Olfaction
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2008-07-09T18:35:15Z
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Reverted edits by [[Special:Contributions/81.157.56.223|81.157.56.223]] to last version by Whipmaster (using [[WP:HG|Huggle]])
'''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 [[odor]]s.
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== History ==
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. 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 [[gene]]s expressing for [[Olfactory receptors|odor reception]].<ref>Buck, Linda and Richard Axel. (1991). A Novel Multigene Family May Encode Odorant Receptors: A Molecular Basis for Odor Recognition. ''Cell'' 65:175-183.</ref> Of these genes, only a portion are functional odor receptors. Humans have far fewer active odor receptor [[gene]]s than other mammals and primates<ref>Gilad Y, Man O, Pääbo S, Lancet D (2003) Human specific loss of olfactory receptor genes. Proc Natl Acad Sci U S A 100:3324–3327.</ref>
Each [[olfactory receptor neuron]] expresses only one functional odor receptor.{{Fact|date=June 2007}} 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 [[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)<ref> need citation! </ref>. An alternative theory, the [[vibration theory]] proposed by [[Luca Turin]]<ref>Turin, Luca. (1996). A spectroscopic mechanism for primary olfactory reception. Chemical Senses, 21, 773-791.</ref><ref>Turin, Luca. (2002). A method for the calculation of odor character from molecular structure. Journal of Theoretical Biology, 216, 367-385.</ref>, posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by [[quantum tunneling|electron tunnelling]]. However, the behavioral predictions of this theory have been called into question.<ref>Keller, A and Vosshall, LB. (2004). A psychophysical test of the vibration theory of olfaction. Nature Neuroscience 7:337-338. See also the editorial on p. 315.</ref> As of yet, there is no theory that explains olfactory perception completely.
However, research is still being done, and institutes like the [[Monell Chemical Sense Center]] are working to uncover the secrets of olfactory perception.
==Olfactory system==
===Olfactory epithelium===
In [[vertebrate]]s smells are sensed by [[olfactory sensory neuron]]s in the [[olfactory epithelium]]. The proportion of olfactory [[epithelium]] compared to respiratory epithelium (not innervated) gives an indication of the animal's olfactory sensitivity. Humans have about 10 cm² of olfactory epithelium, whereas some dogs have 170 cm<sup>2</sup>. A dog's olfactory epithelium is also considerably more densely innervated, with a hundred times more receptors per square centimetre.
Molecules of odorants passing through the [[superior nasal concha]] of the nasal passages dissolve in the [[mucus]] lining the superior portion of the cavity and are detected by [[olfactory receptors]] on the [[dendrites]] of the olfactory sensory neurons. This may occur by diffusion or by the binding of the odorant to [[odorant binding protein]]s. The mucus overlying the epithelium contains [[mucopolysaccharides]], salts, [[enzymes]], and [[antibodies]] (these are highly important, as the olfactory neurons provide a direct passage for infection to pass to the [[brain]]).
In [[insects]] smells are sensed by [[olfactory sensory neuron]]s in the chemosensory [[sensilla]], which are present in insect antenna, palps and tarsa, but also on other parts of the insect body. Odorants penetrate into the cuticle pores of chemosensory sensilla and get in contact with insect [[Odorant binding protein]]s (OBPs) or [[Chemosensory protein]]s (CSPs), before activating the sensory neurons.
===Receptor neuron===
The process of how the binding of the [[ligand]] (odor molecule or odorant) to the receptor leads to an [[action potential]] in the receptor neuron is via a [[second messenger]] pathway depending on the organism. In mammals the odorants stimulate [[adenylate cyclase]] to synthesize [[cyclic adenosine monophosphate|cAMP]] via a [[G protein]] called G<sub>olf</sub>. cAMP, which is the second messenger here, opens a [[cyclic nucleotide-gated ion channel]] (CNG) producing an influx of [[cation]]s (largely [[Calcium|Ca]]<sup>2+</sup> with some [[sodium|Na]]<sup>+</sup>) into the cell, slightly depolarising it. The Ca<sup>2+</sup> in turn opens a Ca<sup>2+</sup>-activated [[chloride channel]], leading to efflux of [[chlorine|Cl]]<sup>-</sup>, further depolarising the cell and triggering an [[action potential]]. Ca<sup>2+</sup> is then extruded through a [[sodium-calcium exchanger]]. A calcium-[[calmodulin]] complex also acts to inhibit the binding of cAMP to the cAMP-dependent channel, thus contributing to olfactory adaptation.'''
