Olfaction or olfactory perception1 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 lobe2 Many vertebrates including most mammals and reptiles have two distinct olfactory systemsthe 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 chemicals3 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 sense4

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 Taste sensations are caused by small organic molecules and proteins5

Contents

1 Study of olfaction
2 Main olfactory system
21 Receptor neuron
22 Olfactory bulb projections
3 Accessory olfactory system
4 Human olfactory system
5 Olfactory coding and perception
6 Genetics of olfaction
7 Interactions of olfaction with other senses
71 Olfaction and taste
72 Olfaction and audition
8 Disorders of olfaction
9 Quantifying olfaction in industry
10 Olfaction in plants and animals
101 Insect olfactory system
11 See also
12 References
13 Further reading
14 External links

Study of olfaction

The Lady and the Unicorn a Flemish tapestry depicting the smell

As the Epicurean and atomistic Roman philosopher Lucretius 1st Century BCE speculated different odors are attributed to different shapes and sizes of odor molecules that stimulate the olfactory organ2 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 reception6 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 mammals7

In mammals each olfactory receptor neuron expresses only one functional odor receptor8 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 perceptioncitation needed An alternative theory the vibration theory proposed by Luca Turin910 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 question11 There is no theory yet that explains olfactory perception completely

Main olfactory system

In vertebrates smells are sensed by olfactory sensory neurons in the olfactory epithelium The olfactory epithelium is made up of at least six morphologically and biochemically different cell types 12The proportion of olfactory epithelium compared to respiratory epithelium not innervated gives an indication of the animals olfactory sensitivity Humans have about 10 cm2 16 sq in of olfactory epithelium whereas some dogs have 170 cm2 26 sq in A dogs olfactory epithelium is also considerably more densely innervated with a hundred times more receptors per square centimetre13

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 This mucus acts as a solvent for odor molecules flows constantly and is replaced approximately every 10 minutes

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 binding of the ligand odor molecule or odorant to the receptor leads to an action potential in the receptor neuron 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 electro-olfactogram 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

Olfactory bulb projections

Early Olfactory System

Olfactory sensory neurons project axons to the brain within the olfactory nerve cranial nerve I These nerve fibers lacking myelin sheaths pass to the olfactory bulb of the brain through perforations in the cribriform plate which in turn projects olfactory information to the olfactory cortex and other areas14 The axons from the olfactory receptors converge in the outer layer of the olfactory bulb within small 50 micrometers in diameter structures called glomeruli Mitral cells located in the inner layer of the olfactory bulb form synapses with the axons of the sensory neurons within glomeruli and send the information about the odor to other parts of the olfactory system where multiple signals may be processed to form a synthesized olfactory perception A large degree of convergence occurs with twenty-five thousand axons synapsing on twenty-five 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 fibers 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 has two major divisions with anatomically distinct organizations and functions The anterior piriform cortex APC is better associated with determining the chemical structure of the odorant molecules and whereas the posterior piriform cortex PPC is best known for its strong role in categorizing odors and assessing similarities between odors eg minty woody citrus are odors which can be distinguished via the PPC despite being highly-variant chemicals and in a concentration-independent manner15 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 stored in long-term memory and has strong connections to emotional memory This is possibly due to the olfactory systems 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 to only 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

Inputs from the two nostrils have separate inputs to the brain with the result that it is possible for humans to experience perceptual rivalry in the olfactory sense akin to that of binocular rivalry when there are two different inputs into the two nostrils16

In insects smells are sensed by sensilla located on the antenna and maxillary palp and first processed by the antennal lobe analogous to the olfactory bulb and next by the mushroom bodies and lateral horn

Accessory olfactory system

Many animals including most mammals and reptiles but not humans 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 facial expression called flehmen to direct stimuli to this organ

The sensory receptors of the accessory olfactory system are located in the vomeronasal organ As in the main olfactory system the axons of these sensory neurons project from the vomeronasal organ to the accessory olfactory bulb which in the mouse is located on the dorsal-posterior portion of the main olfactory bulb Unlike in the main olfactory system the axons that leave the accessory olfactory bulb do not project to the brains cortex but rather to targets in the amygdala and bed nucleus of the stria terminalis and from there to the hypothalamus where they may influence aggressive and mating behavior

Human olfactory system

See also Body odor

In female humans the sense of olfaction is strongest around the time of ovulation significantly stronger than during other phases of the menstrual cycle and stronger than the sense in males17

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 own1819

Humans can detect individuals that are blood-related kin mothersfathers and children but not husbands and wives from olfaction20 Mothers can identify by body odor their biological children but not their stepchildren Preadolescent children can olfactorily detect their full siblings but not half-siblings or step siblings and this might explain incest avoidance and the Westermarck effect21 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 cortex22

Olfactory coding and perception

How olfactory information is coded in the brain to allow for proper perception is still being researched and the process is not completely understood However what is known is that the chemical nature of the odorant is particularly important as there may be a chemotopic map in the brain this map would show specific activation patterns for specific odorants When an odorant is detected by receptors the receptors in a sense break the odorant down and then the brain puts the odorant back together for identification and perception23 The odorant binds to receptors which only recognize a specific functional group or feature of the odorant which is why the chemical nature of the odorant is important24

