Smell

The olfactory system is a phylogenetically ancient sensory system capable of detecting and discriminating among a vast number of different odorants. Olfaction is critical to the survival of a variety of lower animals ranging from insects to mammals, while in humans it has been considered less important than the other senses. Understanding how the olfactory system encodes and decodes information is not an easy task, given the lack of a clear physical energy continuum to characterize and control stimulus presentation, like wavelength for COLOR VISION (see VISUAL ANATOMY AND PHYSIOLOGY) or frequency for auditory pitch (see AUDITION and AUDITORY PHYSIOLOGY). The situation is made even more complex by the findings that similar chemical substances can sometimes have quite different odors and some substances with altogether different chemical formulas can smell alike.

The olfactory organ of vertebrates is a complex structure designed to collect odorant molecules and direct them to the sensory neurons. Although the chemoreceptive endings and neural projections of the olfactory nerve are primary to the sense of smell, other cranial nerves are involved, namely, the trigeminal, glossopharyngeal, and vagus. These accessory cranial nerves possess at least some chemoreceptive endings which line the nose, pharynx, and larynx, giving rise to the pungent or irritating quality often experienced as part of an odor sensation.

For much of the animal kingdom olfaction is basic to the maintenance of life, regulating reproductive physiology, food intake, and social behavior. In fact, the essence of ANIMAL COMMUNICATION is chemical, relying on odors produced by body glands, feces, and urine. For example, the male silk moth uses olfactory cues to find his mate, as does the adult salmon to return to the place where it was spawned. Many mammalian species, ranging from deer to cats and dogs, mark their territory with urine or other secretions. These chemical messages provide the animal sampling the scent mark with information regarding whether it came from a conspecific, if the depositor was male or female, dominant or submissive, and even its reproductive status.

There is also a dependency of reproductive and sexual behavior on olfactory cues. Introducing the odor of a male mouse or his urine can induce and accelerate the estrus cycle of a female. Moreover, appropriate odor cues from a female are important in attracting the male's interest during estrus and promoting copulation. In some species sexual dysfunction and even retarded development of the sex organs results when olfaction is compromised.

In humans, the sense of smell is considered less critical to survival than the other special senses, although the detection of stimuli such as smoke, gas, or decaying food prevents bodily harm. Instead, civilized society appears to emphasize the importance of olfaction on the quality of life. People attempt to modify attractiveness by adding perfumes to their bodies and incense to their homes. Consider the plethora of commercial products for use against "bad" odors. One instance in which smell plays a major role is in flavor perception and the recognition of tastes. Much of what people think they TASTE they actually smell and a large percentage of people coming to chemosensory clinics complaining of taste problems actually have smell dysfunction (consider what happens to food appreciation when a cold strikes). In fact, disorders of the sense of smell can often be profoundly distressing, as well as harbingers of more general disease states.

Although not as extensively documented as in animals, a relationship between olfaction and sex seems likely in humans. Olfactory acuity in women seems better at ovulation than during menstruation and there is evidence that olfactory cues (i.e., human pheromones) among women can synchronize the menstrual cycle and that odors serve as attractants to the opposite sex.

In vertebrates, olfactory receptor neurons (ORNs) differ in the number and profile of odorants to which they respond (see NEURON). For example, one electrophysiological study of single ORN responses to twenty different stimuli demonstrated that individual neurons responded to as few as two of the odorants within the panel. Furthermore, despite sampling over fifty neurons, each had a distinct odorant response profile. Thus, ORN responses define the range of odorants that can elicit a response in a given cell (termed its molecular receptive range [MRR], analogous to the spatial receptive field in the visual system). Emerging evidence further suggests that a cell's MRR may reflect interactions with particular ligand determinants (compounds with similar organization of carbon atoms or functional groups). Accordingly, if ORNs are to be classified as to type on the basis of their response to the odorant universe, then the number of such types must likely exceed fifty.

Recently, a large olfactory-specific gene family (numbering 500 to 1000 genes in humans) has been identified. The proteins encoded by these genes are expressed in ORNs and are considered to be putative odorant receptors (PORs) based on their structural and sequence homology with other known G protein - activating receptors (and G protein cascades have been implicated in the transduction of olfactory stimuli). At present, these PORs remain functionally anonymous. Nonetheless, the extremely large size of the gene family is considered strong evidence that these are odorant receptors, fulfilling the criterion of diversity to interact with an immense number of different odorants.

In other sensory systems (i.e., audition, vision, and somesthesis; see HAPTIC PERCEPTION), receptor cells encode specificity about the sensory stimulus by virtue of their exact placement in the receptor sheet. In contrast, the receptor sheet in olfaction does not form a spatial map about the environment. Instead, as previously noted, the responsivity of an ORN may result from the affinity of its receptor for a particular odorant ligand. So how is this molecular information mapped into the nervous system? Studies of the ensemble properties of the olfactory epithelium suggest that odorant quality information is encoded in large-scale spatial patterns of neural activity. That is, direct presentation of odorants to the exposed olfactory epithelium reveal intrinsic spatial differences in the sensitivity to different odorants across this neural tissue. The biological basis for these intrinsic patterns likely stems from the differential distribution of POR expression across the epithelium. Neurons expressing a particular POR are segregated to one of four broad nonoverlapping zones. Within a zone some PORs are dispersed throughout the anterior-posterior extent of the epithelium, while others are clustered in more limited areas.

Despite the fact that the vertebrate olfactory system does not process information about odors by virtue of a single type of ORN's placement in the receptor sheet, a degree of rhinotopy (analogous to retinotopy in the visual system) does exist in the organization of the central projection. The foundation for this rhinotopy lies in the topographical organization of bulbar glomeruli (neuropil structures comprised of ORN axon terminals and the distal dendrites of mitral, tufted, and periglomerular cells) in relation to the expression of POR type. An entire subset of ORNs expressing a particular POR send their axons to converge on a single glomerulus in the olfactory bulb. As a result, the odorant information contained in the large-scale differential activation of subsets of ORNs can be encoded by producing differential spatial patterns of activity across the glomerular layer of the olfactory bulb. This activity is further sharpened in the relay neurons of the bulb via complicated local circuits that act both locally and laterally on the signals impinging on the bulb.

Exactly how the aforementioned parameters of neural excitation lead to perception is unknown. However, recent work combining animal PSYCHOPHYSICS with neurophysiological techniques suggest a predictive relationship between the large-scale spatial patterning of neural excitation and odorant quality perception.

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-- Steven Youngentob

Smell

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