Thesense of smell, orolfaction,[nb 1] is thespecial sense through which smells (orodors) are perceived.[2] The sense of smell has many functions, including detecting desirable foods, hazards, andpheromones, and plays a role in taste.
In humans, it occurs when anodor binds to areceptor within thenasal cavity, transmitting a signal through theolfactory system.[3]Glomeruli aggregate signals from these receptors and transmit them to theolfactory bulb, where the sensory input will start to interact with parts of the brain responsible for smell identification,memory, andemotion.[4]
Early scientific study of the sense of smell includes the extensive doctoral dissertation ofEleanor Gamble, published in 1898, which compared olfactory to otherstimulus modalities, and implied that smell had a lower intensity discrimination.[7]
As the Epicurean and atomistic Roman philosopherLucretius (1stcentury BC) speculated, different odors are attributed to different shapes and sizes of "atoms" (odor molecules in the modern understanding) that stimulate the olfactory organ.[8]
A modern demonstration of that theory was the cloning of olfactory receptor proteins byLinda B. Buck andRichard Axel (who were awarded theNobel Prize in 2004), and subsequent pairing of odor molecules to specific receptor proteins.[9] Each odor receptor molecule recognizes only a particular molecular feature or class of odor molecules.Mammals have about a thousandgenes that code forodor reception.[10] 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 mammals.[11] In mammals, eacholfactory receptor neuron expresses only one functional odor receptor.[12] 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 theshape theory, each receptor detects a feature of the odormolecule. The weak-shape theory, known as theodotope 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).[13]
According to a new study, researchers have found that a functional relationship exists between molecular volume of odorants and the olfactory neural response.[14] An alternative theory, thevibration theory proposed byLuca Turin,[15][16] posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range byquantum tunnelling. However, the behavioral predictions of this theory have been called into question.[17] There is no theory yet that explains olfactory perception completely.
Flavor perception is an aggregation ofauditory,taste,haptic, and smell sensory information.[18]Retronasal smell plays the biggest role in the sensation of flavor. During the process ofmastication, the tongue manipulates food to release odorants. These odorants enter the nasal cavity during exhalation.[19] The smell of food has the sensation of being in the mouth because of co-activation of the motor cortex and olfactory epithelium during mastication.[18]
Smell,taste, andtrigeminal receptors (also calledchemesthesis) together contribute toflavor. The humantongue can distinguish only among five distinct qualities of taste, while the nose can distinguish among hundreds of substances, even in minute quantities. It is duringexhalation that the smell's contribution to flavor occurs, in contrast to that of proper smell, which occurs during theinhalation phase of breathing.[19] The olfactory system is the only human sense that bypasses thethalamus and connects directly to theforebrain.[20]
Smell andsound information has been shown to converge in the olfactory tubercles ofrodents.[21] This neural convergence is proposed to give rise to a perception termedsmound.[22] Whereas aflavor results from interactions between smell and taste, a smound may result from interactions between smell and sound.
TheMHC genes (known asHLA in humans) are a group of genes present in many animals and important for theimmune 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.[23][24] However, some research suggests that takinghormonal contraception can alter women's preference for partners with dissimilar MHC genes, thus resulting in a greater likelihood to choose partners with relatively similar MHC genes to their own.[25][26] Sexual orientation can also influence preference for different body odors, and some studies suggest that preference may be influenced by the putative pheromonesAND andEST.[27]
Humans can detect blood relatives from olfaction.[28] Mothers can identify by body odor their biological children but not their stepchildren. Pre-adolescent children can olfactorily detect their full siblings but not half-siblings or step siblings, and this might explainincest avoidance and theWestermarck effect.[29] Functional imaging shows that this olfactory kinship detection process involves the frontal-temporal junction, theinsula, and the dorsomedialprefrontal cortex, but not the primary or secondary olfactory cortices, or the relatedpiriform cortex ororbitofrontal cortex.[30]
Sinceinbreeding is detrimental, it tends to be avoided. In the house mouse, themajor urinary protein (MUP) gene cluster provides a highly polymorphic scent signal of genetic identity that appears to underliekin recognition and inbreeding avoidance. Thus, there are fewer matings between mice sharing MUP haplotypes than would be expected if there were random mating.[31]
Some animals usescent trails to guide movement, for example social insects may lay down a trail to a food source, or atracking dog may follow the scent of its target. A number ofscent-tracking strategies have been studied in different species, including gradient search orchemotaxis, anemotaxis, klinotaxis, and tropotaxis. Their success is influenced by theturbulence of the air plume that is being followed.[32][33]
Different people smell various odors, and most of these differences are caused by genetic variation.[34] Althoughodorant receptor genes make up one of the largest gene families in the human genome, only a handful of genes have been conclusively linked to particular smells. For instance, the odorant receptorOR5A1 and its genetic variants (alleles) determine the ability to smell β-ionone, a key aroma compound in foods and beverages.[35] Similarly, the odorant receptorOR2J3 is associated with the ability to detect the "grassy" odor, cis-3-hexen-1-ol.[36] The preference (or dislike) ofcilantro (coriander) has been linked to the olfactory receptorOR6A2.[37]
The importance and sensitivity of smell varies among different organisms; mostmammals have a good sense of smell, whereas mostbirds do not, except thetubenoses (e.g.,petrels andalbatrosses), certain species of new worldvultures, and thekiwis. Also, birds have hundreds of olfactory receptors.[38] Although, recent analysis of the chemical composition ofvolatile organic compounds (VOCs) fromking penguin feathers suggest that VOCs may provide olfactory cues, used by the penguins to locate their colony and recognize individuals.[39] Among mammals, it is well developed in thecarnivores andungulates, which must always be aware of each other, and in those that smell for their food, such asmoles. Having a strong sense of smell is referred to asmacrosmatic in contrast to having a weak sense of smell which is referred to asmicrosmatic.
