Chapter 030. Disorders of Smell, Taste, and Hearing (Part 4)

Definitions Disturbances of the sense of taste may be categorized as total ageusia, total absence of gustatory function or inability to detect the qualities of sweet, salt, bitter, or sour; partial ageusia, ability to detect some but not all of the qualitative gustatory sensations; specific ageusia, inability to detect the taste quality of certain substances; total hypogeusia, decreased sensitivity to all tastants; partial hypogeusia, decreased sensitivity to some tastants; and dysgeusia or phantogeusia, distortion in the perception of a tastant, i.e., the perception of the wrong quality when a tastant is presented or the perception of a taste when...
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Chapter 030. Disorders of Smell, Taste, and Hearing (Part 4) Chapter 030. Disorders of Smell, Taste, and Hearing (Part 4) Definitions Disturbances of the sense of taste may be categorized as total ageusia, totalabsence of gustatory function or inability to detect the qualities of sweet, salt,bitter, or sour; partial ageusia, ability to detect some but not all of the qualitativegustatory sensations; specific ageusia, inability to detect the taste quality of certainsubstances; total hypogeusia, decreased sensitivity to all tastants; partialhypogeusia, decreased sensitivity to some tastants; and dysgeusia or phantogeusia,distortion in the perception of a tastant, i.e., the perception of the wrong qualitywhen a tastant is presented or the perception of a taste when there has been notastant ingested. Confusion between sour and bitter, and less commonly betweensalty and bitter, may represent a semantic misunderstanding or have a truepathophysiologic basis. It may be possible to differentiate between the loss offlavor recognition in patients with olfactory losses who complain of a loss of tasteas well as smell by asking if they are able to taste sweetness in sodas, saltiness inpotato chips, etc. Physiology of Taste The taste receptor cells are located in the taste buds, spherical groups ofcells arranged in a pattern resembling the segments of a citrus fruit (Fig. 30-2). Atthe surface, the taste bud has a pore into which microvilli of the receptor cellsproject. Unlike the olfactory system, the receptor cell is not the primary neuron.Instead, gustatory afferent nerve fibers contact individual taste receptor cells. Thepapillae lie along the lateral margin and dorsum of the tongue; at the junction ofthe dorsum and the base of the tongue; and in the palate, epiglottis, larynx, andesophagus. Figure 30-2 Taste. A. The taste buds of the anterior two-thirds of the tongue areinnervated by the gustatory fibers that travel in a branch of the facial nerve (VII)called the chorda tympani. The taste buds of the posterior third of the tongue areinnervated by gustatory fibers that travel in the lingual branch of theglossopharyngeal nerve (IX). [Adapted from ER Kandel et al (eds): Principles ofNeural Science, 4th ed, New York, McGraw-Hill, 2000; with permission.] B. Themain types of taste papillae are shown in schematic cross sections. Each typepredominates in specific areas of the tongue, as indicated by the arrows from A. C.Each taste bud contains 50–150 taste cells that extend from the base of the tastebud to the taste pore, where the apical microvilli of taste cells have contact withtastants dissolved in saliva and taste pore mucus. Access of tastants to thebasolateral regions of these cells is generally prevented by tight junctions betweentaste cells. Taste cells are short-lived cells that are replaced from stem cells at thebase of the taste bud. Three types of taste cells in each taste bud (light cells, darkcells, and intermediate cells) may represent different stages of differentiation ordifferent cell lineages. Taste stimuli, detected at the apical end of the taste cell,induce action potentials that cause the release of neurotransmitter at synapsesformed at the base of the taste cell with gustatory fibers that transmit signals to thebrain. Tastants gain access to the receptor cells through the taste pore. Fourclasses of taste have been traditionally recognized: sweet, salt, sour, and bitter, andmore recently umami (monosodium glutamate, disodium gluanylate, disodiuminosinate). Tastants enter the taste pore in a solution and initiate transduction byeither activating receptors coupled to G-proteins or by directly activating ionchannels on the microvillae within the taste bud. Individual gustatory afferentfibers almost always respond to a number of different chemicals. As with olfactionand other sensory systems, intensity appears to be encoded by the quantity ofneural activity. The sense of taste is mediated through the facial, glossopharyngeal, andvagal nerves. The chorda tympani branch of the facial nerve subserves taste fromthe anterior two-thirds of the tongue. The posterior third of the tongue is suppliedby the lingual branch of the glossopharyngeal nerve. Afferents from the palatetravel with the greater superficial petrosal nerve to the geniculate ganglion andthen via the facial nerve to the brainstem. The internal branch of the superiorlaryngeal nerve of the vagus nerve contains the taste afferents from the larynx,including the epiglottis and esophagus. The central connections of the nerves terminate in the brainstem in thenucleus of the tractus solitarius. The central pathway from the nucleus of thetractus solitarius ...