After binding the odorant the receptor is activated and will send a signal to the glomeruli24 Each glomerulus receives signals from multiple receptors that detect similar odorant features Because multiple receptor types are activated due to the different chemical features of the odorant multiple glomeruli will be activated as well All of the signals from the glomeruli will then be sent to the brain where the combination of glomeruli activation will encode the different chemical features of the odorant The brain will then essentially put the pieces of the activation pattern back together in order to identify and perceive the odorant24

Odorants that are similar in structure activate similar patterns of glomeruli which lead to a similar perception in the brain2425 Data from animal models suggest that the brain may have a chemotopic map A chemotopic map is an area in the brain to be specific the olfactory bulb in which glomeruli project their signals onto the brain in a specific pattern The idea of the chemotopic map has been supported by the observation that chemicals containing similar functional groups have similar responses with overlapped areas in the brain This is important because it allows the possibility to predict the neural activation pattern from an odorant and vice versa24

Genetics of olfaction

Different people smell different odors and most of these differences are caused by genetic differences26 Although odorant receptor genes make up one of the largest genes families in the human genome only a handful of genes have been linked conclusively to particular smells For instance the odorant receptor OR5A1 and its genetic variants alleles are responsible for our ability or failure to smell -ionone a key aroma in foods and beverages27 Similarly the odorant receptor OR2J3 is associated with the ability to detect the grassy smelling odor cis-3-hexen-1-ol28 The preference or dislike of cilantro coriander has been linked to the olfactory receptor OR6A229

Interactions of olfaction with other senses

Olfaction and taste

Olfaction taste and trigeminal receptors also called Chemesthesis 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 It is during exhalation that the olfaction contribution to flavor occurs in contrast to that of proper smell which occurs during the inhalation phase30 The neurons of the olfactory system is the only one of the human senses that bypasses the thalamus and connects directly to the forebrain31

Olfaction and audition

Olfaction and sound information has been shown to converge in the olfactory tubercles of rodents32 This neural convergence is proposed to give rise to a percept termed smound33 Whereas a flavor results from interactions between smell and taste a smound may result from interactions between smell and sound

Disorders of olfaction

The following are disorders of olfaction34

Anosmia inability to smell
Dysosmia things smell different 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 or she has strong body odor
Parosmia things smell worse than they should35
Phantosmia hallucinated smell often unpleasant in nature

Quantifying olfaction in industry

Nasal Ranger an olfactometer in use

Scientists have devised methods for quantifying the intensity of odors in particular 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 plants and animals

The tendrils of plants are especially sensitive to airborne volatile organic compounds Parasites such as dodder make use of this in locating their preferred hosts and locking on to them36 The emission of volatile compounds is detected when foliage is browsed by animals Threatened plants are then able to take defensive chemical measures such as moving tannin compounds to their foliage see Plant perception

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 eg petrels and albatrosses certain species of vultures and the kiwis Among mammals it is well-developed in the carnivores and ungulates which must always be aware of each other and in those that smell for their food like moles Having a strong sense of smell is referred to as macrosmatic

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 brains 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 humans 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 dogscitation needed have noses ten- to one-hundred-million times more sensitive than a humans 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 that of 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 tastecitation needed 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

Insect olfactory system

Insects have been used as a model system to study olfaction Insects use primarily their antennae for detecting odors Sensory neurons in the antenna generate odor-specific electrical signals called spikes in response to binding of odors The sensory neurons send this information via their axons to the antennal lobe where they synapse with other neurons in semidelineated with membrane boundaries structures called glomeruli The antennal lobes have two kinds of neurons projection neurons mostly excitatory and local neurons inhibitory and some excitatory The projection neurons send their axon terminals to mushroom bodies and the lateral horn both of which are part of the protocerebrum of the insects 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

See also

Portal icon Neuroscience portal

Chemesthesis
Electronic nose
Machine olfaction
Major histocompatibility complex and sexual selection
Nasal administration olfactory transfer
Odor
Olfactometer
Olfactory fatigue
Phantosmia
Scent transfer unit

References

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Further reading

Gordon Ms Shepherd Neurogastronomy How the Brain Creates Flavor and Why It Matters New York Columbia University Press 2012 ISBN 978-0-231-15910-4

External links
Wikimedia Commons has media related to Olfactory system

Mammalian Odor Perception through Genetics
Research on Interesting Questions About Smells
Insect Olfaction of Plant Odour
Smells and Odours – How Smell Works at thenakedscientistscom
Olfaction at cfacuk
Structure-odor relations a modern perspective at flexitralcom PDF
Chirality Odour Perception at leffingwellcom
ScienceDaily Artille 08032006 Quick — Whats That Smell Time Needed To Identify Odors Reveals Much About Olfaction at sciencedailycom
Scents and Emotions Linked by Learning Brown Study Shows at browneducom
Sense of Smell Institute at senseofsmellorg Research arm of international fragrance industrys The Fragrance Foundation
Olfactory Systems Laboratory at Boston University
Smells Database
Burghart
Olfaction and Gustation Neuroscience Online electronic neuroscience textbook by UT Houston Medical School