 100 –200μm thick, situated high in the back of the nose cavity and on the thin bony partition (the central septum) of the nasal passage. The olfactory system responds to airborne, volatile molecules that gain access to the olfactory epithelium with the in-and-out airflow through and behind the nose. The odour molecules are distributed over the receptor sheet in an irregular pattern by the turbulence of the airflow set up by the turbinate bones in the side walls of the nose. The molecules diffuse through the surface layer of mucus and stimulate the olfactory receptors. Hydrophilic (water-soluble) molecules dissolve readily in the mucus, but the diffusion of less soluble molecules is assisted by ‘odour binding proteins’ in the mucus. These odour binding proteins are also thought to assist in removing odour molecules from the receptor cells. The mucus layer moves across the surface of the olfactory mucosa at 10 to 60 mm per minute toward the nasopharynx (the continuous of the nasal cavity backwards and downwards to link to the pharynx. This flow of mucus (which is increased and becomes more watery in such conditions as infection of the nasal cavity and hay fever), also assists in the removal of odours after they have been sensed.
The olfactory epithelium contains specialized, elongated nerve cells (olfactory receptors). These cells have very thin fibres that run upwards in bundles through perforations in the skull (the cribriform plate) above the roof of the nasal cavity, below the frontal lobes of the brain. These bundles of nerves constitute the Ist cranial nerve, the olfactory nerve. They extend only a very short distance, ending in the olfactory bulbs, which are a pair of swellings underneath the frontal lobes.
The other end of each olfactory receptor, pointing down into the nasal cavity, is extended into a long process, ending in a knob carrying several hairs (cilia) between 20 and 200 μm in length. These cilia are bathed in a thin (35 μm-thick) layer of mucus, secreted by specialized cells in the olfactory epithelium, in which the molecules of odorous substances dissolve. In the membrane of the cilia are olfactory receptor proteins, which interact with the smelly molecules, and initiate a cascade reaction inside the cell that leads to a change in the rate of impulses (action potentials) passing along the nerve fibre.
Human beings are able to distinguish 10 000 or more different odours. There have been valiant attempts to classify these into a smaller number (usually 10–20) basic or primary smells, comparable to the four or so primary tastes, but no scheme is universally accepted. The human nose (not to mention that of a sniffer dog) can be incredibly sensitive to very low concentrations of odorous substances. Certain male moths use similar receptor cells on their antennae to detect even single molecules of a pheromone secreted by female moths.
Individual olfactory receptor neurons fire off spontaneously at between 3 and 60 impulses per second. When stimulated with particular odours they increase their firing frequency. Each receptor cell responds, but not equally, to many different types of odour. As in the gustatory system, the successive nerve cells in the pathway become more selective, each responding to fewer odours. Interestingly, despite the poor selectivity of individual receptor cells, different regions of the olfactory sheet (consisting of hundred or thousands of receptor cells) are maximally responsive to particular odours. The overall pattern of activity in the olfactory epithelium can be mapped with electrical recording methods (electro-olfactogram) or other techniques for detecting active regions. Each distinctive odour produces its own ‘fingerprint’ of activity across the epithelium. This mapping is thought to reflect the patterns of expression (activation) of genes that make the receptor proteins in the receptor cell membranes. A huge family of odour receptor genes exists in the mouse, perhaps as many as 5% of all the genes.
The spatial coding of odour quality is transmitted to the first relay of the olfactory pathway, the olfactory bulb. There is a loose topographical projection from the receptor sheet to the bulb, where the axons form synapses with neurons called mitral cells. The olfactory bulb contains a complex network of nerve cells and is responsible for a considerable amount of sensory processing. Hence, neurons in the olfactory bulb respond with one distinct temporal pattern of impulses to one odour and different patterns to another smells. The mitral cells send their fibres into the olfactory tracts, which run backwards. Some end in the thalamus, which in turn sends fibres up to the olfactory cortex. The neurons of the olfactory cortex are still not highly specific for particular odours. Other fibres of the olfactory tract have direct connections to areas of the limbic system around the region of the hypothalamus. Since the limbic system is thought to be responsible for regulating emotions, this might explain the fact that smells can evoke strong feelings of enjoyment or aversion (the hedonic component of sensation).
Unlike other stimuli, olfactory stimuli are not very time-dependent. The effects of visual, tactile and auditory stimulation follow the stimulus immediately, whereas some olfactory stimuli, such as those left by animals when marking their territory, remain when the animal is long gone. In this way olfactory stimuli, and their behavioural and social effects, can have more lasting consequences.
The olfactory system occupies a smaller fraction of the brain in humans than in many other species, and this is part of the evidence for the commonly-held belief that people are generally inferior in their sense of smell. Studies in other animals, from insects to hamsters to monkeys, have revealed the importance of olfaction for many aspects of behaviour, especially reproduction. For example, male rhesus monkeys use smell to sense the hormonal status of females (ovulating or not), with a marked effect on their level of sexual activity. But even in humans, there is growing evidence that olfaction (mainly unconscious) is important in such functions as sexual preference, and recognition of other people.
