- The sense of smell.
- The act or process of smelling.
[Latin olfactus, past participle of olfacere, to smell; see olfactory + -ION.]
olfaction
Dictionary:
ol·fac·tion (ŏl-făk'shən, ōl-)  |
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One of the chemical senses, specifically the sense of smell. Olfaction registers chemical information in organisms ranging from insects to humans, including marine organisms. For terrestrial animals, its stimuli comprise airborne molecules. The typical stimulus is an organic chemical with molecular weight below 300 daltons. A few inorganic chemicals can also stimulate olfaction, notably hydrogen sulfide, ozone, ammonia, and the halogens.
The anatomy of olfactory structures and the neurophysiology of olfaction differ significantly among different animal groups. For examples, insect olfactory receptors exist within sensory hairs on the antennae. The olfactory organ of fishes resides typically in tubular chambers on either side of the mouth. In terrestrial vertebrates, the olfactory receptors reside within a sac or cavity more or less similar to the human nasal cavity. The olfactory mucosa patch in the cavity characteristically contains millions of receptor cells, though in some olfactory-dominated mammals, such as the dog and rabbit, it contains tens of millions. The location of the olfactory mucosa relative to air currents in the cavity plays some role in the ongoing olfactory vigilance of the organism. In the human the mucosa sits out of the main airstream. During quiet breathing eddy currents may carry just enough stimulus to evoke a sensation, whereupon sniffing will occur. Sniffing amplifies the amount of stimulus reaching the receptors by as much as tenfold.
Reception of the chemical stimulus and transduction into a neural signal apparently occur on the olfactory receptor cilia. The ciliary membrane contains receptor protein molecules that interact with stimulating molecules through reversible binding. Vertebrate receptor cells show broad tuning, that is, they respond to many odorants.
Adjacent points in the mucosa generally project to adjacent points in the olfactory bulb of the brain. The synapses between the incoming olfactory nerve fibers and the second-order cells, mitral cells, occur in basketlike structures called glomeruli. On average, a glomerulus receives about 1000 receptor cell fibers for each mitral cell. The location of cells within the bulb seems to play a role in encoding odor quality: each odorant stimulates a more or less unique spatial array.
The central neural pathways of the olfactory system have a complexity unmatched among the sensory systems. One pathway carries information to the pyriform cortex (paleocortex of the temporal lobe), to a sensory relay in the thalamus (dorsomedial nucleus), and to the frontal cortex (orbitofrontal region). This pathway seems rather strictly sensory. Another pathway carries information to the pyriform cortex, the hypothalamus, and other structures of the limbic system. The latter have much to do with the control of emotions, feeding, and sex. The strong affective and motivational consequences of olfactory stimulation seem compatible with projections to the limbic system and with the role of olfaction in certain types of physiological regulation. In many vertebrate species, reception of pheromones occurs via an important accessory olfactory organ, known as the vomeronasal organ, which characteristically resides in the hard palate of the mouth or floor of the nasal cavity. See also Pheromone.
Human olfactory sensitivity varies from odorant to odorant over several orders of magnitude. A common range of thresholds for materials used in fragrances and flavors is 1 to 100 parts per 109 parts of air. Thresholds gathered from various groups of human subjects permit certain generalities about how the state of the organism affects olfaction. For instance, persons aged 70 and above are about tenfold less sensitive than young adults. Males and females have about equal sensitivity, except perhaps in old age, where females are more sensitive. Persons with certain medical disorders, such as multiple sclerosis, Parkinson's disease, paranasal sinus disease, Kallmann's syndrome, and olfactory tumors, exhibit decreased sensitivity (hyposmia) or complete absence of sensitivity (anosmia).
Above its threshold, the perceived magnitude of an odor changes by relatively small amounts as concentration increases. A tenfold increment in concentration will cause, on average, about a twofold change in perceived magnitude. The perceived magnitude of an odor is often greatly influenced by olfactory adaptation, a process whereby during continuous short-term exposure to a stimulus its perceived magnitude falls to about one-third of its initial value.
The stimuli for olfaction are commonly complex, that is, they are mixtures. Such products as coffee, wine, cigarettes, and perfumes contain at least hundreds of odor-relevant constituents. Only rarely does the distinctive quality of a natural product, such as a vegetable, arise from only a single constituent. A chemical analysis of most products will not usually allow a simple prediction of odor intensity or quality. One general rule, however, is that the perceived intensity of the mixture falls well below the sum of the intensities of the unmixed components.
