Difference between revisions of "Olfaction" - New World Encyclopedia

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Olfactory perception

Olfaction (the sense of smell) is the detection of chemicals dissolved in air. The chemicals themselves are called odors. Olfaction is one of the five senses originally described by Aristotle. (It is now estimated that humans have between 9 and 23 senses, depending on the precise definition of what constitutes a sense).

How olfaction works

Receptors

Mammals have about 1,000 genes for odor reception,as discovered by Linda B. Buck and Richard Axelwho were awarded the Nobel Prize in 2004. Of these genes, only a portion are functional odor receptors. Humans have 347 functional odor receptor genes; the other genes have nonsense mutations. This number was determined by analyzing the genome in the Human Genome Project; the number may vary among ethnic groups, and does vary among individuals. For example, not all people can smell androstenone, a component of male and female sweat.

Each olfactory receptor neuron in the nose expresses only one functional odor receptor. 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. 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 create 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 (1996, 2002), 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 found lacking (Keller and Vosshall, 2004).

Receptor neuron

The process of how the binding of the ligand (odor molecule) to the receptor leads to an action potential in the receptor neuron is via second messenger pathway depending on the organism. In mammals the odorants stimulate Adenyl cyclase to synthesize cAMP via a G protein. cAMP which is the second messenger here opens a Cyclic nucleotide-gated ion channel (CNG) which produces an influx of Ca++ in to the cell slightly depolarising it. These Ca++ inturn opens a Ca++ activated chloride channel leading to efflux of Cl- and thus further depolarises it and triggers an action potential.

Averaged activity of the receptor neuron to an odor can be measured by an electroolfactogram in vertebrates or an electroantenogram in insects.

In the brain

In vertebrates smells are sensed by olfactory sensory neurons in the olfactory epithelium. Molecules passing through the superior nasal concha of the nasal passages mix with the mucus lining the superior portion of the cavity and are detected by olfactory receptors on the dendrites of the olfactory sensory neurons. Olfactory sensory neurons project axons to the brain within the olfactory nerve, (cranial nerve I). These axons target the olfactory bulb, which in-turn projects olfactory information to the olfactory cortex.

The axons from all the thousands of cells expressing the same olfactory receptor 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 other parts of the olfactory system in the brain where multiple features of the odor may be combined to form a synthesized olfactory perception. Since olfactory receptors can detect many chemical features of an odor molecule, the combination of features gives the olfactory system a broad range of odors that it can detect.

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.

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

Some pheromones are detected by the olfactory system, although in many vertebrates pheromones are also 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.

Olfaction and taste

Olfaction, taste and trigeminal receptors together contribute to flavor. The human tongue can only distinguish among 7-8 distinct types of taste, while the nose can distinguish among hundreds of substances, even in minute quantities. Olfaction amplifies the sense of taste, as can be proved by a simple "kitchen" experiment. If peeled pieces of apple are placed in one bowl, and peeled pieces of potato in another, and then the nostrils are held completely closed while a piece from one bowl is sampled, the taste of apple and potato are indistinguishable.

Disorders of Olfaction

Anosmia: Lack of ability to smell
Hyposmia: Decreased ability to smell
Phantosmia: "hallucinated smell", often unpleasant in nature
Dysosmia: Things do smell differently than they should

(Hirsch, 2003)

Quantifying olfaction

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.

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 Bill Graham ampitheatre, 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, excepting 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, such as the moles, who smell for their food.

Dogs in general have a nose approximately a hundred thousand to a million times more sensitive than a human's. Scenthounds as a group can smell one to ten million times more acutely than a human, and the Bloodhound, which has the keenest sense of smell of any dog, has a nose ten to a hundred million times more sensitive than a human's. It was bred for the specific purpose of tracking human beings, 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 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.

Schematic of the olfactory system of insects

Insects primarily use their antennae for olfaction. Sensory neurons in the antenna generate odor-specific electrical signals called spikes in response to odour. 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.

