The Chemical Senses - Taste and Smell

The Chemical Senses—Taste and Smell The senses of taste and smell allow us to separate undesirable or even lethal foods from those that are pleasant to eat and nutritious. They also elicit physiological responses involved in the digestion and utilization of foods. The sense of smell allows animals to recognize the proximity of other animals or even individual animals. Finally, both senses are strongly tied to primitive emotional and behavioral functions of our nervous systems. In this chapter, we discuss how taste and smell stimuli are detected and how they are encoded in neural signals transmitted to the brain. SENSE OF TASTE Taste is mainly a function of the taste buds in the mouth, but it is common experience that one’s sense of smell also contributes strongly to taste perception. In addition, the texture of food, as detected by tactual senses of the mouth, and the presence of substances in the food that stimulate pain endings, such as pepper, greatly alter the taste experience. The importance of taste lies in the fact that it allows a person to select food in accord with desires and often in accord with the body tissues’ metabolic need for specific substances. PRIMARY TASTE SENSATIONS The identities of the many specific chemicals that excite different taste receptors are not all known. For practical analysis, the primary sensations of taste have been grouped into five general categories—sour, salty, sweet, bitter, and “umami.” A person can perceive hundreds of different tastes. They are all thought to be combinations of the elementary taste sensations, just as all the colors we can see are combinations of the three primary colors, as described in Chapter 51. Sour Taste. The sour taste is caused by acids—that is, by the hydrogen ion concentration—and the intensity of this taste sensation is approximately proportional to the logarithm of the hydrogen ion concentration (i.e., the more acidic the food, the stronger the sour sensation becomes). Salty Taste. The salty taste is elicited by ionized salts, mainly by the sodium ion concentration. The quality of the taste varies somewhat from one salt to another because some salts elicit other taste sensations in addition to saltiness. The cations of the salts, especially sodium cations, are mainly responsible for the salty taste, but the anions also contribute to a lesser extent. Sweet Taste. The sweet taste is not caused by any single class of chemicals. Some of the types of chemicals that cause this taste include sugars, glycols, alcohols, aldehydes, ketones, amides, esters, some amino acids, some small proteins, sulfonic acids, halogenated acids, and inorganic salts of lead and beryllium. Note specifically that most of the substances that cause a sweet taste are organic chemicals. It is especially interesting that slight changes in the chemical structure, such as the addition of a simple radical, can often change the substance from sweet to bitter. Bitter Taste. The bitter taste, like the sweet taste, is not caused by any single type of chemical agent. Here again, the substances that give the bitter taste are almost entirely organic substances. Two particular classes of substances are especially likely to cause bitter taste sensations: (1) long-chain organic substances that contain nitrogen; and (2) alkaloids. The alkaloids include many of the drugs used in medicines, such as quinine, caffeine, strychnine, and nicotine. Some substances that initially taste sweet have a bitter aftertaste. This characteristic is true of saccharin, which makes this substance objectionable to some people. High concentrations of salts may also result in a bitter taste. The bitter taste, when it occurs in high intensity, usually causes the person or animal to reject the food. This reaction is undoubtedly an important function of the bitter taste sensation because many deadly toxins found in poisonous plants are alkaloids, and virtually all these alkaloids cause an intensely bitter taste, usually followed by rejection of the food. Umami Taste. Umami, a Japanese word meaning “deli- cious,” designates a pleasant taste sensation that is qualitatively different from sour, salty, sweet, or bitter. Umami is the dominant taste of food containing l-glutamate, such as meat extracts and aging cheese. The pleasurable sensation of umami taste is thought to be important for nutrition by promoting