Oral chemoreception includes the senses of taste (gustation), smell (olfaction), and the chemical somatic senses, which will be covered in Chapter 9, ”Oral Mucosa and Mucosal Sensation,” and Chapter 12, “Orofacial Pain, Touch and Thermosensation, and Sensorimotor Functions.” This current chapter, not solely a compilation of current knowledge on the gustatory and olfactory systems, integrates into the text experimental techniques used to develop that knowledge. Perceptual (human objective and subjective psychophysics, animal conditioned taste aversion and preference), physiological (receptor potential, multi- and single- neuron electrophysiology; two-photon calcium imaging in vivo, single dissociated olfactory sensory neuron (OSN) calcium imaging, event-related fMRI brain imaging) and morphological (light and electron microscopy) methodologies are cited.

The independent senses of taste and smell have evolved to monitor chemical environments for health and wellbeing. With them, as well as general somatosensations, distinct sources of chemosensory information, it can be decided that foods need more or less sugar or salt, whether the scent of dinner is foul or appetizing, and whether lemonade is too sharp. One of the first things learned from patients at the opening of the Connecticut Taste and Smell Clinic in 1980 was that people often confuse smells with tastes. They may feel certain that they cannot taste after an automobile accident involving head trauma but have no measurable loss of gustation. They are perfectly capable of distinguishing salt from sugar. Retronasal smelling, a recent topic of interest, helps explain the patients’ confusions. Odors from the outside are sensed orthonasally during inspiration; but on expiration oral odors are sensed, retronasally. (See also Disorders in Glossary). Retronasal odors from the mouth, closely allied with tastes, contribute to the specific flavor of a food such as asparagus, which also gives urine an unusual orthonasal odor.

Tastes and smells of chemosensory compounds are attached to innate and developed preferences and aversions. Odors’ primal pleasure/displeasure dimension (for example) is hardwired in olfactory mucosal patches of sensitivity to volatile compounds with appealing (+) or repulsive (–) odors. This is elaborated on in Box 10.1(1). Also, single chemosensory compounds may activate several chemosensory systems. For example, acetic acid has a vinegary smell, a sour taste, and painfully irritates as concentration increases. Anosmic individuals, without a sense of smell, can detect acetic acid orthonasally but only at a higher concentration than normosmic individuals. Correspondingly, individuals taste-blind for specific compounds such as phenylthiocarbamide (PTC) and related bitter compounds detect them only at high concentrations, as shown in Box 10.2(1). Moreover, chemosensory stimulus recognition is fleeting, fading with time through peripheral adaptation and mutual central inhibition evident in mixture suppression among separate neural pathways. This coding strategy emphasizes the identification of recently appearing and strong tasting or smelling chemical compounds.

Taste

Fundamentals and historical perspective

Taste evaluates beneficial and dangerous water-soluble chemicals taken into the mouth to assist nutriture through the pleasure of eating and poisoning through the awfulness of tainted foods.

A century ago, Hans Henning represented the relationships between pure human taste perceptions geometrically on the surface of a tetrahedron: “primaries” at the four corners and mixtures of two or three taste “primaries” elsewhere on edges and faces (Fig. 10.1). In the tetrahedron drawn, compounds with each taste (sweet, salty, sour, and bitter) are positioned at corners, compounds with binary mixtures of tastes (saccharin, KCl, NH₄Cl) are positioned appropriately on edges between pure tastes, and a compound with a mixture of three tastes (NaSO₄) is positioned on a tetrahedral face. Single chemical compounds with one taste quality, single compounds with two or three taste qualities, and, by extension, mixtures of two or three compounds with different pure qualities are represented by the tetrahedral model.

Chemical stimuli and taste quality

Sugars, some amino acids, and artificial sweeteners such as saccharin and aspartame are sweet. Halide sodium salts NaCl and NaBr are salty. Non-sodium halide salts are salty and bitter. KCl, NH₄Cl, MgSO₄ and salts with large organic cations such as denatonium benzoate and quinine hydrochloride and zwitter-ionic amino acids as well as nonionic alkaloids (thioureas, bile acids and glycosides) are bitter, a broad category of unpleasant tastes that may include several distinct experiences. Sour-tasting acids, which are also irritants, may also have characteristic odors, such as the vinegary acetic acid. Ethanol is the epitome multi-chemosensory stimulus, tasting bitter and sweet and having an odor, as well as being an irritant. The non-gustatory aspects of taste stimuli were intentionally not considered in Henning’s pure-taste tetrahedron. Structures of a variety of water-soluble compounds with a broad range of tastes are shown in Fig. 10.2.