References
ISBN links support NWE through referral fees

Buck, Linda and Richard Axel. (1991). A Novel Multigene Family May Encode Odorant Receptors: A Molecular Basis for Odor Recognition. Cell 65:175-183.
Hirsch, Alan R. (2003) Life's a Smelling Success
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.
Turin, Luca. (1996). A spectroscopic mechanism for primary olfactory reception. Chemical Senses, 21, 773-791.
Turin, Luca. (2002). A method for the calculation of odor character from molecular structure. Journal of Theoretical Biology, 216, 367-385.
Stopfer, M, Jayaraman, V, Laurent, G (2003) Intensity versus Identity Coding in an Olfactory System, Neuron 39, 991-1004.
Stopfer, M. and Laurent, G. (1999). Short-term memory in olfactory network dynamics, Nature 402, 664-668.
Chandler Burr. (2003). The Emperor of Scent : A Story of Perfume, Obsession, and the Last Mystery of the Senses. ISBN 0-375-50797-3

External links

{{credit2|Olfaction|85046328|Odor|85144218]]

@@ Line 1: / Line 1: @@
-[[Image:Luna_antennae_2971.JPG|right|frame|Odor receptors on the antennae of a [[Actias luna|Luna moth]]]]
+[[Image:Olf2.gif|thumb|300px|Olfactory perception ]]
 '''Olfaction''' (the [[sense]] of [[#Odor|smell]]) is the detection of chemicals dissolved in air. The chemicals themselves are called [[#Odor|odors]]. Olfaction is one of the five senses originally described by Aristotle. (It is now estimated that humans have between 9 and 23 senses, depending on the precise definition of what constitutes a sense).
-==Odor==
-An '''odor''' or '''odour''' is a chemical dissolved in air, generally at a very low concentration, which we perceive by the [[sense]] of olfaction.  Odors are also called '''smells''', which can refer to both pleasant and unpleasant odors.  In contrast, '''stench''' and '''stink''' are used specifically to describe an unpleasant odor. The terms '''fragrance''', '''scent''', or '''aroma''' are used primarily by the [[food]] and cosmetic industry to describe a pleasant odor, and is sometimes used to refer to [[perfume]]s.
-=== Basics ===
-Odor is a [[sensation]] caused by [[odorant]] molecules dissolved in air.
-The widest range of odors consist of organic compounds although some inorganic substances, such as [[hydrogen sulphide]] and [[ammonia]], are also odorants.
-The perception of an odour effect is a two step process.
-First, there is the physiological part; the sense of the [[stimulus]] by receptors in the nose.
-After that the psychological part follows. The stimuli are processed by the region of the human brain which is responsible for smelling.
-Because of this a [[Objectivity (science)|objective]] and [[analytical]] measure of odor is impossible.
-While odor feelings are very personal [[perceptions]], individual reactions are related to [[gender]], [[age]], state of health and private affectations. Common odors that people are used to, such as their own body odor, are less noticeable to individuals than external or uncommon odors.
-For most people, the process of smelling gives little information concerning the [[ingredients]] of a substance. It only offers information related to the emotional impact. However, experienced people, such as [[flavorist]]s and [[perfumer]]s can pick out individual chemicals in complex mixes through smell alone.
-=== Types of odors ===
-Some odors such as perfumes and flowers are sought after, elite varieties commanding high prices.  Whole industries have developed products to remove unpleasant odors (see [[deodorant]]). The perception of odors is also very much dependent upon circumstance and culture. The odor of [[cooking]] processes may be agreeable while cooking but not necessarily after the [[meal]].
-The odor molecules send messages to the limbic system, the area of the brain that governs emotional responses. Some believe that these messages have the power to alter moods, evoke distant memories, raise their spirits, and boost self-confidence. This belief has led to the concept of “[[aromatherapy]]” wherein fragrances are claimed to cure a wide range of psychological and physical problems. Aromatherapy claims fragrances can positively affect sleep, stress, alertness, social interaction, and general feelings of well-being. However, the evidence for the effectiveness of aromatherapy consists mostly of [[Anecdotal evidence|anecdotes]] and lacks controlled [[science|scientific]] studies to back up its claims.