ingestion of proteins. THRESHOLD FOR TASTE The molar threshold for stimulation of the sour taste by hydrochloric acid averages 0.0009 M, for stimulation of the salty taste by sodium chloride, 0.01 M, for the sweet taste by sucrose, 0.01 M, and for the bitter taste by quinine, 0.000008 M. Note especially that the bitter taste sense is much more sensitive than all the others, which provides an important protective function against many dangerous toxins in food. Table 54-1 lists the relative taste indices (the reciprocals of the taste thresholds) of different substances. In this table, the intensities of four of the primary sensations of taste are referred, respectively, to the intensities of the taste of hydrochloric acid, quinine, sucrose, and sodium chloride, each of which is arbitrarily chosen to have a taste index of 1. Taste Blindness. Some people are taste blind for certain substances, especially for different types of thiourea compounds. A substance used frequently by psychologists for demonstrating taste blindness is phenylthiocarbamide, for which about 15% to 30% of all people exhibit taste blindness; the exact percentage depends on the method of testing and the concentration of the substance. TASTE BUDS AND THEIR FUNCTION Figure 54-1B shows a taste bud, which has a diameter of about 1⁄30 of a millimeter and a length of about 1⁄16 of a millimeter. The taste bud is composed of epithelial cells; some are supporting cells called sustentacular cells and others are called taste cells. There are about 100 Table 54-1 taste cells in each taste bud. The taste cells are continually being replaced by mitotic division of surrounding epithelial cells, so some taste cells are young cells. Others are mature cells that lie toward the center of the bud; these cells soon break up and dissolve. The average life span of each taste cell is estimated to be about 10 days, although there is considerable variation, with some taste cells being eliminated in only 2 days while others may survive for over 3 weeks. The outer tips of the taste cells are arranged around a minute taste pore, shown in Figure 54-1B. From the tip of each taste cell, several microvilli, or taste hairs, protrude outward into the taste pore to approach the cavity of the mouth. These microvilli provide the receptor surface for taste. Interwoven around the bodies of the taste cells is a branching terminal network of taste nerve fibers that are stimulated by the taste receptor cells. Some of these fibers invaginate into folds of the taste cell membranes. Many vesicles form beneath the cell membrane near the fibers. These vesicles are believed to contain a neurotransmitter substance that is released through the cell membrane to excite the nerve fiber endings in response to taste stimulation. Location of the Taste Buds. The taste buds are found on three types of papillae of the tongue, as follows (see Figure 54-1A): (1) a large number of taste buds are on the walls of the troughs that surround the circumvallate papillae, which form a V line on the surface of the posterior tongue; (2) moderate numbers are on the foliate papillae located in the folds along the lateral surfaces of the tongue; and (3) moderate numbers of taste buds are on the fungiform papillae over the flat anterior surface of the tongue. Additional taste buds are located on the palate, and a few are found on the tonsillar pillars, Relative Taste Indices of Different Substances Sour Substances Index Bitter Substances Index Sweet Substances Hydrochloric acid 1 Quinine 1 Formic acid 1.1 Brucine 11 Chloroacetic acid 0.9 Strychnine 3.1 Saccharin Acetoacetic acid 0.85 Nicotine 1.3 Chloroform Sucrose 1-Propoxy-2-amino4-nitrobenzene Salty Substances Index NaCl 1 NaF 2 675 CaCl2 1 40 NaBr 0.4 Index 1 5000 Lactic acid 0.85 Phenylthiourea 0.9 Fructose 1.7 NaI 0.35 Tartaric acid 0.7 Caffeine 0.4 Alanine 1.3 LiCl 0.4 Malic acid 0.6 Veratrine 0.2 Glucose 0.8 NH4Cl 2.5 Potassium H tartrate 0.58 Pilocarpine 0.16 Maltose 0.45 KCl 0.6 Acetic acid 0.55 Atropine 0.13 Galactose 0.32 Citric acid 0.46 Cocaine 0.02 Lactose 0.3 Carbonic acid 0.06 Morphine 0.02 CaCl2, Calcium chloride; KCl, potassium chloride; LiCl, lithium chloride; NaBr, sodium bromide; NaCl, sodium chloride; NaF, sodium fluoride; NaI, sodium iodide; NH4Cl, ammonium chloride. Data from Pfaffman C: Handbook of Physiology, vol 1. Baltimore: Williams & Wilkins, 1959, p 507. on the epiglottis, and