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Olfaction (also known as olfactics or smell) refers to the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates. For air-breathing animals, the olfactory system detects volatile or, in the case of the accessory olfactory system, fluid-phase chemicals. For water-dwelling organisms, e.g., fish or crustaceans, the chemicals are present in the surrounding aqueous medium. Olfaction, along with taste, is a form of chemoreception. The chemicals themselves which activate the olfactory system, generally at very low concentrations, are called odors. As described by the Roman philosopher Lucretius (1st Century BCE), different odors are attributed to different shapes and sizes of odor molecules that stimulate the olfactory organ[citation needed]. The modern counterpart to 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 expressing for odor reception.[1] Of these genes, only a portion are functional odor receptors. Humans have far fewer active odor receptor genes than other mammals and primates[2] Each olfactory receptor neuron expresses only one functional odor receptor.[citation needed] 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)[3]. An alternative theory, the vibration theory proposed by Luca Turin[4][5], posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by electron tunnelling. However, the behavioral predictions of this theory have been called into question.[6] As of yet, there is no theory that explains olfactory perception completely. However, research is still being done, and institutes like the Monell Chemical Senses Center are working to uncover the secrets of olfactory perception.
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