Olfaction odor discrimination

L Etoile N. D. and Bargmann C. I. (2000) Olfaction and odor discrimination are mediated by the C. elegans guanylyl cyclase ODR-1. Neuron 25, 575-586. 

The inability to taste food is a common complaint when nasal congestion reduces the sense of smell. Thus, smell greatly augments our sense of taste (also known as gustation), and taste is, in many ways, the sister sense to olfaction. Nevertheless, the two senses differ from each other in several important ways. First, we are able to sense several classes of compounds by taste that we are unable to detect by smell salt and sugar have very little odor, yet they are primary stimuli of the gustatory system. Second, whereas we are able to discriminate thousands of odorants, discrimination by taste is much more modest. Five primary tastes are perceived bitter, sweet, sour, salty, and umami (the taste of glutamate from the Japanese word for "deliciousness"). These five tastes serve to classify compounds into potentially... 

An interaction between main olfaction and the trigeminal somatosensory system has been proposed to facilitate directional smelling. For carbon dioxide and menthol, human subjects were able to tell the odor direction in 96% of cases, while the results for hydrogen sulfide and vanillin were random (Kobal et ah, 1989). Rats can discriminate odor direction in one sniff (Rajan et ah, 2006). 

Vertebrates possess three primary chemosensory systems gustation ( taste ), trigeminal, and olfaction ( smell ) but only one of these, the olfactory system, mediates responses to pheromones. Chemicals that stimulate the olfactory system are known as odorants and comprise one type of biological cue (any entity that stimulates a sensory system). Bouquets of odorants that can be discriminated as specific entities are termed odors. The olfactory system contains olfactory receptor neurons (ORNs) that comprise cranial nerve I and project directly to the forebrain. ORNs are now known to express only one to a few olfactory receptor proteins ( receptors ), which means that the chemoreceptive range of each neuron can be very narrow. The olfactory system also has several subcomponents including the vomeronasal organ, which is described below. 

Olfaction, the sense of smell, is an important neural system in various animal species, including fish, for their life. Fish can detect a variety of odorants emitted from objects and dissolved in the water, such as amino acids, bile salts, nucleotides, polyamines, prostaglandins, and steroids. The fish olfactory system is extensively developed to receive and discriminate these odorant molecules, to transmit their signals to the brain, and to mediate fundamental behaviors such as food finding, alarm response, predator avoidance, social communication, reproductive activity, and spawning migration (Sorensen and Caprio 1998 Zielinski and Hara 2007). 

The ability to discriminate different molecules constitutes a criterion for olfaction. Because, as mentioned, anosmic persons can tell some pairs of odors apart based on nonolfactory cues, an experimenter must choose with care the compounds for study. 3-Phenethyl alcohol has an odor that many people find reminiscent of roses, and vapors from dilute solutions are widely accepted as an olfactory stimulus that does not interact with other chemosensory modalities in humans (Betcher Doty, 1998). Consider a human subject who can detect J3-phenethyl alcohol with the same sensitivity as nor-mosraics and can also detect -butanol (another alcohol often used for testing olfactory sensitivity (Hummel et al., 1997), which has an odor very different from that of P-phenethyl alcohol) with normal acuity. Suppose this subject cannot distinguish the two odors. How can an experimenter assess whether the subject exhibits the sense called olfaction ... 

H. M. clearly understands how to discriminate chemical stimuli. His impairment limits only his recognition of odors, but no other aspect of his sense of smell. He is "odor deaf," by analogy to stroke victims who can read and write and retain an intact sense of hearing but cannot recognize words aurally (and are said to be "word deaf ) (Takahashi et al., 1992). Based on the evidence discussed so far, H. M. s behavior does not satisfy the fourth of the guidelines for olfaction. 

By considering the role of timing of odorant delivery in biological olfaction (Rubin Cleland 2006), we have recently built a novel machine olfaction technology, termed an artificial olfactory mucosa , which demonstrates clearly a third principle of odour discrimination in artificial olfactory systems ... 

Olfaction is the primary modality for social recognition and communication in nocturnal rodents (Johnston, 1983 Halpin, 1986, 1991). For example, most mammals that have been tested discriminate between the odors of individual conspecifics that are not close relatives (e.g., Johnston, et al., 1993 Johnston Jernigan, 1994 Todrank Heth, 1996) and many species may use these individually distinctive odor cues for recognition of kin versus non-kin as well (e.g. Block, et al., 1981 Hepper, 1983). Most secretions that are individually discriminated also contain information about other attributes of the individual, such as the sex, reproductive state, and species of the odor donor (Johnston, 1983 Heth, Beauchamp, Nevo Yamazaki, 1996). 

For both olfaction and chemesthesis, sensitivity, and any differences in sensitivity, where they exist, can be determined in populations of interest. Furthermore, by establishing olfactory detection and chemesthetic localization thresholds, one can then know when an odorant becomes an irritant. These quantitative measures then allow discrimination between eognitive and sensory irritation. 

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