The second chemical sense is gestation, or taste. For the sense of taste, chemicals called tastants are detected and transduced by chemoreceptors located in taste buds. Tastes are mixtures of five elementary taste qualities: salty, sweet, sour, bitter, and umami (savory, including monosodium glutamate).
Disorders associated with the sense of taste are not life threatening, but they can impair the quality of life, impair nutritional status, and increase the possibility of accidental poisoning. Taste disorders include ageusia (absence of taste), hypogeusia (decreased taste sensitivity), hypergeusia (increased taste sensitivity), and dysgeusia (distortion of taste, including taste sensation in the absence of taste stimuli).
Taste Buds and Receptors
Taste receptor cells are located within taste buds on the tongue, palate, pharynx, and larynx. The taste buds on the tongue are found in taste papillae, which include as many as several hundred taste buds. The taste buds are anatomically similar to the olfactory epithelium and consist of three cell types: supporting cells, basal cells, and receptor cells (Fig. 3-27).
Figure 3–27 Structure of a taste bud.
Supporting cells are found among the taste receptor cells. These cells do not respond to taste stimuli, and their function is not known.
Basal cells are undifferentiated stem cells that serve as precursors to taste receptor cells (just as basal cells serve as precursors to olfactory receptor cells). Basal cells undergo continuous replacement. New cells, which are generated approximately every 10 days, migrate toward the center of the taste bud and differentiate into new receptor cells. New receptor cells are needed to replace those cells that are sloughed from the tongue.
Taste receptor cells are the chemoreceptors of the taste system. They line the taste buds and extend microvilli into the taste pores. These microvilli provide a large surface area for detection of chemical stimuli. In contrast to the olfactory system (in which the receptor cells are the primary afferent neurons), in the gustatory system, the receptor cells are not neurons. They are specialized epithelial cells that function as chemoreceptors, transducing chemical stimuli into electrical signals. Afferent fibers innervate the taste receptor cells and transmit this information to the CNS.
Taste buds on the tongue are organized in specialized papillae (Fig. 3-28). Three types of papillae contain taste buds: circumvallate, foliate, and fungiform.
Figure 3–28 Structure of taste papillae lined with taste buds.
Circumvallate papillae are the largest in size but fewest in number. They are arranged in rows at the base of the tongue. Each circumvallate papilla is surrounded by a trench, with taste buds located along the sides of the trenches. Because of their large size, approximately half the total number of taste buds are found in circumvallate papillae. The taste cells in circumvallate papillae are innervated by CN VII and IX.
Foliate papillae are located on the lateral borders of the tongue. Taste buds are located in folds on the sides of the papillae.
Fungiform papillae are scattered on the dorsal surface of the tongue and are most numerous near the anterior tip. They are mushroom shaped (“fungiform”), with each papilla containing anywhere from three to five taste buds. The fungiform papillae are translucent with a dense blood supply, making them appear as red spots on the surface of the tongue. The taste cells in fungiform papillae are innervated exclusively by the chorda tympani branch of CN VII.
Detection of the five basic taste qualities involves differential sensitivity of areas of the tongue (Fig. 3-29). Although all five taste qualities can be detected over the full surface of the tongue, different regions of the tongue do have different thresholds. The tip of the tongue is most responsive to sweet, salty, and umami, whereas the posterior tongue is most responsive to bitter, and the sides of the tongue are most responsive to sour.
Figure 3–29 Organization of taste papillae on the tongue. The circumvallate, foliate, and fungiform papillae and the chemicals they detect are shown.
The chemical signals for the five taste qualities are transduced by the mechanisms shown in Figure 3-30. In most cases, transduction ultimately results in depolarization of the taste receptor membrane (i.e., a depolarizing generator potential). This depolarization leads to action potentials in afferent nerves innervating that portion of the tongue. For bitter sensation, the tastant molecules bind to G protein–coupled receptors on the taste receptor membrane and, mediated by an inositol 1,4,5-triphosphate (IP3)/Ca2+ mechanism, opens so-called transient receptor potential (TRP) channels and results in depolarization. For sweet and umami sensations, molecules bind to a different class of G protein–coupled receptors on the taste receptor cell membrane and, mediated by IP3/Ca2+, open TRP channels and cause depolarization. For sour sensation (mediated by H+), H+ enters the taste receptor through epithelial Na+channels (ENaC), leading to depolarization. For salty sensation (mediated by Na+), Na+ enters the taste receptor through the same epithelial Na+ channels, leading directly to depolarization.
Figure 3–30 Mechanisms of transduction in taste receptor cells. ENaC, Epithelial Na+ channel; IP3, inositol 1,4,5-triphosphate; TRP, transient receptor potential.
Encoding Taste Stimuli
How taste qualities are encoded in the CNS is not precisely known. One theory states there is an across-fiber pattern code in which each taste fiber responds best to one stimulus but also responds to a lesser extent to other stimuli. Thus, an afferent taste fiber might respond best to salt but also responds to acid. Another taste fiber might respond best to acid but also responds to bitter. Thus, each afferent taste fiber receives input from a population of taste receptors with a distinctive pattern of responses. The response pattern across many fibers then encodes for a particular taste sensation.
As noted, taste begins with transduction of chemical signals in the taste receptor cells, which are located in taste buds. Transduction leads to depolarizing receptor potentials, which lead to action potentials in primary afferent neurons innervating specific regions of the tongue. Different regions of the tongue are innervated by branches of three cranial nerves. The posterior one third of the tongue (where bitter and sour sensations are most sensitive) is innervated by the glossopharyngeal nerve (CN IX). The anterior two thirds of the tongue (where sweet, umami, and salty sensations are most sensitive) is innervated by the facial nerve (CN VII). The back of the throat and epiglottis are innervated by the vagus nerve (CN X). These three cranial nerves (CN VII, IX, and X) enter the brain stem, ascend in the solitary tract, and terminate on second-order neurons in the solitary nucleus of the medulla. The second-order neurons project ipsilaterally to the ventral posteromedial nucleus of the thalamus. Third-order neurons leave the thalamus and terminate in the taste cortex.