Guyton and Hall Physiology Chapter 54 - The Chemical Senses—Taste and Smell PDF

CHAPTER 54 UNIT X The Chemical Senses—Taste and Smell The senses of taste and smell allow us to separate undesir- Salty Taste. Th...

CHAPTER 54 UNIT X The Chemical Senses—Taste and Smell The senses of taste and smell allow us to separate undesir- Salty Taste. The salty taste is elicited by ionized salts, able or even lethal foods from those that are pleasant to mainly by the sodium ion concentration. The quality of eat and nutritious. They also elicit physiological responses the taste varies somewhat from one salt to another be- involved in the digestion and utilization of foods. The cause some salts elicit other taste sensations in addition sense of smell allows animals to recognize the proximity to saltiness. The cations of the salts, especially sodium of other animals or even individual animals. Finally, both cations, are mainly responsible for the salty taste, but the senses are strongly tied to primitive emotional and behav- anions also contribute to a lesser extent. ioral functions of our nervous systems. In this chapter, Sweet Taste. The sweet taste is not caused by any single we discuss how taste and smell stimuli are detected and class of chemicals. Some of the types of chemicals that how they are encoded in neural signals transmitted to the cause this taste include sugars, glycols, alcohols, alde- brain. hydes, ketones, amides, esters, some amino acids, some small proteins, sulfonic acids, halogenated acids, and in- SENSE OF TASTE organic salts of lead and beryllium. Note specifically that Taste is mainly a function of the taste buds in the mouth, most of the substances that cause a sweet taste are organic but it is common experience that one’s sense of smell chemicals. It is especially interesting that slight changes also contributes strongly to taste perception. In addition, in the chemical structure, such as the addition of a simple the texture of food, as detected by tactual senses of the radical, can often change the substance from sweet to bit- mouth, and the presence of substances in the food that ter. stimulate pain endings, such as pepper, greatly alter the Bitter Taste. The bitter taste, like the sweet taste, is not taste experience. The importance of taste lies in the fact caused by any single type of chemical agent. Here again, that it allows a person to select food in accord with desires the substances that give the bitter taste are almost entirely and often in accord with the body tissues’ metabolic need organic substances. Two particular classes of substances for specific substances. are especially likely to cause bitter taste sensations: (1) long-chain organic substances that contain nitrogen; and PRIMARY TASTE SENSATIONS (2) alkaloids. The alkaloids include many of the drugs The identities of the many specific chemicals that excite used in medicines, such as quinine, caffeine, strychnine, different taste receptors are not all known. For practi- and nicotine. cal analysis, the primary sensations of taste have been Some substances that initially taste sweet have a bitter grouped into five general categories—sour, salty, sweet, aftertaste. This characteristic is true of saccharin, which bitter, and “umami.” makes this substance objectionable to some people. High A person can perceive hundreds of different tastes. concentrations of salts may also result in a bitter taste. They are all thought to be combinations of the elemen- The bitter taste, when it occurs in high intensity, usu- tary taste sensations, just as all the colors we can see are ally causes the person or animal to reject the food. This combinations of the three primary colors, as described in reaction is undoubtedly an important function of the bit- Chapter 51. ter taste sensation because many deadly toxins found in poisonous plants are alkaloids, and virtually all these alka- Sour Taste. The sour taste is caused by acids—that is, loids cause an intensely bitter taste, usually followed by by the hydrogen ion concentration—and the intensity rejection of the food. of this taste sensation is approximately proportional to the logarithm of the hydrogen ion concentration (i.e., the Umami Taste. Umami, a Japanese word meaning “deli- more acidic the food, the stronger the sour sensation be- cious,” designates a pleasant taste sensation that is qualita- comes). tively different from sour, salty, sweet, or bitter. Umami is 675 UNIT X The Nervous System: B. The Special Senses the dominant taste of food containing l-glutamate, such taste cells in each taste bud. The taste cells are continually as meat extracts and aging cheese. The pleasurable sensa- being replaced by mitotic division of surrounding epithe- tion of umami taste is thought to be important for