Neuroscience 6e Chapter 15 Summary
The chemical senses—olfaction, vomeronasal sensation, and taste—all contribute to sensing airborne or soluble molecules from a variety of sources. Humans and other mammals rely on this information for behaviors as diverse as attraction, reproduction, feeding, and avoiding potentially dangerous circumstances. Receptor neurons in the olfactory epithelium transduce chemical stimuli into neuronal activity by stimulation of a large family of G-protein-coupled receptors that elicit second-messenger-mediated regulation of Na+, Ca2+, and Cl– ion channels. These events generate olfactory receptor potentials, and ultimately action potentials, in the afferent axons of these cells. The large number of odorant receptor molecules in most species is believed to establish sensitivity to the myriad odors that animals can discriminate. In most vertebrates except humans, the vomeronasal pathway provides a parallel pathway for the detection of pheromones from conspecifics (attractive cues) and kairomones from predators or prey (aversive or attractive cues). The peripheral vomeronasal receptor neurons appear similar to ORNs, but they express a different family of G-protein-coupled receptors and the intercellular transduction mechanisms for generating action potentials is distinct from that in ORNs. Taste receptor cells, in contrast, use a variety of mechanisms for transducing a more limited range of chemical stimuli. Each of the five perceptual categories of taste—salty, sour, sweet, amino acids (also known as umami), and bitter—are encoded by receptor cells that express distinct receptor proteins. Salts and protons (acids, which elicit a sour taste) directly activate two different ion channels, and for sweet, amino acid, and bitter tastes there are specific sets of G-protein-coupled receptors. Olfaction, vomeronasal sensation, and taste are all relayed via specific pathways in the CNS. ORNs project directly from the periphery to the olfactory bulb, and the olfactory bulb projects primarily to the pyriform cortex. Vomeronasal receptor neurons project directly to the accessory olfactory bulb, which in turn projects to targets in the hypothalamus and amygdala. In the taste system, cranial sensory ganglion neurons relay information from taste cells to the solitary nucleus in the brainstem. The solitary nucleus projects, via the thalamus, to the taste area of the cerebral cortex, where each of the five taste categories is represented in a distinct, non-overlapping domain. Each of these systems ultimately processes chemosensory information in ways that give rise to some of the most sublime pleasures humans can experience.