General notions about the properties that endow a molecule with its quality have spawned more than two dozen theories of olfaction, including various chemical and vibrational theories. Most modern theories hold that the key to quality lies in the size and shape of molecules, with some influence of chemical functionality. For molecules below about 100 daltons, functional group has obvious importance: for example, thiols smell skunky, esters fruity, amines fishy-uriny, and carboxylic acids rancid. For larger molecules, the size and shape of the molecule seem more important. Shape detection is subtle enough to enable easy discrimination of some optical isomers. Progressive changes in molecular architecture along one or another dimension often lead to large changes in odor quality. No current theory makes testable predictions about such changes. See also Chemical senses; Chemoreception.
| Thesaurus: olfaction |
noun
- The sense by which odors are perceived: nose, scent, smell. See smells/good smells/bad smells/smell.
| Veterinary Dictionary: olfaction |
1. the act of smelling.
2. the sense of smell.
| Wikipedia: Olfaction |
Olfaction (also known as olfactics or more commonly as smell) is 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.
Contents |
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[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 primates and other mammals.[2]
In mammals, each olfactory receptor neuron expresses only one functional odor receptor.[3] 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)[4]. An alternative theory, the vibration theory proposed by Luca Turin[5][6], 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.[7] 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.
Olfactory system
Olfactory epithelium
In vertebrates smells are sensed by olfactory sensory neurons 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 cm². 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 proteins. 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 neurons 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 proteins (OBPs) or Chemosensory proteins (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 cAMP via a G protein called Golf. cAMP, which is the second messenger here, opens a cyclic nucleotide-gated ion channel (CNG) producing an influx of cations (largely Ca2+ with some Na+) into the cell, slightly depolarising it. The Ca2+ in turn opens a Ca2+-activated chloride channel, leading to efflux of Cl-, further depolarising the cell and triggering an action potential. Ca2+ 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 photoreceptors, in which the second messenger 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 axons 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 receptors converge in the olfactory bulb within small (~50 micrometers in diameter) structures called glomeruli. Mitral cells 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 cells and granular cells 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 insects 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 pheromones, 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.[8]
The MHC genes (known as 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.[9][10]
Humans can detect individuals that are blood related kin (mothers and children but not husbands and wives) from olfaction.[11] Mothers can identify by body odor their biological children but not their stepchildren. Preadolescent children can olfactory detect their full siblings but not half-siblings or step siblings and this might explain incest avoidance and the Westermarck effect.[12] Functional imaging shows that this olfactory kinship detection process involves the frontal-temporal junction, the insula, and the dorsomedial prefrontal cortex but not the primary or secondary olfactory cortices, or the related piriform cortex or orbitofrontal cortex.[13]
Olfaction and taste
Olfaction, taste and 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:[14]
- Anosmia – lack of ability to smell
- Cacosmia – things smell like feces[15]
- Dysosmia – things smell differently than they should
- Hyperosmia – an abnormally acute sense of smell.
- Hyposmia – decreased ability to smell
- Olfactory Reference Syndrome – psychological disorder which causes the patient to imagine he has strong body odor
- Parosmia – things smell worse than they should
- Phantosmia – "hallucinated smell", often unpleasant in nature
Quantifying olfaction in industry
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.
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 animals
The importance and sensitivity of smell varies among different organisms; most mammals have a good sense of smell, whereas most birds do not, except the tubenoses (e.g., petrels and albatrosses), and the kiwis. Among mammals, it is well-developed in the carnivores and ungulates, who must always be aware of each other, and in those that smell for their food, like moles.
Figures suggesting greater or lesser sensitivity in various species reflect experimental findings from the reactions of animals exposed to aromas in known extreme dilutions. These are, therefore, based on perceptions by these animals, rather than mere nasal function. That is, the brain's smell-recognizing centers must react to the stimulus detected, for the animal to show a response to the smell in question. It is estimated that dogs in general have an olfactory sense approximately a hundred thousand to a million times more acute than a human's. That is, they have a greater acuity. This does not mean they are overwhelmed by smells our noses can detect; rather, it means they can discern a molecular presence when it is in much greater dilution in the carrier, air. Scenthounds as a group can smell one- to ten-million times more acutely than a human, and Bloodhounds, which have the keenest sense of smell of any dogs[citation needed], 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.
Bears, such as the Silvertip Grizzly found in parts of North America, have a sense of smell seven times stronger than the bloodhound, essential for locating food underground. Using their elongated claws, bears dig deep trenches in search of burrowing animals and nests as well as roots, bulbs, and insects. Bears can detect the scent of food from up to 18 miles away; because of their immense size they often scavenge new kills, driving away the predators (including packs of wolves and human hunters) in the process.
The sense of smell is less-developed in the catarrhine primates (Catarrhini), and nonexistent in cetaceans, which compensate with a well-developed sense of taste. In some prosimians, such as the Red-bellied Lemur, scent glands occur atop the head. In many species, olfaction is highly tuned to pheromones; 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 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 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.