nutri- lial cells, so some taste cells are young cells. Others are tion by promoting ingestion of proteins. mature cells that lie toward the center of the bud; these cells soon break up and dissolve. The average life span of THRESHOLD FOR TASTE each taste cell is estimated to be about 10 days, although The molar threshold for stimulation of the sour taste by there is considerable variation, with some taste cells being hydrochloric acid averages 0.0009 M, for stimulation of eliminated in only 2 days while others may survive for the salty taste by sodium chloride, 0.01 M, for the sweet over 3 weeks. taste by sucrose, 0.01 M, and for the bitter taste by qui- The outer tips of the taste cells are arranged around a nine, 0.000008 M. Note especially that the bitter taste minute taste pore, shown in Figure 54-1B. From the tip of sense is much more sensitive than all the others, which each taste cell, several microvilli, or taste hairs, protrude provides an important protective function against many outward into the taste pore to approach the cavity of the dangerous toxins in food. mouth. These microvilli provide the receptor surface for Table 54-1 lists the relative taste indices (the reciprocals taste. of the taste thresholds) of different substances. In this table, Interwoven around the bodies of the taste cells is a the intensities of four of the primary sensations of taste branching terminal network of taste nerve fibers that are are referred, respectively, to the intensities of the taste of stimulated by the taste receptor cells. Some of these fibers hydrochloric acid, quinine, sucrose, and sodium chloride, invaginate into folds of the taste cell membranes. Many each of which is arbitrarily chosen to have a taste index of 1. vesicles form beneath the cell membrane near the fibers. These vesicles are believed to contain a neurotransmit- Taste Blindness. Some people are taste blind for certain ter substance that is released through the cell membrane substances, especially for different types of thiourea com- to excite the nerve fiber endings in response to taste pounds. A substance used frequently by psychologists for stimulation. demonstrating taste blindness is phenylthiocarbamide, for which about 15% to 30% of all people exhibit taste Location of the Taste Buds. The taste buds are found blindness; the exact percentage depends on the method on three types of papillae of the tongue, as follows (see of testing and the concentration of the substance. 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 pos- TASTE BUDS AND THEIR FUNCTION terior tongue; (2) moderate numbers are on the foliate Figure 54-1B shows a taste bud, which has a diame- papillae located in the folds along the lateral surfaces ter of about 1⁄30 of a millimeter and a length of about 1⁄16 of the tongue; and (3) moderate numbers of taste buds of a millimeter. The taste bud is composed of epithe- are on the fungiform papillae over the flat anterior sur- lial cells; some are supporting cells called sustentacular face of the tongue. Additional taste buds are located on cells and others are called taste cells. There are about 100 the palate, and a few are found on the tonsillar pillars, Table 54-1 Relative Taste Indices of Different Substances Salty Sour Substances Index Bitter Substances Index Sweet Substances Index Substances Index Hydrochloric acid 1 Quinine 1 Sucrose 1 NaCl 1 Formic acid 1.1 Brucine 11 1-Propoxy-2-amino- 5000 NaF 2 4-nitrobenzene Chloroacetic acid 0.9 Strychnine 3.1 Saccharin 675 CaCl2 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 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. 676  Chapter 54 The Chemical Senses—Taste and Smell on the epiglottis, and even in the proximal esophagus. Mechanism of Stimulation of Taste Buds Adults have 3000 to 10,000 taste buds, and children Receptor Potential. The membrane of the taste cell, like have a few more. Beyond the age of 45 years, many taste that of most other sensory receptor cells, is negatively buds degenerate, causing taste sensitivity to decrease in charged on the inside with respect to the outside. Appli- old age. cation of a taste substance to the taste hairs causes par- UNIT X Specificity of Taste Buds for a Primary Taste Stimulus. tial loss of this negative potential—that is, the taste cell Microelectrode studies from single taste buds show that becomes depolarized. In most cases, the decrease in po- each taste bud usually responds mostly to one of the five tential, within a wide range, is approximately proportional primary taste stimuli when the taste substance is in low to the logarithm of concentration of the stimulating sub- concentration. However, at high concentration, most buds stance. This change in electrical potential in the taste cell can be excited by two or more of the primary taste stimu- is called the receptor potential for taste. li, as well as by a few other taste stimuli that do not fit into The mechanism whereby most stimulating substances the “primary” categories. 