Tastes surely have four pure qualities that signal nutritious carbohydrates and minerals or harmful substances. Another taste, savory (umami in Japanese), has been legitimized with discovery of a receptor. It is evoked by monosodium L-glutamate (MSG), or better, a mixture of MSG and inosine monophosphate (IMP), and may signal protein sources. Other stimulus domains, free fatty acids (for example, linoleic acid), glucose polymers (for example, the nutritional supplement Polycose®), and low osmolarity (water) may also be detected orally; possibly with involvement of gustatory receptors.

Species-specific taste worlds

Species differences in the gustatory quality of chemicals can be great, especially for the many compounds with bitter tastes associated with the T2R bitter genes. The variation may reflect the importance of detecting the different poisons and food items used for nourishment as habitats change and species evolve. For example, carnivorous felines are unable to taste sugar, which is consistent with pseudogenization of the T1R2 sweet-taste receptor gene during evolutionary time owing to a release from positive evolution. Conversely, ethanol (bitter-sweet and poisonous to humans at high concentrations) is primarily sweet to golden hamsters, who utilize its caloric value without inebriation poisoning when feeding on fermented grain stored in their deep burrow domiciles.

Studies on rodents are used as models for human taste. However, before linking human to animal taste, differences in perceptual “taste worlds” must be experimentally addressed. To this end, confusability of taste stimuli, quantitatively measured psychophysically in humans with taste confusion matrix methodology and behaviorally in animals with taste aversion generalization tests, reveals how taste mixtures are evaluated. Multivariate analysis of human confusions, Box 10.3(1), distributed the prototypic sweet sucrose, salty NaCl, sour citric acid, bitter quinine, and five binary taste mixtures in a two-dimensional space. All taste stimuli were distinct from insipid water, some mixtures fell between mixture components but some were much closer to a dominant component. Correspondingly, intake suppression seen in hamster generalization tests, Box 10.3(2), differed for hamsters that learned aversions to three individual compounds (sucrose, NaCl, quinine) and three binary or one ternary mixture of the compounds. Like humans, the hamsters detected components in binary mixtures, and the ability to recognize components deteriorated with the higher level ternary mixture, mixture suppression reminiscent of human tasting. (See also Fig. 10.10 under “Peripheral Nerves” below.)

Peripheral activation

Taste buds detect water-soluble chemicals

About 50 taste cells (TC) reside in taste bud cellular clusters located in lingual epithelia and elsewhere in oropharyngeal mucosa. Taste cells turn over every 10 days, which reflects their epithelial origin. Fig. 10.5a is a photomicrograph of a taste bud within a fungiform papilla. Mature taste buds depend on neurotrophism; atrophied anlage remain after injury to the taste nerve. Taste-bud cells project membrane extensions, microvilli, into the gustatory pore, which contains dense mucosubstance, as pictured in the diagram in Fig. 10.5b. Type I, II, and III TC with different lineages each have unique morphologies and biochemical features, suggestive of distinct functions. Glial-like, GLAST (glutamate aspartate transporter) containing, type I dark cells with sheet-like processes intermingle among other TC, possibly serving physical and biochemical supporting roles. Type II TC (light cells) contain the biochemical necessities for taste stimulus transduction. This includes the G protein α-gustducin, T1R and T2R G protein-coupled receptors (GPCR), as well as downstream elements phospholipase C (PLCβ2) and transient receptor potential (TRP) cation channel TRPM5 necessary for TC depolarization. All must be operational for the tasting of sweet, savory and bitter compounds, which is elaborated on in the next few paragraphs. Type III TC, which project one broad apical microvillus into the gustatory pore and may be directly activated by ionic taste stimuli, are the only TC known to be synaptically associated with the axons of taste nerves at the base of the taste bud. Multiple trigeminal nerve (CN V) endings are seen apically in the papilla outside the taste bud that are capable of mediating touch, temperature and chemical irritation via TRP channels, a topic covered in Chapter 9, “Oral Mucosa and Mucosal Sensation.”

Taste receptors are related to perceptual taste quality

Interactions between taste stimuli and taste receptors (TR) occur within gustatory pores. Nonionic taste stimuli first bind to receptor proteins in apical membranes of TC microvilli that are coupled to second-messenger systems. Members of two families of GPCR detect taste stimuli, 3 T1R and 30 T2R. The G protein gustducin with an α subunit quite similar to retinal rod transducin (which can substitute for gustducin) is ubiquitously involved. Sweet ligands activate the single heterodimeric GPCR T1R2/T1R3; its structure is drawn in Fig. 10.6. Umami (savory) ligands may activate heterodimer T1R1/T1R3. Each of these receptors is restricted to a distinct subset of taste cells. Structurally varied bitter ligands activate the more numerous T2R that appear together in a single TC variant. Most human T2R genes are tightly clustered on chromosomes 12 or 7. T2R38 on chromosome 7q36 is one of few human bitter receptors for which function is definitively established due to discovery and documentation of variation in tasting PTC and related compounds nearly 80 years ago. Of the two common haplotypes, PAV, found in primates, is thought to be the ancestral. (See also Box 10.2(2) above.) Stimulus binding of ligands such as sweet sucralose and bitter salicin is associated with extracellular portions of T1R2 and T2R16, respectively.

