As we saw in Chapter 13, the receptors for the sense of taste in vertebrates are grouped together in taste buds, each of which contains about 100 taste receptor cells. Taste buds in mammals are located primarily on the tongue, but also on the pharynx, the laryngeal epiglottis, and at the entrance of the esophagus. Some fish have taste buds over the entire surface of the body (Butler and Hodos 1996).
The surface of each taste cell is covered with microvilli, which protrude through a pore in the cell (the taste pore) to be exposed to the oral cavity. They are specialized to detect chemicals dissolved in the saliva on the tongue. The microvilli in turn are clustered into papillae that are embedded in the epithelial layer of the tongue. Afferent nerve axons enter the taste bud at the base and each one synapses with multiple receptor cells in the taste bud.
The taste cells can detect five basic stimuli: bitter, salty, sour, sweet, and umami. The ability to taste amino acids seems to be particularly acute in fishes, but it is not obvious why this particular sense developed. Combinations of the five basic tastant stimuli lead to perceptions of complex tastes. Each stimulus is transduced by a different mechanism, and the same taste may be elicited by different stimuli and different mechanisms. In addition, the mechanism used to sense the same stimulus may differ between vertebrate species.
Sour and salty tastants do not have a receptor molecule in the receptor cell in the usual sense; these tastes are due to the hydrogen or sodium ions in the substance. They are not detected with the usual receptors but by changing the membrane potential of ion channels in the taste receptor cell. Sweet, bitter, and umami reception are more complex, and transduction of these tastes is more similar to phototransduction and olfactory transduction: the tastant is bound by a G protein membrane receptor, which in turn activates intracellular signals that affect the ionic permeability of the taste receptor cell. Because many molecules can be perceived as sweet or bitter, receptors for these tastants are many and varied.
Two nerves carry input from the tongue to the brain: the facial nerve (cranial nerve VII) conveys information from the front of the tongue and the glossopharyngeal nerve (cranial nerve IX) from the back. These two nerves also contain the sensory fibers for transduction of touch, pressure, and temperature. Axons from these nerve bundles enter the brain stem in the medulla and synapse in a thin line of cells called the nucleus of the solitary tract. As with other sensory systems, the somatotopic organization of taste is maintained in the solitary, or gustatory, nucleus, such that gustatory nerves from different regions of the head, and skin in animals with taste buds on the body, enter this area in the order in which they are located on the body. From there, depending on the extent of development of the animal's thalamocortical system, input is conveyed to the gustatory region of the somatosen-sory cortex, the primary gustatory cortex, where the perception of taste is formed, and, in animals with more fully developed forebrains, to the hypothalamus, amygdala, and insula, the limbic area, where behavioral responses to the perception of taste—aversion, salivation, gastric secretion, pleasure, etc.—are triggered.
Fish have a very well developed sense of taste in general, with taste receptors on their lips and in their mouth and pharynx, but several suborders have evolved taste receptors over the entire surface of their bodies. In these fish, the gustatory nervous system is very complex and supplies them with a detailed topographical taste map of their surroundings. These silurids (catfishes, of which there are more than 1,000 species) and cyprinids (carps, minnows, chubs, and goldfish) are bottom feeders and have evolved an elaborate system for separating inedible particles from food as they pick up mouthfuls of sediment from the bottom of the stream or ocean (Butler and Hodos 1996).
In insects, detection of soluble chemicals by gustatory neurons can elicit feeding behavior, but mating behavior as well (Scott et al. 2001). Drosophila, for example, have chemosensory hairs on their legs and proboscis that activate proboscis extension and feeding when they detect sweet compounds. Female Drosophila have bristles on their genitalia that elicit ovipositing upon detection of nutrients, which probably maximizes the probability that eggs are laid in an environment in which the newly hatched can feed.
Taste receptors in Drosophila are found in sensory sensilla located on the fly wing, legs, proboscis, and genitalia. Two types of sensilla have been characterized: taste bristles, which are located on the legs, wings, ovipositor, and mouthparts, and taste pegs, on the oral surface and in the pharynx. Bristles are hollow hairlike lymph-filled structures with a pore at the terminal end, through which nutrients enter to dissolve in the lymph. Each bristle contains neurons that are specific for sugar, salt, or water, as well as a single mechanoreceptor neuron (Shanbhag et al. 2001). Taste pegs, in contrast, do not have obvious pores in the termini or side-walls, but their role in gustation has been shown behaviorally in blowflies, and electrophysiological assays suggest that they have sugar and salt receptors, as well as a mechanorecep-tor. Stimulation of these cells induces feeding (Shanbhag et al. 2001).
Adult Drosophila have about 2,000 chemosensory neurons in the sensilla. Up to four neurons innervate each sensillum. Taste pegs are innervated by two receptor cells: a chemoreceptor and a mechanoreceptor (Shanbhag et al. 2001). Sensory neurons from the proboscis extend to the subesophogeal ganglion (SOG) in the brain and then are relayed to other gustatory areas for further processing. Gustatory neurons from other parts of the body project locally to peripheral ganglia. How taste is represented in the Drosophila brain is not yet known.
Differential taste responses could arise from different locations on the body or tongue as a consequence of the developmental patterning mechanism that generates the taste buds or receptors. That there is a topographic map of taste receptors in terms of their location of the body is no surprise; it provides a sense of where a given tastant is being detected, but this need not have any particular evolutionary function and instead may just be a useful result of developmental patterning. But how different and variable locations on insect bodies are all interpreted as taste, or whether something "tastes" different depending on which sensors it activates, are more intriguing questions.
Odorant detection is odorant specific at the level of the cell in many animal species. This has to do with the developmental allelic and gene exclusion in generating the olfactory neuron. Taste is generally a different kind of detection, more general, and it is easy to understand why each taste bud might be sensitive to multiple tastant characters: it could enable the organism to determine the generic "flavor" of the stimulating substance, rather than its specifics. At the same time, it could also simply be the consequence of the way taste-sensing units develop.
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