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 recep- Chorda tympani (VII) tor cell, near to or protruding through a villus membrane. This action, in turn, opens ion channels, which allows Glossopharyngeal n. (IX) positively charged sodium ions or hydrogen ions to enter and depolarize the normal negativity of the cell. Then, the Epiglottis taste chemical is gradually washed away from the taste Root of tongue villus by the saliva, which removes the stimulus. Foliate papilla The type of receptor protein in each taste villus deter- mines 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 spe- cific ion channels, likely the epithelial sodium channel (ENaC), in the apical membranes of the taste cells, thereby IX activating the receptors. However, for the sweet and bit- Circumvallate papilla ter taste sensations, the portions of the G-protein coupled receptors that protrude through the apical membranes Taste bud activate second-messenger transmitter substances inside the taste cells; these second messengers cause intracellu- VII Serous gland lar chemical changes that elicit the taste signals. Sweet-tasting compounds are detected by a combi- Fungiform papilla nation 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 A T1R3 proteins. Thus, T1R3 appears to function as a co- receptor for sweet and umami tastes. Taste hairs (microvilli) Bitter taste is sensed by another family (T2R) of approximately 30 different G-protein coupled receptors. Taste pore Individual bitter-sensing taste receptor cells express mul- tiple T2Rs, each of which recognizes a unique set of bitter Epithelial cells compounds. This pattern of receptor expression permits detection of a variety of bitter compounds through a sin- Taste cell gle 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 Basal cell not fully understood. Recent studies suggest that an acid- Sensory sensitive potassium channel (KIR2.1) and a hydrogen afferent ion–selective ion channel (otopetrin 1) may mediate acid B nerve fiber responses in taste receptor cells. Subepithelial connective tissue Generation of Nerve Impulses by the Taste Bud. On Figure 54-1. (A) Distribution of taste buds in papillae of the tongue and neuronal pathways for transmission of taste signals. (B) Structure first application of the taste stimulus, the rate of discharge of a taste bud. n., Nerve. of the nerve fibers from taste buds rises to a peak in a 677 UNIT X The Nervous System: B. The Special Senses small fraction of a second but then adapts within the next the thalamus, third-order neurons are transmitted to the few seconds back to a lower steady level as long as the lower tip of the postcentral gyrus in the parietal cerebral taste stimulus remains. Thus, a strong immediate signal is cortex, where it curls deep into the sylvian fissure, and into transmitted by the taste nerve, and a weaker continuous the adjacent opercular insular area. This area lies slightly signal is transmitted as long as the taste bud is exposed to lateral, ventral, and rostral to the area for tongue tactile the taste stimulus. signals in cerebral somatic area I. From this description of the taste pathways, it is evident that they closely parallel TRANSMISSION OF TASTE SIGNALS INTO the somatosensory pathways from the tongue. THE CENTRAL NERVOUS SYSTEM Taste Reflexes Are Integrated in the Brain Stem. From Figures 54-1 and 54-2 show the neuronal pathways for the tractus solitarius, many taste signals are transmitted transmission of taste signals from the tongue and pharyn- within the brain stem itself directly into the superior and geal region into the central nervous system. Taste impulses inferior salivatory nuclei. These areas transmit signals to from the anterior two-thirds of the tongue pass first into the submandibular, sublingual, and parotid glands to help the lingual nerve, then through the chorda tympani into control the secretion of saliva during the ingestion and the facial nerve, and finally into the tractus solitarius in digestion of food. the brain stem. Taste sensations from the circumvallate Rapid Adaptation of Taste. Everyone is familiar with papillae on the back of the tongue and from other pos- the fact that taste sensations adapt rapidly, often almost terior regions of the mouth and throat are transmitted completely, within a minute or so of continuous stimula- through the glossopharyngeal nerve also into the tractus tion. Yet, from electrophysiological studies of taste nerve solitarius, but at a slightly more posterior level. Finally, a fibers, it is clear that adaptation of