R. W. A. Linden
See also brain; cerebral cortex; limbic system; nose; pheromones; sensation; sensory receptors; tongue.
Taste and Smell
Taste and Smell
The topics of taste and smell are frequently combined for discussion because the nature of the stimulus in each case is defined in chemical terms. Except for this general similarity, the two modalities appear to represent quite distinct phenomena, and it should not be assumed that they are slightly different manifestations of the same basic mechanism. For purposes of scientific discussion, taste refers to those sensations arising from stimulation of specialized receptors in the mouth, primarily on the tongue, while smell refers to those sensations arising from the stimulation of receptors in the upper portion of the nasal cavity. In everyday experience they are typically so intermingled that a large part of sensations commonly called taste are actually olfactory sensations arising from odorous molecules from ingested material in the mouth reaching the olfactory epithelium by way of the posterior nares . The absence of these sensations is recognized during periods of nasal congestion that result in the so-called flat taste of foods.
Among the senses, taste and smell have been relatively neglected as areas of research. Man regulates his behavior only to a small extent by differential sensory input over either of these two routes. In the case of many other animals, however, a different situation prevails. This difference is especially notable for smell, which provides detailed information for the highly specific responses of many species. Instead of being an occasionally useful but primarily affective distance sense, as in man, it serves in a capacity comparable to the other principal distance senses, vision and audition. The male gypsy moth, for instance, responds to the scent of a female one-quarter of a mile upwind. Similarly, the ability of many mammals to track their prey by means of odorous cues is well known.
Both taste and smell participate in the regulation of feeding activities of animals, including man. Although normal caged rats maintain their caloric intake in the absence of taste and smell sensations, rats with dietary deficiencies or with reduced hunger and thirst induced by brain lesions are highly dependent on these sensations for regulation of intake. Dietary deficiencies in man may result in changes in food preferences without affecting taste thresholds. Highly preferred tastes may cause us to overeat to the point of discomfort, and almost everyone has experienced the arousal of appetite by the smell of desirable food as well as the analogous loss of appetite produced by objectionable odors.
The major topics of concern in studying sensation are (1) the qualitative classes into which the sensations can be divided; (2) the quantitative sensitivity of the modality, expressed as threshold measurements; and (3) the nature of the adequate stimulus and the basic mechanism of excitation. Methods of study include both psychophysical and electrophysiological procedures. Detailed coverage and bibliographies can be found in Adey (1959), Geldard (1953, pp. 270-323), Michels et al. (1962), Pf aff mann (1959 ), Taylor (1937 ), and the bibliography published by Airkem (1952). Recent research findings are presented in the symposium edited by Zotterman (1963).
Receptors and neural transmission . The receptor cells for taste are known as taste buds and, in mammals, are distributed throughout the oral cavity, including the larynx and pharynx, with the major concentration on the papillae of the upper and lateral surfaces of the tongue. Man has about 9,000 taste buds. The life of an individual taste cell may be as short as one week, which raises some provocative questions concerning the neural organization of a continuously shifting population that provides constant patterns of information. Three cranial nerves (the seventh, ninth, and tenth) are involved in the transmission of nerve impulses from the taste cells to the brain. The fifth cranial nerve subserves the cutaneous sensations associated with ingestión of sapid substances. There is no area in the cerebral cortex uniquely associated with taste; rather, the incoming impulses are represented along with the sensory and motor mechanisms of the face area.
Taste thresholds and stimulus attributes . The primary qualities of taste are sour, sweet, salty, and bitter. Sensitivity to these qualities is distributed differentially, with the tip of the tongue most sensitive to sweet, the back to bitter, the posterior portion of the sides to sour; the salty taste is perceived equally well in all receptive areas. Table 1 presents some absolute thresholds for representatives of the different qualities.
|Table 1 — Absolute thresholds for taste|
|Substance||Per cent concentration (Approx.)||Molar concentration|
|Source: Geldard 1953, p.306.|
Changes in the temperature of a substance appear to affect its threshold, but the exact nature of this relationship for each quality is not clear at present. In general, absolute thresholds increase with age. Estimates of differential sensitivity, the percentage of change in intensity that can be just perceived, vary from 10 per cent to 100 per cent and are dependent upon such parameters as the qualitative dimension being studied, the level of intensity of stimuli, the size of the area being stimulated, the psychophysical method employed, etc. The measurements so far suggest that differential sensitivity to the salty taste is slightly greater than that to sweet and sour, and sensitivity to biter is slightly less. More intense stimuli are judged more pleasant in the case of sweet but less pleasant in the case of the other qualities. Two subjective intensity scales have been devised for taste (Geldard 1953, pp. 308-311). Specific ageusias (absences of taste) have been discovered in man; one of them, to phenyl thiocarbimide (PTC), is especially interesting to geneticists, for it is inherited as a simple Mendelian recessive and can be used as an aid in classifying ethnic groups.