even in the proximal esophagus. Adults have 3000 to 10,000 taste buds, and children have a few more. Beyond the age of 45 years, many taste buds degenerate, causing taste sensitivity to decrease in old age. Specificity of Taste Buds for a Primary Taste Stimulus. Microelectrode studies from single taste buds show that each taste bud usually responds mostly to one of the five primary taste stimuli when the taste substance is in low concentration. However, at high concentration, most buds can be excited by two or more of the primary taste stimuli, as well as by a few other taste stimuli that do not fit into the “primary” categories. Chorda tympani (VII) Glossopharyngeal n. (IX) Epiglottis Root of tongue Foliate papilla IX Circumvallate papilla Taste bud VII Serous gland Fungiform papilla A Taste hairs (microvilli) Taste pore Epithelial cells Taste cell Basal cell Sensory afferent nerve fiber B Subepithelial connective tissue Figure 54-1. (A) Distribution of taste buds in papillae of the tongue and neuronal pathways for transmission of taste signals. (B) Structure of a taste bud. n., Nerve. Mechanism of Stimulation of Taste Buds Receptor Potential. The membrane of the taste cell, like that of most other sensory receptor cells, is negatively charged on the inside with respect to the outside. Application of a taste substance to the taste hairs causes partial loss of this negative potential—that is, the taste cell becomes depolarized. In most cases, the decrease in potential, within a wide range, is approximately proportional to the logarithm of concentration of the stimulating substance. This change in electrical potential in the taste cell is called the receptor potential for taste. The mechanism whereby most stimulating substances react with the taste villi to initiate the receptor potential is by binding of the taste chemical to a protein receptor molecule that lies on the outer surface of the taste receptor cell, near to or protruding through a villus membrane. This action, in turn, opens ion channels, which allows positively charged sodium ions or hydrogen ions to enter and depolarize the normal negativity of the cell. Then, the taste chemical is gradually washed away from the taste villus by the saliva, which removes the stimulus. The type of receptor protein in each taste villus determines the type of taste that will be perceived. For sodium ions and hydrogen ions, which elicit salty and sour taste sensations, respectively, the receptor proteins open specific ion channels, likely the epithelial sodium channel (ENaC), in the apical membranes of the taste cells, thereby activating the receptors. However, for the sweet and bitter taste sensations, the portions of the G-protein coupled receptors that protrude through the apical membranes activate second-messenger transmitter substances inside the taste cells; these second messengers cause intracellular chemical changes that elicit the taste signals. Sweet-tasting compounds are detected by a combination of two closely related G-protein-coupled taste receptors, T1R2 and T1R3. The receptors responsible for umami taste is believed to be a complex of T1R1 and T1R3 proteins. Thus, T1R3 appears to function as a coreceptor for sweet and umami tastes. Bitter taste is sensed by another family (T2R) of approximately 30 different G-protein coupled receptors. Individual bitter-sensing taste receptor cells express multiple T2Rs, each of which recognizes a unique set of bitter compounds. This pattern of receptor expression permits detection of a variety of bitter compounds through a single type of taste receptor cell. Sour taste, associated with acidic food or drink, is believed to be sensed by ion channels that are opened by hydrogen ions although the precise mechanisms are not fully understood. Recent studies suggest that an acidsensitive potassium channel (KIR2.1) and a hydrogen ion–selective ion channel (otopetrin 1) may mediate acid responses in taste receptor cells. Generation of Nerve Impulses by the Taste Bud. On first application of the taste stimulus, the rate of discharge of the nerve fibers from taste buds rises to a peak in a small fraction of a second but then adapts within the next few seconds back to a lower steady level as long as the taste stimulus remains. Thus, a strong immediate signal is transmitted by the taste nerve, and a weaker continuous signal is transmitted as long as the taste