the taste buds usually few taste signals are transmitted into the tractus solitarius accounts for no more than about half of this rapid taste from the base of the tongue and other parts of the pharyn- adaptation. Therefore, the final extreme degree of adapta- geal region by way of the vagus nerve. tion that occurs in the sensation of taste almost certainly All taste fibers synapse in the posterior brain stem occurs in the central nervous system, although the mech- in the nuclei of the tractus solitarius. These nuclei send anisms are not known. This mechanism of adaptation is second-order neurons to a small area of the ventral pos- different from that of many other sensory systems, which terior medial nucleus of the thalamus, located slightly adapt mainly at the receptors. medial to the thalamic terminations of the facial regions of the dorsal column–medial lemniscal system. From TASTE PREFERENCE AND CONTROL OF THE DIET Gustatory cortex (anterior insula- Taste preference simply means that an animal will choose frontal operculum) certain types of food in preference to others, and the animal automatically uses this preference to help con- trol 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 abil- ity of animals to choose food in accord with the needs of their bodies. First, adrenalectomized, salt-depleted Ventral posterior medial nucleus of animals automatically select drinking water with a high thalamus concentration of sodium chloride in preference to pure Geniculate water, and the amount of sodium chloride in the water Chorda ganglion tympani is often sufficient to supply the needs of the body and Tongue prevent death due to salt depletion. Second, an animal N. VII given injections of excessive amounts of insulin develops Nucleus of solitary tract a depleted blood sugar level, and the animal automatically Glossopharyngeal N. IX Petrosal chooses the sweetest food from among many samples. ganglion Gustatory area Third, calcium-depleted, parathyroidectomized animals N. X automatically choose drinking water with a high concen- tration of calcium chloride. Pharynx Nodose The same phenomena are also observed in everyday ganglion 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 sensa- Figure 54-2. Transmission of taste signals into the central nervous tion, which in many cases protects our bodies from unde- system. N., nerve. sirable substances. 678  Chapter 54 The Chemical Senses—Taste and Smell The phenomenon of taste preference almost certainly cilia and then it binds with receptor proteins in the mem- results from some mechanism located in the central brane of each cilium (Figure 54-4). Each receptor protein nervous system and not from a mechanism in the taste is actually a long molecule that threads its way through the receptors, although the receptors often become sensitized membrane about seven times, folding inward and outward. in favor of a needed nutrient. An important reason for believing that taste preference is mainly a central nervous UNIT X Olfactory tract system phenomenon is that previous experience with unpleasant or pleasant tastes plays a major role in deter- Olfactory bulb mining one’s taste preferences. For example, if a person becomes sick soon after eating a particular type of food, Mitral cell the person then generally develops a negative taste prefer- ence, or taste aversion, for that particular food; the same Glomerulus Bowman’s gland effect can be demonstrated in lower animals. SENSE OF SMELL Sustentacular Smell is the least understood of our senses, partly because cells the sense of smell is a subjective phenomenon that cannot Olfactory cell be studied with ease in lower animals. Another compli- cating problem is that the sense of smell is poorly devel- oped in human beings compared with the sense of smell Olfactory cilia in many other mammals. Mucus layer OLFACTORY MEMBRANE Figure 54-3. Organization of the olfactory membrane and olfactory bulb and connections to the olfactory tract. The olfactory membrane, the histology of which is shown in Figure 54-3, lies in the superior part of the nasal cav- ity. Medially, the olfactory membrane folds downward along the surface of the superior septum; laterally, it folds over the superior turbinate and even over a small portion of the upper surface of the middle turbinate. The olfac- tory membrane has a total surface area of about 5 square centimeters in humans. Olfactory Cells Are the Receptor Cells for Smell Sen- sation. The olfactory cells (see Figure 53-3) are actually bipolar nerve cells derived originally from the central nervous system. There are about 100 million of these cells in the olfactory epithelium interspersed among sus- Extracellular side tentacular cells, as shown in Figure 54-3. The mucosal Odorant Adenyl cyclase end of