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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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Neuroscience. 2nd edition.

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Odorant Receptors and Olfactory Coding

Olfactory receptor molecules (Figure 15.6B) are homologous to a large family of other G-protein-linked receptors that includes β-adrenergic receptors and the photopigment rhodopsin. Odorant receptor proteins have seven membrane-spanning hydrophobic domains, potential odorant binding sites in the extracellular domain of the protein, and the ability to interact with G-proteins at the carboxyl terminal region of their cytoplasmic domain. The amino acid sequences for these molecules also show substantial variability, particularly in regions that code for the membrane-spanning domains.

The specificity of olfactory signal transduction is presumably the result of this variety of odorant receptor molecules present in the nasal epithelium. In rodents (the mouse has been the animal of choice for such studies because of its well-established genetics), genes identified from an olfactory epithelium cDNA library have defined about 1000 different odorant receptors, making this the largest known gene family. In humans, the number of olfactory receptor genes is smaller (about 500–750). Since approximately 75% of these genes do not encode full-length proteins, the number of functional human receptors is about 100–200. This relatively small number of odorant receptor types may reflect our poor sense of smell compared to other species. Nevertheless, the combined activity of this number of receptors is easily large enough to account for the number of distinct odors that can be discriminated by the human olfactory system (estimated to be about 10,000).

Messenger RNAs for different olfactory receptor genes are expressed in subsets of olfactory neurons that occur in bilaterally symmetric patches of olfactory epithelium defined by the expression of receptors. Genetic analysis shows that each olfactory receptor neuron expresses only one or at most a few of the 1000 or so odorant receptor genes. Thus, different odors activate molecularly and spatially distinct subsets of olfactory receptor neurons. In short, individual odorants can activate multiple receptors, and individual receptors can be activated by multiple odorants.

Like other sensory receptor cells, olfactory receptor neurons are sensitive to a subset of chemical stimuli that define a “tuning curve.” Depending on the particular olfactory receptor molecules they contain, some olfactory receptor neurons exhibit marked selectivity to particular chemical stimuli, whereas others are activated by a number of different odorant molecules (Figure 15.7A). In addition, olfactory receptor neurons can exhibit different thresholds for a particular odorant. That is, receptor neurons that are inactive at concentrations sufficient to stimulate some neurons are activated when exposed to higher concentrations of an odorant. These characteristics suggest why the perception of an odor can change as a function of its concentration (Figure 15.7B).

Figure 15.7. Responses of olfactory receptor neurons to selected odorants.

Figure 15.7

Responses of olfactory receptor neurons to selected odorants. (A) Neuron 1 responds similarly to three different odorants. In contrast, neuron 2 responds to only one of these odorants. Neuron 3 responds to two of the three stimuli. The responses of these (more...)

How these olfactory responses convey the type and concentration of a given odorant is a complex issue that is unlikely to be explained at the level of the primary neurons. Nevertheless, neurons with specific receptors are located in particular parts of the olfactory epithelium. These neurons project to specific subsets of glomeruli in the olfactory bulb. Thus, the regions of the olfactory epithelium and bulb that are stimulated by particular odorants are clearly significant (Figure 15.8). As in other sensory systems, this topographical arrangement is referred to as space coding, although the meaning of this phrase in the olfactory system is much less clear than in vision, for example (where a topographical map correlates with visual space). The coding of olfactory information also has a temporal dimension. Sniffing, for example, is a periodic event that elicits trains of action potentials and synchronous activity of populations of neurons. Information conveyed by timing is called temporal coding and occurs in a variety of species (Box B). How, and whether, spatial or temporal coding contributes to olfactory perception is just beginning to be elucidated.

Figure 15.8. The organization of the mammalian olfactory bulb.

Figure 15.8

The organization of the mammalian olfactory bulb. (A) When the bulb is viewed from its dorsal surface (visualized here in a living mouse in which the overlying bone has been removed), olfactory glomeruli can be seen. The dense accumulation of dendrites (more...)

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Box B

Temporal “Coding” of Olfactory Information in Insects. Most studies of olfaction in mammals have emphasized the spatial patterns of receptors in the nose and glomeruli in the bulb that are activated by specific odorants. However, beginning (more...)

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By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2001, Sinauer Associates, Inc.
Bookshelf ID: NBK10824

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