This mechanism of transduction is somewhat unique, in that cAMP works by directly binding to the [[ion channel]] rather than through activation of [[protein kinase A]]. It is similar to the transduction mechanism for [[photoreceptor]]s, in which the second messenger [[Cyclic guanosine monophosphate|cGMP]] works by directly binding to ion channels, suggesting that maybe one of these receptors was evolutionarily adapted into the other. There are also considerable similarities in the immediate processing of stimuli by [[lateral inhibition]].
Averaged activity of the receptor neurons can be measured in several ways. In vertebrates responses to an odor can be measured by an [[electroolfactogram]] or through calcium imaging of receptor neuron terminals in the olfactory bulb. In insects, one can perform [[electroantenogram]] or also calcium imaging within the olfactory bulb.
The receptor neurons in the nose are particularly interesting because they are the only direct recipient of stimuli in all of the senses which are nerves. Senses like hearing, tasting, and, to some extent, touch use cilia or other indirect pressure to stimulate nerves, and sight uses the chemical Rhodopsin to stimulate the brain.
===Olfactory bulb projections===
Olfactory sensory neurons project [[axon]]s to the brain within the [[olfactory nerve]], ([[cranial nerve]] I). These axons pass to the [[olfactory bulb]] through the [[cribriform plate]], which in-turn projects olfactory information to the [[olfactory cortex]] and other areas. The axons from the [[olfactory receptor]]s converge in the [[olfactory bulb]] within small (~50 [[micrometer (unit)|micrometers]] in diameter) structures called [[glomeruli]]. [[Mitral cell]]s in the olfactory bulb form synapses with the axons within glomeruli and send the information about the [[odor]] to multiple other parts of the olfactory system in the brain, where multiple signals may be processed to form a synthesized olfactory perception. There is a large degree of convergence here, with twenty-five thousand axons synapsing on one hundred or so mitral cells, and with each of these mitral cells projecting to multiple glomeruli. Mitral cells also project to [[periglomerular cell]]s and [[granular cell]]s that inhibit the mitral cells surrounding it ([[lateral inhibition]]). Granular cells also mediate inhibition and excitation of mitral cells through pathways from centrifugal fibres and the anterior olfactory nuclei.
The mitral cells leave the olfactory bulb in the [[lateral olfactory tract]], which synapses on five major regions of the cerebrum: the [[anterior olfactory nucleus]], the [[olfactory tubercle]], the [[amygdala]], the [[piriform cortex]], and the [[entorhinal cortex]]. The anterior olfactory nucleus projects, via the [[anterior commissure]], to the contralateral olfactory bulb, inhibiting it. The piriform cortex projects to the [[medial dorsal nucleus]] of the thalamus, which then projects to the orbitofrontal cortex. The orbitofrontal cortex mediates conscious perception of the odor. The 3-layered piriform cortex projects to a number of thalamic and hypothalamic nuclei, the hippocampus and amygdala and the orbitofrontal cortex but its function is largely unknown. The entorhinal cortex projects to the [[amygdala]] and is involved in emotional and autonomic responses to odor. It also projects to the hippocampus and is involved in motivation and memory. Odor information is easily stored in [[long-term memory]] and has strong connections to [[emotional memory]]. This is possibly due to the olfactory system's close anatomical ties to the [[limbic system]] and [[hippocampus]], areas of the brain that have long been known to be involved in emotion and place memory, respectively.
Since any one receptor is responsive to various odorants, and there is a great deal of convergence at the level of the olfactory bulb, it seems strange that human beings are able to distinguish so many different odors. It seems that there must be a highly-complex form of processing occurring; however, as it can be shown that, while many neurons in the olfactory bulb (and even the pyriform cortex and amygdala) are responsive to many different odors, half the neurons in the orbitofrontal cortex are responsive only to one odor, and the rest to only a few. It has been shown through microelectrode studies that each individual odor gives a particular specific spatial map of excitation in the olfactory bulb. It is possible that, through spatial encoding, the brain is able to distinguish specific odors. However, temporal coding must be taken into account. Over time, the spatial maps change, even for one particular odor, and the brain must be able to process these details as well.
In [[insect]]s smells are sensed by sensilla located on the antenna and first processed by the [[antennal lobe]] (analogous to the [[olfactory bulb]]), and next by the mushroom bodies.