the olfactory cell forms a knob from which 4 to Odorant Na+ receptor 25 olfactory hairs (also called olfactory cilia), measuring 0.3 micrometer in diameter and up to 200 micrometers in length, project into the mucus that coats the inner surface of the nasal cavity. These projecting olfactory cilia form a dense mat in the mucus, and it is these cilia that react to odors in the air and stimulate the olfactory cells, as dis- cussed later. Spaced among the olfactory cells in the olfac- γ α tory membrane are many small Bowman glands that se- β Na+ crete mucus onto the surface of the olfactory membrane. G-protein ATP cAMP Cytoplasmic side STIMULATION OF THE OLFACTORY CELLS Figure 54-4. Summary of olfactory signal transduction. Binding of Mechanism of Excitation of the Olfactory Cells. The the odorant to a G-protein–coupled receptor causes the activation of portion of each olfactory cell that responds to the olfac- adenylate cyclase, which converts adenosine triphosphate (ATP) to cy- clic adenosine monophosphate (cAMP). The cAMP activates a gated tory chemical stimuli is the olfactory cilia. The odorant sodium channel that increases sodium influx and depolarizes the cell, substance, on coming in contact with the olfactory mem- exciting the olfactory neuron and transmitting action potentials to brane surface, first diffuses into the mucus that covers the the central nervous system. 679 UNIT X The Nervous System: B. The Special Senses The odorant binds with the portion of the receptor protein the stimulus strength, which demonstrates that the olfac- that folds to the outside. The inside of the folding protein is tory receptors obey principles of transduction similar to coupled to a G protein, itself a combination of three subu- those of other sensory receptors. nits. On excitation of the receptor protein, an alpha subu- Rapid Adaptation of Olfactory Sensations. The olfac- nit breaks away from the G protein and activates adenylyl tory receptors adapt about 50% in the first second or so cyclase, which is attached to the inside of the ciliary mem- after stimulation. Thereafter, they adapt very little and brane near the receptor cell body. The activated cyclase, very slowly. Yet, we all know from our own experience in turn, converts many molecules of intracellular adeno- that smell sensations adapt almost to extinction within sine triphosphate (ATP) into cyclic adenosine monophos- a minute or so after entering a strongly odorous atmos- phate (cAMP). Finally, this cAMP activates another nearby phere. Because this psychological adaptation is far greater membrane protein, a gated sodium ion channel, that opens than the degree of adaptation of the receptors, it is almost its “gate” and allows large numbers of sodium ions to pour certain that most of the additional adaptation occurs in through the membrane into the receptor cell cytoplasm. the central nervous system, which seems to be true for the The sodium ions increase the electrical potential in the adaptation of taste sensations as well. positive direction inside the cell membrane, thus exciting The following neuronal mechanism for the adaptation the olfactory neuron and transmitting action potentials is postulated: large numbers of centrifugal nerve fibers into the central nervous system via the olfactory nerve. pass from the olfactory regions of the brain backward The importance of this mechanism for activating olfac- along the olfactory tract and terminate on special inhibi- tory nerves is that it greatly multiplies the excitatory effect tory cells in the olfactory bulb, the granule cells. After the of even the weakest odorant. To summarize: (1) activation onset of an olfactory stimulus, the central nervous system of the receptor protein by the odorant substance activates quickly develops strong feedback inhibition to suppress the G-protein complex, which, in turn (2) activates mul- relay of the smell signals through the olfactory bulb. tiple molecules of adenylyl cyclase inside the olfactory cell membrane, which (3) causes the formation of many times Search for the Primary Sensations of more molecules of cAMP, and finally, (4) the cAMP opens Smell still many times more sodium ion channels. Therefore, In the past, most physiologists were convinced that the even a minute concentration of a specific odorant initiates many smell sensations are subserved by a few rather dis- a cascading effect that opens extremely large numbers of crete primary sensations in the same way that vision and sodium channels. This process accounts for the exquisite taste are subserved by a few select primary sensations. On sensitivity of the olfactory neurons to even the slightest the basis of psychological studies, one attempt to classify amount of odorant. these sensations is the following: In