Ionic taste stimuli directly activate dedicated subsets of TC by passing through ion channels in cell membranes, but the TC types involved are uncertain. In rodents and humans, a Na⁺ taste is transduced by influx of Na⁺ through an amiloride-sensitive ion channel (ENaC); its structure is drawn in Fig. 10.6. Box 10.4(2) below shows a hamster nerve recording demonstrating the effect of 10-μM amiloride on a response to 100-mM NaCl. The taste of NaCl is not completely accounted for by ENaC sensitivity; a quinine-like taste remains. Sour taste may be generated by influx of H⁺ through TRP PKD2L1 + PKD1 channels (structure drawn in Fig. 10.6) in membranes of type III TC. It also has been suggested that intracellular acidification has a role in the production of acid taste.

Peripheral nerves

GSN are pseudo-unipolar neurons with cell bodies in the three peripheral ganglia listed in Table 10.2. Central and peripheral coursing axonal processes of the neurons are small, either myelinated or unmyelinated. GSN of CN VII travel in the chorda tympani (CT), which joins the lingual branch of the mandibular division of the trigeminal nerve (CN V) before reaching the tongue, and the greater superficial petrosal (GSP) nerve, which reaches the soft palate via the vidian nerve as traced in Fig. 10.8. Geniculate central axonal processes travel through the internal auditory meatus to enter the brainstem as the intermediate nerve root, which lies between CN VII and VIII roots. Petrosal central axonal processes of lingual sensory branches of CN IX and nodose central axonal processes of superior laryngeal branches of CN X enter the brain stem within the roots of CN IX and X, respectively.

Visualization of the CT medial to the mandible coursing from the tympanic bulla to join the lingual nerve is achieved with removal of soft tissue after disarticulation of the temporomandibular joint (anatomy described in detail in Chapter 8, “Temporomandibular Joint”), with removal of its condylar and coronoid processes and the zygomatic arch. This surgical approach and, as important, the ease with which the fungiform taste buds on the tongue tip can be stimulated, has resulted in taste function being defined primarily for a single taste bud field. Box 10.4 contains more information on the techniques used for GSN electrophysiological analysis. Function of individual GSN is measured with single-unit recording (an example is included in Box 10.4(1)); whereas, gustatory sensory nerve function, the combined responses of hundreds of single GSN, is measured by whole-nerve recording (an example is included in Box 10.4(2)).

Gustatory sensory neuron function

Single hamster GSN respond selectively to taste compounds as illustrated for the CT in Fig. 10.9. The neural responses of single neurons in this one taste nerve are compatible with (but less complex than) perceptual “taste worlds” based on all oral sensory nerves (Fig. 10.3). This means more information than received from CT GSN is used by hamsters to make behavioral discriminations among the tested taste stimuli. The CT contributes sucrose-sensitive (S) and NaCl-sensitive (N) CT GSN that are taste quality specialists, the former responding primarily to sweeteners (illustrated in the third column of recordings in Fig. 10.9b) and the latter responding to Na⁺ and Li⁺ halides (illustrated in the second column of recordings in Fig. 10.9b). T1R GPCR and ENaC ion channels (Fig. 10.6), their dedicated TC, and upstream S and N neurons form two peripheral lines to the brain that identify nutrients. In the CT stimulus space in Fig. 10.9c, note tightly clustered sweets and sodium salts falling distant from each other and a looser cluster for other stimuli. Responses to this broader cluster as well as the amiloride-insensitive response to NaCl (illustrated in the first column of recordings shown in Fig. 10.9b), occur in CT H GSN (also known as E GSN because of broad sensitivity to electrolytes). These more variable electrolyte-sensitive CT generalists respond to sour HCl and other acids, salty NaCl, bitter quinine and the bitter salts KCl and NH₄Cl. Thus, each cluster in Fig. 10.9c corresponds to activation of the distinct hamster CT neuron types: the highly correlated responses of each of two types of specialists and loosely correlated responses of electrolyte generalists that are possibly composed of several more-specific subtypes.