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 be said to show a response to the smell in question. It is estimated that dogs, in general, have an olfactory sense approximately ten thousand to a hundred thousand times more acute than a human's.[40] 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, andbloodhounds, which have the keenest sense of smell of any dogs,[41] 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 ascent trail a few days old. The second-most-sensitive nose is possessed by theBasset Hound, which was bred to track and hunt rabbits and other small animals.
Grizzly bears 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 eighteen 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.
Fish, too, have a well-developed sense of smell, even though they inhabit an aquatic environment.[citation needed] 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.
Although conventional wisdom and lay literature, based on impressionistic findings in the 1920s, have long presented human smell as capable of distinguishing between roughly 10,000 unique odors, recent research has suggested that the average individual is capable of distinguishing over one trillion unique odors.[42] Researchers in the most recent study, which tested the psychophysical responses to combinations of over 128 unique odor molecules with combinations composed of up to 30 different component molecules, noted that this estimate is "conservative" and that some subjects of their research might be capable of deciphering between a thousand trillion odorants, adding that their worst performer could probably still distinguish between 80million scents.[43] Authors of the study concluded, "This is far more than previous estimates of distinguishable olfactory stimuli. It demonstrates that the human olfactory system, with its hundreds of different olfactory receptors, far out performs the other senses in the number of physically different stimuli it can discriminate."[44] However, it was also noted by the authors that the ability to distinguish between smells is not analogous to being able to consistently identify them, and that subjects were not typically capable of identifying individual odor stimulants from within the odors the researchers had prepared from multiple odor molecules. In November 2014 the study was strongly criticized by Caltech scientist Markus Meister, who wrote that the study's "extravagant claims are based on errors of mathematical logic."[45][46] The logic of his paper has in turn been criticized by the authors of the original paper.[47]
In humans and othervertebrates, smells are sensed byolfactory sensory neurons in theolfactory epithelium. The olfactory epithelium is made up of at least six morphologically and biochemically different cell types.[20] The proportion of olfactoryepithelium compared to respiratory epithelium (not innervated, or supplied with nerves) gives an indication of the animal's olfactory sensitivity. Humans have about 10 cm2 (1.6 sq in) of olfactory epithelium, whereas some dogs have 170 cm2 (26 sq in). A dog's olfactory epithelium is also considerably more densely innervated, with a hundred times more receptors per square centimeter.[48] The sensory olfactory system integrates with other senses to form the perception offlavor.[18] Often, land organisms will have separate olfaction systems for smell and taste (orthonasal smell andretronasal smell), but water-dwelling organisms usually have only one system.[49]
Molecules of odorants passing through thesuperior nasal concha of the nasal passages dissolve in themucus that lines the superior portion of the cavity and are detected byolfactory receptors on thedendrites of the olfactory sensory neurons. This may occur by diffusion or by the binding of the odorant toodorant-binding proteins. The mucus overlying the epithelium containsmucopolysaccharides, salts,enzymes, andantibodies (these are highly important, as the olfactory neurons provide a direct passage for infection to pass to thebrain). This mucus acts as a solvent for odor molecules, flows constantly, and is replaced approximately every ten minutes.
Ininsects, smells are sensed by olfactory sensory neurons in the chemosensorysensilla, 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) orChemosensory proteins (CSPs), before activating the sensory neurons.