addition to the basic chemical mechanism whereby 1. Camphoraceous the olfactory cells are stimulated, several physical factors 2. Musky affect the degree of stimulation. First, only volatile sub- 3. Floral stances that can be sniffed into the nasal cavity can be 4. Pepperminty smelled. Second, the stimulating substance must be at 5. Ethereal least slightly water-soluble so that it can pass through the 6. Pungent mucus to reach the olfactory cilia. Third, it is helpful for 7. Putrid the substance to be at least slightly lipid-soluble, presum- It is certain that this list does not represent the true ably because lipid constituents of the cilium are a weak primary sensations of smell. Multiple clues, including barrier to non–lipid-soluble odorants. specific studies of the genes that encode for the recep- Membrane Potentials and Action Potentials in tor proteins, suggest the existence of at least 100 primary Olfactory Cells. The membrane potential inside unstim- sensations of smell—a marked contrast to only three pri- ulated olfactory cells, as measured by microelectrodes, mary sensations of color detected by the eyes and only averages about −55 millivolts. At this potential, most of five primary sensations of taste detected by the tongue. the cells generate continuous action potentials at a very Some studies suggest that there may be as many as 1000 slow rate, varying from once every 20 seconds up to two different types of odorant receptors. Further support for or three per second. the many primary sensations of smell is that people have Most odorants cause depolarization of the olfactory been found who have odor blindness for single substances; cell membrane, decreasing the negative potential in the such discrete odor blindness has been identified for more cell from the normal level of −55 millivolts to −30 milli- than 50 different substances. It is presumed that odor volts or less. Along with this, the number of action poten- blindness for each substance represents lack of the appro- tials increases to 20 to 30 per second, which is a high rate priate receptor protein in olfactory cells for that particular for the minute olfactory nerve fibers. substance. Over a wide range, the rate of olfactory nerve impulses Affective Nature of Smell. Smell, even more so than changes approximately in proportion to the logarithm of taste, has the affective quality of either pleasantness or 680  Chapter 54 The Chemical Senses—Taste and Smell unpleasantness, and thus smell is probably even more terminating in multiple globular structures in the olfac- important than taste for the selection of food. A person tory bulb called glomeruli. Each bulb has several thousand who has previously eaten food that disagreed with him or such glomeruli, each of which is the terminus for about her is often nauseated by the smell of that same food on a 25,000 axons from olfactory cells. Each glomerulus also second occasion. Conversely, perfume of the right quality is the terminus for dendrites from about 25 large mitral can be a powerful stimulant of human emotions. In addi- cells and about 60 smaller tufted cells, the cell bodies of UNIT X tion, in some animals, odors are the primary excitant of which lie in the olfactory bulb superior to the glomeruli. sexual drive. These dendrites receive synapses from the olfactory cell neurons; the mitral and tufted cells send axons through Threshold for Smell. One of the principal characteris- the olfactory tract to transmit olfactory signals to higher tics of smell is the minute quantity of stimulating agent in levels in the central nervous system. the air that can elicit a smell sensation. For example, the Some research has suggested that different glomeruli substance methylmercaptan can be smelled when only respond to different odors. It is possible that specific one 25 trillionth of a gram is present in each milliliter of glomeruli are the real clue to the analysis of different odor air. Because of this very low threshold, this substance is signals transmitted into the central nervous system. mixed with natural gas to give the gas an odor that can be detected when even small amounts of gas leak from a Primitive and Newer Olfactory Pathways pipeline. Into the Central Nervous System Gradations of Smell Intensities. Although the thresh- The olfactory tract enters the brain at the anterior junc- old concentrations of substances that evoke smell are tion between the mesencephalon and cerebrum; there, the extremely slight, for many (if not most) odorants, con- tract divides into two pathways, as shown in Figure 54-5, centrations only 10 to 50 times above the threshold evoke one passing medially into the medial olfactory area of the maximum intensity of smell. This range of intensity dis- brain stem and the other