The problem of how the molecules of the stimulating substance affect the receptor cell so that a nerve impulse is produced in the associated afferent neuron is still unsolved. The substance must first of all be soluble in water, i.e., saliva. The critical event might be an enzymatic mechanism. Beidler (1961) has suggested that the adsorption of the stimulus onto the surface of the taste cell may result in a change in the membrane potential of the cell such that a nervous impulse can be generated. Inasmuch as all taste receptors appear to be responsive to stimuli of more than one quality, the mechanism underlying the distinctiveness of the different qualities must, be sought in some aspect of the patterning of the afferent neural signals.
The specific stimulus characteristics that are responsible for the initiation of neural patterns that permit qualitative discrimination have not been completely identified. Stimulus substances possessing hydrogen ions taste sour, although hydrogen-ion concentration is not the only determining factor. The salty taste is associated with both the anión and cation of soluble salts. Bitter and sweet tastes occur for representatives of many chemical classes whose essential characteristics are not yet recognized.
Receptors and neural transmission . The receptors for smell are unique among the special senses, for they are simply rod-shaped endings of the olfactory nerve fibers rather than specialized cells. The nerve endings project into the epithelium of the olfactory clefts, 2.5 square centimeter areas at the upper rear of each nostril. The olfactory, or first cranial, nerve is similarly unique in that its fibers pass directly from the olfactory mucosa into the cerebral cortex, i.e., the olfactory bulb, without a thalamic relay. The cortical projection zone for smell lies on the lower surface of the frontal lobe. From there, the pathway ramifies diffusely to many other cortical and thalamic areas as a complex inherited from man’s phylogenetic ancestors, who made more use of smell than does man.
Olfactory thresholds and stimulus attributes . The adequate stimulus for olfaction must be volatile, presumably water-soluble, and possibly lipoid-soluble as well. As yet, no one has succeeded in identifying the primary smell qualities, although many ingenious suggestions have been made. The difficulties arise from the paucity of good electro-physiological data and the subjective complexity of most odors. The latest attempt at identification (Amoore et al. 1964) is based on a new hypothesis concerning the nature of the adequate stimulus and not on a direct analysis of sensory data. Amoore has proposed that the olfactory process is a stereochemical phenomenon, with all odor-producing molecules represented by seven basic shapes. He assumes, in addition, that the olfactory membrane is composed of receptor sites with configurations corresponding to the basic molecular shapes. The surface of the olfactory epithelium is known to have strong adsorptive qualities. The olfactory process, therefore, would be initiated when the molecular shape and the receptor site correspond to some critical degree. Amoore identifies these seven basic shapes with the odor qualities of ethereal, camphoraceous, musky, floral, pepper-minty, pungent, and putrid. Although detailed validation of this proposal remains to be provided, there is no satisfactory alternative at present.
Olfactory sensitivity varies widely with the stimulating substance, but it can be so acute as to enable detection of the odor of vanillin at a concentration of 2 x 10-7 milligram per cubic centimeter of air. Even with such a low concentration, the number of odorous molecules actually entering the nose is probably of the order of billions. What proportion reaches the olfactory receptors is difficult to state precisely because they can be reached only by variable air currents, which divert the molecules to the olfactory membrane and away from the main airway. Some olfactometric techniques have been devised that attempt to control this factor. No one has any direct estimates of stimulus concentration at the olfactory area, however. Differential sensitivity seems poor, with estimates ranging from 20 per cent to 100 per cent. A notable characteristic of olfaction is the rapidity with which adaptation occurs, so that repeated threshold measurements must be widely separated. Different degrees of anosmia (absence of smell) occur in human beings, but anosmia is not well understood. Sex hormones influence olfactory sensitivity to certain substances, such as one of the synthetic musks, which is a strong odor for women but a very weak one for men.
Bernice M. Wenzel
[Other relevant material may be found in Nervous Systemand Senses
Adey, W. R. 1959 The Sense of Smell. Volume 1, pages 535-548 in Handbook of Physiology. Section 1: Neurophysiology. Washington: American Physiological Society.
Airkem, Inc. 1952 Odors and the Sense of Smell: A Bibliography, 320 B.C.-1947. New York: Airkem.
Amoore, John E.; Johnston, James W. JR.; and Rubin, Martin 1964 The Stereochemical Theory of Odor. Scientific American 210, Feb.: 42-49.
Beidler, Lloyd M. 1961 Biophysical Approaches to Taste. American Scientist 49:421-431.
Geldard, Frank A. 1953 The Human Senses. New York: Wiley.
Michels, K. M. et al. 1962 Odor and Olfaction: A Bibliography, 1948-1960. Perceptual and Motor Skills 15:475-529.
Pfaffmann, Carl 1959 The Sense of Taste. Volume 1, pages 507-534 in Handbook of Physiology. Section 1: Neurophysiology. Washington: American Physiological Society.
Taylor, Norman B. (1937) 1961 The Senses of Taste and Smell: Common Chemical Senses. Pages 1458-1469 in Charles H. Best and Norman B. Taylor, The Physiological Basis of Medical Practice. 7th ed. Baltimore: Williams & Wilkins.
Zotterman, Yngve (editor) 1963 Olfaction and Taste: Proceedings of the First International Symposium Held at the Wenner-Gren Center, Stockholm, September, 1962. Oxford: Pergamon Press.