The binding of theligand (odor molecule or odorant) to the receptor leads to anaction potential in the receptor neuron, via asecond messenger pathway, depending on the organism. In mammals, the odorants stimulateadenylate cyclase to synthesizecAMP via aG protein called Golf. cAMP, which is the second messenger here, opens acyclic nucleotide-gated ion channel (CNG), producing an influx ofcations (largelyCa2+ with someNa+) into the cell, slightly depolarising it. The Ca2+ in turn opens a Ca2+-activatedchloride channel, leading to efflux ofCl−, further depolarizing the cell and triggering an action potential. Ca2+ is then extruded through asodium-calcium exchanger. A calcium-calmodulin complex also acts to inhibit the binding of cAMP to the cAMP-dependent channel, thus contributing to olfactory adaptation.
The main olfactory system of some mammals also contains small subpopulations of olfactory sensory neurons that detect and transduce odors somewhat differently. Olfactory sensory neurons that use trace amine-associated receptors (TAARs) to detect odors use the same second messenger signaling cascade as do the canonical olfactory sensory neurons.[50] Other subpopulations, such as those that express the receptor guanylyl cyclase GC-D (Gucy2d)[51] or the soluble guanylyl cyclase Gucy1b2,[52] use a cGMP cascade to transduce their odorant ligands.[53][54][55] These distinct subpopulations (olfactory subsystems) appear specialized for the detection of small groups of chemical stimuli.
This mechanism of transduction is somewhat unusual, in that cAMP works by directly binding to theion channel rather than through activation ofprotein kinase A. It is similar to the transduction mechanism forphotoreceptors, in which the second messengercGMP 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 bylateral inhibition.
Averaged activity of the receptor neurons can be measured in several ways. In vertebrates, responses to an odor can be measured by anelectro-olfactogram or through calcium imaging of receptor neuron terminals in the olfactory bulb. In insects, one can performelectroantennography or calcium imaging within the olfactory bulb.
Schematic of the early olfactory system including the olfactory epithelium and bulb. Each ORN expresses one OR that responds to different odorants. Odorant molecules bind to ORs on cilia. ORs activate ORNs that transduce the input signal into action potentials. In general, glomeruli receive input from ORs of one specific type and connect to the principal neurons of the OB, mitral and tufted cells (MT cells).
Olfactory sensory neurons projectaxons to the brain within theolfactory nerve, (cranial nerveI). These nerve fibers, lackingmyelin sheaths, pass to theolfactory bulb of the brain through perforations in thecribriform plate, which in turn projects olfactory information to theolfactory cortex and other areas.[56] The axons from theolfactory receptors converge in the outer layer of the olfactory bulb within small (≈50micrometers in diameter) structures calledglomeruli.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 theodor 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 25,000 axons synapsing on 25 or so mitral cells, and with each of these mitral cells projecting to multiple glomeruli. Mitral cells also project toperiglomerular cells andgranular 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. Neuromodulators likeacetylcholine,serotonin andnorepinephrine all send axons to the olfactory bulb and have been implicated in gain modulation,[57] pattern separation,[58] andmemory functions,[59] respectively.
The mitral cells leave the olfactory bulb in thelateral olfactory tract, which synapses on five major regions of the cerebrum: theanterior olfactory nucleus, theolfactory tubercle, theamygdala, thepiriform cortex, and theentorhinal cortex. The anterior olfactory nucleus projects, via theanterior 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) appears to be better at determining the chemical structure of the odorant molecules, and the posterior piriform cortex (PPC) has a strong role in categorizing odors and assessing similarities between odors (e.g. minty, woody, and citrus are odors that can, despite being highly variant chemicals, be distinguished via the PPC in a concentration-independent manner).[60] The piriform cortex projects to themedial dorsal nucleus of the thalamus, which then projects to the orbitofrontal cortex. The orbitofrontal cortex mediates conscious perception of the odor.[citation needed] The three-layered piriform cortex projects to a number of thalamic and hypothalamic nuclei, thehippocampus 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 inlong-term memory and has strong connections toemotional memory. This is possibly due to the olfactory system's close anatomical ties to thelimbic 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 may seem strange that human beings are able to distinguish so many different odors. It seems that a highly complex form of processing must be 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 spatial map of excitation in the olfactory bulb. It is possible that the brain is able to distinguish specific odors through spatial encoding, but temporal coding must also 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 twonostrils have separate inputs to the brain, with the result that, when each nostril takes up a different odorant, a person may experience perceptual rivalry in the olfactory sense akin to that ofbinocular rivalry.[61]
The process by which olfactory information is coded in the brain to allow for proper perception is still being researched, and is not completely understood. When an odorant is detected by receptors, they in a sense break the odorant down, and then the brain puts the odorant back together for identification and perception.[62] The odorant binds to receptors that recognize only a specific functional group, or feature, of the odorant, which is why the chemical nature of the odorant is important.[63]
