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

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.