See also
- Accessory olfactory system
- Barosmia
- Electronic nose
- Machine olfaction
- Olfactory fatigue
- Nasal administration olfactory transfer
References
- ^ Buck L, Axel R (April 1991). "A novel multigene family may encode odorant receptors: a molecular basis for odor recognition". Cell 65 (1): 175–87. doi:. PMID 1840504.
- ^ Gilad Y, Man O, Pääbo S, Lancet D (March 2003). "Human specific loss of olfactory receptor genes". Proceedings of the National Academy of Sciences of the United States of America 100 (6): 3324–7. doi:. PMID 12612342.
- ^ Pinel, John P.J. (2006) Biopsychology. Pearson Education Inc. ISBN 0-205-42651-4 (page 178)
- ^ need citation!
- ^ Turin L (December 1996). "A spectroscopic mechanism for primary olfactory reception". Chemical senses 21 (6): 773–91. PMID 8985605. http://chemse.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=8985605.
- ^ Turin L (June 2002). "A method for the calculation of odor character from molecular structure". Journal of theoretical biology 216 (3): 367–85. doi:. PMID 12183125.
- ^ Keller A, Vosshall LB (April 2004). "A psychophysical test of the vibration theory of olfaction". Nature neuroscience 7 (4): 337–8. doi:. PMID 15034588. See also the editorial on p. 315.
- ^ Navarrete-Palacios E, Hudson R, Reyes-Guerrero G, Guevara-Guzmán R (July 2003). "Lower olfactory threshold during the ovulatory phase of the menstrual cycle". Biological psychology 63 (3): 269–79. doi:. PMID 12853171.
- ^ Boehm T, Zufall F (February 2006). "MHC peptides and the sensory evaluation of genotype". Trends in neurosciences 29 (2): 100–7. doi:. PMID 16337283.
- ^ Santos PS, Schinemann JA, Gabardo J, Bicalho Mda G (April 2005). "New evidence that the MHC influences odor perception in humans: a study with 58 Southern Brazilian students". Hormones and behavior 47 (4): 384–8. doi:. PMID 15777804.
- ^ Porter RH, Cernoch JM, Balogh RD. (1985). Odor signatures and kin recognition. Physiol Behav. 34(3):445–8. PMID 4011726
- ^ Weisfeld GE, Czilli T, Phillips KA, Gall JA, Lichtman CM. (2003). Possible olfaction-based mechanisms in human kin recognition and inbreeding avoidance. J Exp Child Psychol. 85(3):279–95. PMID 12810039
- ^ Lundström JN, Boyle JA, Zatorre RJ, Jones-Gotman M. (2009). The neuronal substrates of human olfactory based kin recognition. Hum Brain Mapp. 30:2571–2580 PMID 19067327
- ^ Hirsch, Alan R. (2003) Life's a Smelling Success
- ^ Gilbert, Avery (2008). "Freaks, Geeks, and Prodigies". What the Nose Knows. Crown Publishers. p. 52. ISBN 9781400082346.
External links
- Smells and Odours - How Smell Works at thenakedscientists.com
- MaxxScent - Scent Marketing
- Olfaction at cf.ac.uk
- The importance of smell, and pheromones, to Humans and other Animals at thenakedscientists.com
- Structure-odor relations: a modern perspective at flexitral.com (PDF)
- Olfaction at leffingwell.com
- Chirality & Odour Perception at leffingwell.com
- ScienceDaily Artille 08/03/2006, Quick -- What's That Smell? Time Needed To Identify Odors Reveals Much About Olfaction at sciencedaily.com
- Scents and Emotions Linked by Learning, Brown Study Shows at brown.edu.com
- Sense of Smell Institute at senseofsmell.org. Research arm of international fragrance industry's The Fragrance Foundation
- Nasal Ranger.com
- Olfactory Systems Laboratory at Boston University
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| Translations: Olfaction |
Nederlands (Dutch)
reukvermogen, reuk
Français (French)
n. - olfaction
Deutsch (German)
n. - Geruchsvermögen
Ελληνική (Greek)
n. - όσφρηση, οσφραντικότητα
Português (Portuguese)
n. - olfação (f)
Español (Spanish)
n. - sentido del olfato, olfato
Svenska (Swedish)
n. - luktande, luktsinne
中文（简体）(Chinese (Simplified))
嗅觉
中文（繁體）(Chinese (Traditional))
n. - 嗅覺
العربيه (Arabic)
(الاسم) حاسه الشم
עברית (Hebrew)
n. - הרחה, חוש הריח
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