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

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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) 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 there has been no tastant ingested. Confusion between sour and bitter, and less commonly between salty and bitter, may represent a semantic misunderstanding or have a true pathophysiologic basis. It may be possible to differentiate between the loss of
flavor recognition in patients with olfactory losses who complain of a loss of taste as well as smell by asking if they are able to taste sweetness in sodas, saltiness in potato chips, etc. Physiology of Taste The taste receptor cells are located in the taste buds, spherical groups of cells arranged in a pattern resembling the segments of a citrus fruit (Fig. 30-2). At the surface, the taste bud has a pore into which microvilli of the receptor cells project. Unlike the olfactory system, the receptor cell is not the primary neuron. Instead, gustatory afferent nerve fibers contact individual taste receptor cells. The papillae lie along the lateral margin and dorsum of the tongue; at the junction of the dorsum and the base of the tongue; and in the palate, epiglottis, larynx, and esophagus. Figure 30-2
Taste. A. The taste buds of the anterior two-thirds of the tongue are innervated 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 are innervated by gustatory fibers that travel in the lingual branch of the glossopharyngeal nerve (IX). [Adapted from ER Kandel et al (eds): Principles of
Neural Science, 4th ed, New York, McGraw-Hill, 2000; with permission.] B. The main types of taste papillae are shown in schematic cross sections. Each type predominates 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 taste bud to the taste pore, where the apical microvilli of taste cells have contact with tastants dissolved in saliva and taste pore mucus. Access of tastants to the basolateral regions of these cells is generally prevented by tight junctions between taste cells. Taste cells are short-lived cells that are replaced from stem cells at the base of the taste bud. Three types of taste cells in each taste bud (light cells, dark cells, and intermediate cells) may represent different stages of differentiation or different cell lineages. Taste stimuli, detected at the apical end of the taste cell, induce action potentials that cause the release of neurotransmitter at synapses formed at the base of the taste cell with gustatory fibers that transmit signals to the brain. Tastants gain access to the receptor cells through the taste pore. Four classes of taste have been traditionally recognized: sweet, salt, sour, and bitter, and more recently "umami" (monosodium glutamate, disodium gluanylate, disodium inosinate). Tastants enter the taste pore in a solution and initiate transduction by either activating receptors coupled to G-proteins or by directly activating ion channels on the microvillae within the taste bud. Individual gustatory afferent fibers almost always respond to a number of different chemicals. As with olfaction
and other sensory systems, intensity appears to be encoded by the quantity of neural activity. The sense of taste is mediated through the facial, glossopharyngeal, and vagal nerves. The chorda tympani branch of the facial nerve subserves taste from the anterior two-thirds of the tongue. The posterior third of the tongue is supplied by the lingual branch of the glossopharyngeal nerve. Afferents from the palate travel with the greater superficial petrosal nerve to the geniculate ganglion and then via the facial nerve to the brainstem. The internal branch of the superior laryngeal 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 the nucleus of the tractus solitarius. The central pathway from the nucleus of the tractus solitarius projects to the ipsilateral parabrachial nuclei of the pons. Two divergent pathways project from the parabrachial nuclei. One ascends to the gustatory relay in the dorsal thalamus, synapses, and continues to the cortex of the insula. There is also evidence for a direct pathway from the parabrachial nuclei to the cortex. (Olfaction and gustation appear to be unique among sensory systems in that at least some fibers bypass the thalamus.) The other pathway from the parabrachial nuclei goes to the ventral forebrain, including the lateral hypothalamus, substantia innominata, central nucleus of the amygdala, and the stria terminalis.