Figure 13-7. Location of gustatory receptors on a mammalian tongue; four major classes.

j-foliate papillae j-foliate papillae jS filiform ' papillae

Figure 13-7. Location of gustatory receptors on a mammalian tongue; four major classes.

The taste mechanism in vertebrates is patterned on the tongue and develops separately from the olfactory and vomeronasal systems. Taste receptor cells have voltage-gated ion channels, which in vertebrates synapse into three of the primary cranial nerves (facial, glosopharyngeal, and vagus); remarkably, this is true even of distal taste buds on the body surface of fish (Finger 1997). Taste neurons are secondary receptors, which is in contrast to olfactory neurons, which are primary receptors; that is, taste receptors have no axon but synapse with sensory nerves that go to the brain. Different taste regions use different molecular reception mechanisms because of the chemical properties of the molecules being detected. Salt and acids can pass directly through ion channels, whereas sugars and bitter substances must activate a second messenger to be detected. In insects, the neurons are bipolar with a distal process that extends to the surface of the epithelium, usually ending at an opening in the cuticle or exoskeleton, which chemicals can penetrate, and a central process extending into the CNS (Finger and Simon 2000).

Gustation and olfaction are related in interesting ways, not least of course being their use of related chemoreceptor genes. The organization of taste receptors resembles that of the olfactory epithelium, but this is due to developmentally different patterning events. The classic senses of salt, sweet, sour, and bitter (and umami, the taste elicited by glutamate, found naturally in some foods and added to others as monosodium glutamate, or MSG) are detected by receptors for specific chemicals, and the long-held idea (going back to the classics) that these are located exclusively in different parts of the tongue has been shown to be incorrect. The regional differentiation of the olfactory epithelium and VNO is not highly correlated (if at all) with particular odorant properties; similar ORs are located similarly, but odorant perception is not obviously regionalized in the nose. "Taste" as we usually refer to it, really is an integral use of taste and smell receptors. However, taste receptors map to distinct regions of the vertebrate brain even though the senses lead to related and integrated percepts. A separation of smell and taste in the brain occurs also in insects, but the distinction is less clear and at least some insect GR genes also function in olfactory circuits.

OTHER VERTEBRATE CHEMOSENSORY MECHANISMS Chemosensation is a very general phenomenon, and two mechanisms unrelated to immune detection and olfaction merit mention (e.g., Finger 1997). One is the human "common chemical sense," which is a property of free somatosensory nerve endings in epithelia such as on the exposed surfaces of eyes, nose, mouth, and throat. These respond to substances such as ammonia, mint, and pepper and provide sensations of burning and coolness, often leading to avoidance reactions. A second and perhaps related system involves the solitary chemoreceptor cell (SCC). Cells of this type are secondary sensory neurons, as in taste cells, and are located on external surfaces of nonamniotic aquatic craniates and used in feeding and predator avoidance. The SCC may be related evolutionarily to taste but is connected to the somatosensory system.

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