bud is exposed to the taste stimulus. TRANSMISSION OF TASTE SIGNALS INTO THE CENTRAL NERVOUS SYSTEM Figures 54-1 and 54-2 show the neuronal pathways for transmission of taste signals from the tongue and pharyngeal region into the central nervous system. Taste impulses from the anterior two-thirds of the tongue pass first into the lingual nerve, then through the chorda tympani into the facial nerve, and finally into the tractus solitarius in the brain stem. Taste sensations from the circumvallate papillae on the back of the tongue and from other posterior regions of the mouth and throat are transmitted through the glossopharyngeal nerve also into the tractus solitarius, but at a slightly more posterior level. Finally, a few taste signals are transmitted into the tractus solitarius from the base of the tongue and other parts of the pharyngeal region by way of the vagus nerve. All taste fibers synapse in the posterior brain stem in the nuclei of the tractus solitarius. These nuclei send second-order neurons to a small area of the ventral posterior medial nucleus of the thalamus, located slightly medial to the thalamic terminations of the facial regions of the dorsal column–medial lemniscal system. From Gustatory cortex (anterior insulafrontal operculum) Ventral posterior medial nucleus of thalamus Tongue Chorda tympani Geniculate ganglion N. VII Glossopharyngeal Petrosal ganglion N. IX Nucleus of solitary tract Gustatory area N. X Pharynx Nodose ganglion Figure 54-2. Transmission of taste signals into the central nervous system. N., nerve. the thalamus, third-order neurons are transmitted to the lower tip of the postcentral gyrus in the parietal cerebral cortex, where it curls deep into the sylvian fissure, and into the adjacent opercular insular area. This area lies slightly lateral, ventral, and rostral to the area for tongue tactile signals in cerebral somatic area I. From this description of the taste pathways, it is evident that they closely parallel the somatosensory pathways from the tongue. Taste Reflexes Are Integrated in the Brain Stem. From the tractus solitarius, many taste signals are transmitted within the brain stem itself directly into the superior and inferior salivatory nuclei. These areas transmit signals to the submandibular, sublingual, and parotid glands to help control the secretion of saliva during the ingestion and digestion of food. Rapid Adaptation of Taste. Everyone is familiar with the fact that taste sensations adapt rapidly, often almost completely, within a minute or so of continuous stimulation. Yet, from electrophysiological studies of taste nerve fibers, it is clear that adaptation of the taste buds usually accounts for no more than about half of this rapid taste adaptation. Therefore, the final extreme degree of adaptation that occurs in the sensation of taste almost certainly occurs in the central nervous system, although the mechanisms are not known. This mechanism of adaptation is different from that of many other sensory systems, which adapt mainly at the receptors. TASTE PREFERENCE AND CONTROL OF THE DIET Taste preference simply means that an animal will choose certain types of food in preference to others, and the animal automatically uses this preference to help control what it eats. Furthermore, its taste preferences often change in accord with the body’s need for certain specific substances. The following experiments demonstrate this ability of animals to choose food in accord with the needs of their bodies. First, adrenalectomized, salt-depleted animals automatically select drinking water with a high concentration of sodium chloride in preference to pure water, and the amount of sodium chloride in the water is often sufficient to supply the needs of the body and prevent death due to salt depletion. Second, an animal given injections of excessive amounts of insulin develops a depleted blood sugar level, and the animal automatically chooses the sweetest food from among many samples. Third, calcium-depleted, parathyroidectomized animals automatically choose drinking water with a high concentration of calcium chloride. The same phenomena are also observed in everyday life. For example, the “salt licks” of desert regions are known to attract animals from far and wide. Also, human beings reject food that has an unpleasant affective sensation, which in many cases protects our bodies from undesirable substances.