===Pheromonal olfaction===
Many animals, including most mammals and reptiles, have two distinct and segregated olfactory systems: a main olfactory system, which detects volatile stimuli, and an [[accessory olfactory system]], which detects fluid-phase stimuli. Behavioral evidence suggests that these fluid-phase stimuli often function as [[pheromone]]s, although pheromones can also be detected by the main olfactory system. In the [[accessory olfactory system]], stimuli are detected by the [[vomeronasal organ]], located in the vomer, between the [[nose]] and the [[mouth]]. Snakes use it to smell prey, sticking their tongue out and touching it to the organ. Some mammals make a face called [[flehmen]] to direct air to this organ.
In women, the sense of olfaction is strongest around the time of [[ovulation]], significantly stronger than during other phases of the [[menstrual cycle]] and also stronger than the sense in males.<ref> Navarrete-Palacios E, Hudson R, Reyes-Guerrero G, Guevara-Guzman R. "Lower olfactory threshold during the ovulatory phase of the menstrual cycle." ''Biol Psychol.'' 2003 Jul;63(3):269-79. PMID 12853171</ref>
The [[major histocompatibility complex|MHC]] genes (known as [[human leukocyte antigen|HLA]] in humans) are a group of genes present in many animals and important for the [[immune system]]; in general, offspring from parents with differing MHC genes have a stronger immune system. Fish, mice and female humans are able to smell some aspect of the MHC genes of potential sex partners and prefer partners with MHC genes different from their own.<ref>Boehm T, Zufall F. "MHC peptides and the sensory evaluation of genotype." ''Trends Neurosci.'' 2006 Feb;29(2):100-7. PMID 16337283</ref><ref>Santos PS, Schinemann JA, Gabardo J, Bicalho Mda G. "New evidence that the MHC influences odor perception in humans: a study with 58 Southern Brazilian students." ''Horm Behav.'' 2005 Apr;47(4):384-8.</ref>
===Olfaction and taste===
Olfaction, [[taste]] and [[trigeminal nerve|trigeminal]] receptors together contribute to [[flavor]]. The human [[tongue]] can distinguish only among five distinct qualities of [[taste]], while the nose can distinguish among hundreds of substances, even in minute quantities.
===Disorders of olfaction===
The following are disorders of olfaction:<ref>Hirsch, Alan R. (2003) Life's a Smelling Success</ref>
*[[Anosmia]] - lack of ability to smell
*[[Hyposmia]] - decreased ability to smell
*[[Phantosmia]] - "hallucinated smell", often unpleasant in nature
*[[Dysosmia]] - things smell differently than they should
*[[Hyperosmia]] - an abnormally acute sense of smell
==Quantifying olfaction in industry==
[[Image:Nasal Ranger.JPG|thumb|right|Nasal Ranger in use.]]
Scientists have devised methods for quantifying the intensity of odors, particularly for the purpose of analyzing unpleasant or objectionable odors released by an industrial source into a community. Since the 1800s, industrial countries have encountered incidents where proximity of an industrial source or landfill produced adverse reactions to nearby residents regarding airborne odor. The basic theory of odor analysis is to measure what extent of dilution with "pure" air is required before the sample in question is rendered indistinguishable from the "pure" or reference standard. Since each person perceives odor differently, an "odor panel" composed of several different people is assembled, each sniffing the same sample of diluted specimen air. A field olfactometer can be utilized to determine the magnitude of an odor. One example is the Nasal Ranger field olfactometer, which is often utilized in odor studies. [http://www.nasalranger.com]
Many air management districts in the [[USA]] have numerical standards of acceptability for the intensity of odor that is allowed to cross into a residential property. For example, the [[Bay Area Air Quality Management District]] has applied its standard in regulating numerous industries, landfills, and sewage treatment plants. Example applications this district has engaged are the [[San Mateo, California]] wastewater treatment plant; the [[Shoreline Amphitheatre]] in [[Mountain View, California]]; and the [[IT Corporation]] waste ponds, [[Martinez, California]].
==Olfaction in non-human animals==
The importance and sensitivity of smell varies among different organisms; most [[mammal]]s have a good sense of smell, whereas most [[bird]]s do not, except the [[tubenose]]s (e.g., [[petrel]]s and [[albatross]]es), and the [[kiwi]]s. Among mammals, it is well-developed in the [[carnivore]]s and [[ungulate]]s, who must always be aware of each other, and in those that smell for their food, like [[mole (animal)|mole]]s.
