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Olfaction
Olfaction, also known as olfactics, is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, which can be considered analogous to sensory cells of the antennae of invertebrates. In humans, olfaction occurs when odorant molecules bind to specific sites on the olfactory receptors. These receptors are used to detect the presence of smell. They come together at the glomerulus, a structure which transmits signals to the olfactory bulb (a brain structure directly above the nasal cavity and below the frontal lobe). Many vertebrates, including most mammals and reptiles, have two distinct olfactory systems—the main olfactory system, and the accessory olfactory system (used mainly to detect pheromones). For air-breathing animals, the main olfactory system detects volatile chemicals, and the accessory olfactory system detects fluid-phase chemicals. Olfaction, along with taste, is a form of chemoreception. The chemicals themselves that activate the olfactory system, in general at very low concentrations, are called odorants. Although taste and smell are separate sensory systems in land animals, water-dwelling organisms often have one chemical sense.
Volatile small molecule odorants, non-volatile proteins, and non-volatile hydrocarbons may all produce olfactory sensations. Some animal species are able to smell carbon dioxide in minute concentrations.
==Study of olfaction==
Early scientific study of olfaction includes the extensive doctoral dissertation of Eleanor Gamble, published in 1898, which compared olfactory to other stimulus modalities, and implied that smell had a lower intensity discrimination. As the Epicurean and atomistic Roman philosopher Lucretius (1st Century BCE) speculated, different odors are attributed to different shapes and sizes of "atoms" (odor molecules in the modern understanding) that stimulate the olfactory organ (). A modern demonstration of that theory was the cloning of olfactory receptor proteins by Linda B. Buck and Richard Axel (who were awarded the Nobel Prize in 2004), and subsequent pairing of odor molecules to specific receptor proteins. Each odor receptor molecule recognizes only a particular molecular feature or class of odor molecules. Mammals have about a thousand genes that code for odor reception. Of the genes that code for odor receptors, only a portion are functional. Humans have far fewer active odor receptor genes than other primates and other mammals.
In mammals, each olfactory receptor neuron expresses only one functional odor receptor.〔Pinel, John P.J. (2006) Biopsychology. Pearson Education Inc. ISBN 0-205-42651-4 (page 178)〕 Odor receptor nerve cells function like a key-lock system: If the airborne molecules of a certain chemical can fit into the lock, the nerve cell will respond. There are, at present, a number of competing theories regarding the mechanism of odor coding and perception. According to the shape theory, each receptor detects a feature of the odor molecule. Weak-shape theory, known as odotope theory, suggests that different receptors detect only small pieces of molecules, and these minimal inputs are combined to form a larger olfactory perception (similar to the way visual perception is built up of smaller, information-poor sensations, combined and refined to create a detailed overall perception). An alternative theory, the vibration theory proposed by Luca Turin, posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by quantum tunnelling. However, the behavioral predictions of this theory have been called into question.〔 See also the editorial on p. 315.〕 There is no theory yet that explains olfactory perception completely.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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