The Organization of the Olfactory System - Neuroscience - NCBI Bookshelf

From an evolutionary perspective, the chemical senses—particularly olfaction—are deemed the “oldest” sensory systems; nevertheless, they remain in many ways the least understood of the sensory modalities. The olfactory system (Figure 15.1) is the most thoroughly studied component of the chemosensory triad and processes information about the identity, concentration, and quality of a wide range of chemical stimuli. These stimuli, called odorants, interact with olfactory receptor neurons in an epithelial sheet—the olfactory epithelium—that lines the interior of the nose (Figure 15.1A,B). The axons arising from the receptor cells project directly to neurons in the olfactory bulb, which projects in turn to the pyriform cortex in the temporal lobe (Figure 15.1C). The olfactory system is thus unique among the sensory systems in that it does not entail a thalamic relay en route to the primary cortical region that processes the sensory information. The olfactory tract also projects to a number of other targets in the forebrain, including the hypothalamus and amygdala. Projections from the pyriform cortex and other forebrain regions, via the thalamus, provide olfactory information to several additional regions of the cerebral cortex (see Figure 15.1C). The further processing that occurs in these various regions identifies the odorant and initiates appropriate motor, visceral, and emotional reactions to olfactory stimuli.

Despite its phylogenetic “age” and unusual trajectory to the cortex, the olfactory system abides by the basic principle that governs other sensory modalities: Interactions of chemical stimuli with receptors at the periphery are transduced and encoded into electrical signals, which are then transmitted to higher-order centers. Nevertheless, less is known about the central organization of the olfactory system than other sensory pathways. For example, the somatic sensory and visual cortices described in the preceding chapters all feature spatial maps of the relevant receptor surface, and the auditory cortex features frequency maps. Whether any analogous maps exist in the pyriform cortex (or the olfactory bulb) is not yet known. Indeed, until recently it has been difficult to imagine on what sensory qualities an olfactory map would be based, or what features might be processed in parallel (as occurs in other sensory systems).