Dogs in general have a nose approximately a hundred thousand to a million times more sensitive than a human's. [[Scenthound]]s as a group can smell one- to ten-million times more acutely than a human, and [[Bloodhound]]s, which have the keenest sense of smell of any dogs, have noses ten- to one-hundred-million times more sensitive than a human's. They were bred for the specific purpose of tracking humans, and can detect a scent trail a few days old. The second-most-sensitive nose is possessed by the [[Basset Hound]], which was bred to track and hunt rabbits and other small animals.
The sense of smell is less-developed in the catarrhine [[primate]]s ([[Catarrhini]]), and nonexistent in [[cetacea]]ns, which compensate with a well-developed sense of [[taste]]. In some [[prosimian]]s, such as the [[Red-bellied Lemur]], scent glands occur atop the head. In many species, olfaction is highly tuned to [[pheromone]]s; a male [[silkworm]] moth, for example, can sense a single molecule of [[bombykol]].
Fish too have a well-developed sense of smell, even though they inhabit an aquatic environment. Salmon utilize their sense of smell to identify and return to their home stream waters. Catfish use their sense of smell to identify other individual catfish and to maintain a social hierarchy. Many fishes use the sense of smell to identify mating partners or to alert to the presence of food.
Insects primarily use their [[antenna (biology)|antennae]] for olfaction. Sensory neurons in the antenna generate odor-specific electrical signals called spikes in response to odor. They process these signals from the sensory neurons in the [[antennal lobe]] followed by the [[mushroom body|mushroom bodies]] and [[lateral horn]] of the brain. The antennae have the sensory neurons in the sensilla and they have their [[axons]] terminating in the antennal lobes where they synapse with other neurons there in semidelineated (with membrane boundaries) called glomeruli. These antennal lobes have two kinds of neurons, projection neurons (excitatory) and local neurons (inhibitory). The projection neurons send their axon terminals to mushroom body and lateral horn (both of which are part of the protocerebrum of the insects), and local neurons have no axons. Recordings from projection neurons show in some insects strong specialization and discrimination for the odors presented (especially for the projection neurons of the macroglomeruli, a specialized complex of glomeruli responsible for the pheromones detection). Processing beyond this level is not exactly known though some preliminary results are available.
==References==
<references/>
==See also==
*[[Accessory olfactory system]]
*[[Barosmia]]
*[[Electronic nose]]
*[[Machine olfaction]]
*[[Olfactory fatigue]]
*[[Limbic system]]
==External links==
*[http://www.thenakedscientists.com/HTML/Columnists/peterbrennancolumn.htm Smells and Odours - How Smell Works at thenakedscientists.com]
*[http://www.maxxscent.co.uk MaxxScent - Scent Marketing]
*[http://www.cf.ac.uk/biosi/staff/jacob/teaching/sensory/olfact1.html Olfaction at cf.ac.uk]
*[http://www.thenakedscientists.com/HTML/Columnists/peterbrennancolumn2.htm The importance of smell, and pheromones, to Humans and other Animals at thenakedscientists.com]
*[http://www.flexitral.com/research/review_final.pdf Structure-odor relations: a modern perspective at flexitral.com] (PDF)
*[http://www.leffingwell.com/olfaction.htm Olfaction at leffingwell.com]
*[http://www.leffingwell.com/chirality/chirality.htm Chirality & Odour Perception at leffingwell.com]
*[http://www.sciencedaily.com/releases/2006/08/060803091235.htm ScienceDaily Artille 08/03/2006, Quick -- What's That Smell? Time Needed To Identify Odors Reveals Much About Olfaction at sciencedaily.com]
*[http://www.brown.edu/Administration/News_Bureau/2004-05/04-069.html Scents and Emotions Linked by Learning, Brown Study Shows at brown.edu.com]
*[http://www.senseofsmell.org/feature/odor/index.php Sense of Smell Institute at senseofsmell.org]. Research arm of international fragrance industry's [[The Fragrance Foundation]]
*[http://www.nasalranger.com/ Nasal Ranger.com]
*[http://people.bu.edu/dmattw/ Olfactory Systems Laboratory at Boston University]
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{{Olfactory_system}}
[[Category:Olfaction|*]]
[[Category:Limbic system]]
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