-With some fragrances, such as those found in perfume, scented shampoo, scented deodorant, or similar products, people can be allergic to the ingredients. The reaction, as with other chemical allergies, can be anywhere from a slight [[headache]] to [[anaphylactic shock]], which can result in death.
-Unpleasant odors can arise from certain industrial processes, adversely affecting workers and even residents downwind of the industry.  The most common sources of industrial odor arise from [[sewage treatment]] plants, [[refinery|refineries]], certain animal rendering plants and industries processing chemicals (such as sulphur) which have odorous characteristics.  Sometimes industrial odor sources are the subject of community controversy and scientific analysis.
-=== The study of odors ===
-The study of odors is a growing field but is a complex and difficult one. The human [[olfactory system]] can detect many thousands of scents based on only very minute airborne concentrations of a chemical. The sense of smell of many animals is even better. Some fragrant flowers give off ''[[Bee learning and communication|odor plumes]]'' that move downwind and are detectable by [[bee]]s more than a kilometer away.
-The study of odors can also get complicated because of the complex chemistry taking place at the moment of a smell sensation. For example iron metal objects are perceived to have an odor when touched although iron [[vapor pressure]] is negligible. According to a 2006 study  <ref>''Communication The Two Odors of Iron when Touched or Pickled: (Skin) Carbonyl Compounds and Organophosphines'' Dietmar Glindemann, Andrea Dietrich, Hans-Joachim Staerk, Peter Kuschk [[Angewandte Chemie International Edition]] web release '''2006''' {{DOI|10.1002/anie.200602100}}</ref> this smell is the result of [[aldehyde]]s (for example nonanal) and [[ketone]]s (exampe: 1-octen-3-one) released from the human skin on contact with [[ferrous]] ions that are formed in the sweat-mediated corrosion of iron. The same chemicals are also associated with the smell of [[blood]] as ferrous iron in blood on skin produces the same reaction.
-=== Pheromones ===
-[[Pheromone]]s are odors that are deliberately used for communication. A female [[moth]] may release a pheromone that can entice a male moth that is several kilometers away. [[Honeybee]] queens constantly release pheromones that regulate the activity of the [[hive]]. Workers can release such odors to call other bees into an appropriate cavity when a swarm moves in or to "sound" an alarm when the hive is threatened.
 ==How olfaction works==
 ===Receptors===
-As discovered by [[Linda B. Buck]] and [[Richard Axel]] (who were awarded the [[Nobel Prize]] in 2004), mammals have about 1000 [[gene]]s for [[odor receptor|odor reception]]. Of these genes, only a portion are functional odor receptors. Humans have 347 functional odor receptor genes; the other genes have nonsense [[mutation]]s. This number was determined by analyzing the genome in the [[Human Genome Project]]; the number may vary among ethnic groups, and does vary among individuals. For example, not all people can smell [[androstenone]], a component of male and female sweat.
+Mammals have about 1,000 genes for odor reception,as discovered by Linda B. Buck and Richard Axelwho were awarded the Nobel Prize in 2004.  Of these genes, only a portion are functional odor receptors. Humans have 347 functional odor receptor genes; the other genes have nonsense [[mutation]]s. This number was determined by analyzing the genome in the Human Genome Project; the number may vary among ethnic groups, and does vary among individuals. For example, not all people can smell androstenone, a component of male and female sweat.
 Each [[olfactory receptor neuron]] in the nose expresses only one functional odor receptor. 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. 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 create 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]] (1996, 2002), 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 found lacking (Keller and Vosshall, 2004).