passing laterally into the lateral crimination is in contrast to most other sensory systems olfactory area. The medial olfactory area represents a very of the body, in which the ranges of intensity discrimina- primitive olfactory system, whereas the lateral olfactory tion are tremendous—for example, 500,000 to 1 for the area is the input to the following: (1) a less old olfactory eyes and 1 trillion to 1 for the ears. This difference might system; and (2) a newer system. be explained by the fact that smell is concerned more with The Primitive Olfactory System—The Medial Olfactory detecting the presence or absence of odors rather than Area. The medial olfactory area consists of a group of nu- with quantitative detection of their intensities. clei located in the midbasal portions of the brain imme- diately anterior to the hypothalamus. Most conspicuous TRANSMISSION OF SMELL SIGNALS INTO THE CENTRAL NERVOUS SYSTEM are the septal nuclei, which are midline nuclei that feed into the hypothalamus and other primitive portions of The olfactory portions of the brain were among the first the brain’s limbic system. This is the brain area most con- brain structures developed in primitive animals, and cerned with basic behavior (as described in Chapter 59). much of the remainder of the brain developed around The importance of this medial olfactory area is best these olfactory beginnings. In fact, part of the brain that understood by considering what happens in animals originally subserved olfaction later evolved into the basal brain structures that control emotions and other aspects Hypothalamus Medial olfactory area of human behavior; we call this system the limbic system, as discussed in Chapter 59. Prefrontal Transmission of Olfactory Signals Into the Olfactory cortex Bulb. The olfactory bulb is shown in Figure 54-5. The ol- Olfactory factory nerve fibers leading backward from the bulb are tract called cranial nerve I, or the olfactory tract. In reality, both the tract and the bulb are an anterior outgrowth of brain Mitral tissue from the base of the brain; the bulbous enlargement cell at its end, the olfactory bulb, lies over the cribriform plate, Olfactory separating the brain cavity from the upper reaches of the bulb nasal cavity. The cribriform plate has multiple small per- forations through which an equal number of small nerves Lateral Orbito- olfactory frontal pass upward from the olfactory membrane in the nasal area cortex cavity to enter the olfactory bulb in the cranial cavity. Brain stem Hippocampus Temporal Figure 54-3 demonstrates the close relation between the cortex olfactory cells in the olfactory membrane and the olfac- tory bulb, showing short axons from the olfactory cells Figure 54-5. Neural connections of the olfactory system. 681 UNIT X The Nervous System: B. The Special Senses when the lateral olfactory areas on both sides of the Centrifugal Control of Activity in the Olfactory Bulb brain are removed, and only the medial system remains. by the Central Nervous System. Many nerve fibers The removal of these areas hardly affects the more basic that originate in the olfactory portions of the brain pass responses to olfaction, such as licking the lips, salivation, from the brain in the outward direction into the olfacto- and other feeding responses caused by the smell of food ry tract to the olfactory bulb (i.e., centrifugally from the or by basic emotional drives associated with smell. Con- brain to the periphery). These nerve fibers terminate on versely, removal of the lateral areas abolishes the more a large number of small granule cells located among the complicated olfactory conditioned reflexes. mitral and tufted cells in the olfactory bulb. The granule cells send inhibitory signals to the mitral and tufted cells. The Less Old Olfactory System—The Lateral Olfactory This inhibitory feedback may be a means for sharpening Area. The lateral olfactory area is composed mainly of the a person’s specific ability to distinguish one odor from prepyriform and pyriform cortex plus the cortical portion another. of the amygdaloid nuclei. From these areas, signal path- ways pass into almost all portions of the limbic system, especially into less primitive portions such as the hip- Bibliography pocampus, which seem to be most important for learning Augustine V, Gokce SK, Oka Y: Peripheral and central nutrient to like or dislike certain foods depending on one’s experi- sensing underlying appetite regulation. Trends Neurosci 41:526, ences with them. For example, it is believed that this lateral 2018. olfactory area and its many connections with the limbic Avau B, Depoortere I: The bitter truth about bitter taste receptors: beyond sensing bitter in the oral cavity. 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