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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 “delicious,” 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.
Table 54-1
Relative Taste Indices of Different Substances
Data from Pfaffman C: Handbook of Physiology, vol 1. Baltimore: Williams & Wilkins, 1959, p 507.
| Sour Substances | Index | Bitter Substances | Index | Sweet Substances | Index | Salty Substances | Index |
|---|---|---|---|---|---|---|---|
| Hydrochloric acid | 1 | Quinine | 1 | Sucrose | 1 | NaCl | 1 |
| Formic acid | 1.1 | Brucine | 11 | 1-Propoxy-2-amino-4-nitrobenzene | 5000 | NaF | 2 |
| Chloroacetic acid | 0.9 | Strychnine | 3.1 | Saccharin | 675 | CaCl 2 | 1 |
| Acetoacetic acid | 0.85 | Nicotine | 1.3 | Chloroform | 40 | NaBr | 0.4 |
| 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 | NH 4 Cl | 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 |
CaCl 2 , Calcium chloride; KCl, potassium chloride; LiCl, lithium chloride; NaBr, sodium bromide; NaCl, sodium chloride; NaF, sodium fluoride; NaI, sodium iodide; NH 4 Cl, ammonium chloride.
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 54-1 B 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 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 54-1 B . 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 54-1 A ): (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, 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.
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