After binding the odorant, the receptor is activated and will send a signal to the glomeruli[63] in theolfactory bulb. Each glomerulus receives signals from multiple receptors that detect similar odorant features. Because several receptor types are activated due to the different chemical features of the odorant, several glomeruli are activated as well. The signals from the glomeruli are transformed to a pattern of oscillations of neural activities[64] of themitral cells, the output neurons from the olfactory bulb. Olfactory bulb sends this pattern to theolfactory cortex. Olfactory cortex is thought to have associative memories,[65] so that it resonates to this bulbar pattern when the odor object is recognized.[66] The cortex sends centrifugal feedback to the bulb.[67] This feedback could suppress bulbar responses to the recognized odor objects, causing olfactory adaptation to background odors, so that the newly arrived foreground odor objects could be singled out for better recognition.[66][68] During odor search, feedback could also be used to enhance odor detection.[69][66] The distributed code allows the brain to detect specific odors in mixtures of many background odors.[70]
It is a general idea that the layout of brain structures corresponds to physical features of stimuli (called topographic coding), and similar analogies have been made in smell with concepts such as a layout corresponding to chemical features (called chemotopy) or perceptual features.[71] While chemotopy remains a highly controversial concept,[72] evidence exists for perceptual information implemented in the spatial dimensions of olfactory networks.[71]
Many animals, including most mammals and reptiles, but not humans,[73] 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 aspheromones, although pheromones can also be detected by the main olfactory system. In the accessory olfactory system, stimuli are detected by thevomeronasal organ, located in the vomer, between thenose and themouth. Snakes use it to smell prey, sticking their tongue out and touching it to the organ. Some mammals make a facial expression calledflehmen 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 theaccessory olfactory bulb, which in the mouse is located on the dorsal-posterior portion of the mainolfactory bulb. Unlike in the main olfactory system, the axons that leave the accessory olfactory bulb do not project to the brain's cortex but rather to targets in theamygdala andbed nucleus of the stria terminalis, and from there to thehypothalamus, where they may influence aggression and mating behavior.
Insect olfaction refers to the function ofchemical receptors that enableinsects to detect and identifyvolatile compounds forforaging, predator avoidance, findingmating partners (viapheromones) and locatingoviposition habitats.[74] Thus, it is the most important sensation for insects.[74] Most important insect behaviors must be timed perfectly which is dependent on what they smell and when they smell it.[75] For example, smell is essential for hunting in many species ofwasps, includingPolybia sericea.
The two organs insects primarily use for detecting odors are theantennae and specialized mouth parts called the maxillary palps.[76] However, a recent study has demonstrated the olfactory role of ovipositor in fig wasps.[77] Inside of these olfactory organs there are neurons called olfactory receptor neurons which, as the name implies, house receptors for scent molecules in their cell membranes. The majority ofolfactory receptor neurons typically reside in theantenna. These neurons can be very abundant, for exampleDrosophila flies have 2,600 olfactory sensory neurons.[76]
Insects are capable of smelling and differentiating between thousands ofvolatile compounds bothsensitively and selectively.[74][78] Sensitivity is how attuned the insect is to very small amounts of an odorant or small changes in the concentration of an odorant. Selectivity refers to the insects' ability to tell one odorant apart from another. These compounds are commonly broken into three classes: short chaincarboxylic acids,aldehydes and low molecular weight nitrogenous compounds.[78] Some insects, such as the mothDeilephila elpenor, use smell as a means to find food sources.
The tendrils of plants are especially sensitive to airbornevolatile organic compounds. Parasites such asdodder make use of this in locating their preferred hosts and locking on to them.[79] 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 movingtannin compounds to their foliage.
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 faced incidents where the proximity of an industrial source or landfill caused adverse reactions among nearby residents. These reactions were due to unpleasant 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, eachsniffing the same sample of diluted specimen air. A fieldolfactometer can be utilized to determine the magnitude of an odor.
In western cultures, the amount of value traditionally bestowed on the sense of smell has derived from how it places within themind–body dualism. The mind, held to be superior to the body, has been associated with the "refined" senses of vision and hearing, while sense of smell, along with taste, have been considered "chemical" senses, associated with the body, and less valued. This derision arises in part as it is hard to abstract smell; it is difficult to describe an odor without reference to its source (e.g. describingvanilla). This value system contrasts with that of Japan, where more value is placed on the sense of smell, and whereKōdō, the art of appreciating incense, is practiced.[80]
Aroma is understood to stimulate recall, a characteristic emphasized byProust inIn Search of Lost Time. The smells of home cooking, such as the smells of holiday meals and chocolate chip cookies has been described as particularly evocative.[80]
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