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Sense of smell
Sense of smell
Sense of smell
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Smell
Painting of a woman smelling a carnation. Olfaction uses chemoreceptors that create signals processed in the brain that form the sense of smell.
Details
SystemOlfactory system
Functionsense chemicals in the environment that are used to form the sense of smell
Identifiers
MeSHD012903
Anatomical terminology

The sense of smell, or olfaction,[nb 1] is the special sense through which smells (or odors) are perceived.[2] The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.

In humans, it occurs when an odor binds to a receptor within the nasal cavity, transmitting a signal through the olfactory system.[3] Glomeruli aggregate signals from these receptors and transmit them to the olfactory bulb, where the sensory input will start to interact with parts of the brain responsible for smell identification, memory, and emotion.[4]

There are many different things which can interfere with a normal sense of smell, including damage to the nose or smell receptors, anosmia, upper respiratory infections, traumatic brain injury, and neurodegenerative disease.[5][6]

History of study

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The Lady and the Unicorn, a Flemish tapestry depicting the sense of smell, 1484–1500. Musée national du Moyen Âge, Paris.

Early scientific study of the sense of smell includes the extensive doctoral dissertation of Eleanor Gamble, published in 1898, which compared olfactory to other stimulus modalities, and implied that smell had a lower intensity discrimination.[7]

As the Epicurean and atomistic Roman philosopher Lucretius (1st century BCE) 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 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.[9] 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 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, each olfactory 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 the shape theory, each receptor detects a feature of the odor molecule. The weak-shape theory, known as the 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).[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, the vibration theory proposed by Luca Turin,[15][16] posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by quantum tunnelling. However, the behavioral predictions of this theory have been called into question.[17] There is no theory yet that explains olfactory perception completely.

Function

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Taste

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Flavor perception is an aggregation of auditory, taste, haptic, and smell sensory information.[18] Retronasal smell plays the biggest role in the sensation of flavor. During the process of mastication, 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, 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 smell's contribution to flavor occurs, in contrast to that of proper smell, which occurs during the inhalation phase of breathing.[19] The olfactory system is the only human sense that bypasses the thalamus and connects directly to the forebrain.[20]

Hearing

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Smell and sound information has been shown to converge in the olfactory tubercles of rodents.[21] This neural convergence is proposed to give rise to a perception termed smound.[22] Whereas a flavor results from interactions between smell and taste, a smound may result from interactions between smell and sound.

Inbreeding avoidance

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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.[23][24] However, some research suggests that taking hormonal 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 pheromones AND and EST.[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 explain incest avoidance and the Westermarck effect.[29] 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.[30]

Since inbreeding is detrimental, it tends to be avoided. In the house mouse, the major urinary protein (MUP) gene cluster provides a highly polymorphic scent signal of genetic identity that appears to underlie kin recognition and inbreeding avoidance. Thus, there are fewer matings between mice sharing MUP haplotypes than would be expected if there were random mating.[31]

Guiding movement

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Some animals use scent trails to guide movement, for example social insects may lay down a trail to a food source, or a tracking dog may follow the scent of its target. A number of scent-tracking strategies have been studied in different species, including gradient search or chemotaxis, anemotaxis, klinotaxis, and tropotaxis. Their success is influenced by the turbulence of the air plume that is being followed.[32][33]

Genetics

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Main article: Evolution of olfaction

Different people smell various odors, and most of these differences are caused by genetic variation.[34] Although odorant 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 receptor OR5A1 and its genetic variants (alleles) determine the ability to smell β-ionone, a key aroma compound in foods and beverages.[35] Similarly, the odorant receptor OR2J3 is associated with the ability to detect the "grassy" odor, cis-3-hexen-1-ol.[36] The preference (or dislike) of cilantro (coriander) has been linked to the olfactory receptor OR6A2.[37]

Variability amongst vertebrates

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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), certain species of new world vultures, and the kiwis. Also, birds have hundreds of olfactory receptors.[38] Although, recent analysis of the chemical composition of volatile organic compounds (VOCs) from king 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 the carnivores and ungulates, which must always be aware of each other, and in those that smell for their food, such as moles. Having a strong sense of smell is referred to as macrosmatic in contrast to having a weak sense of smell which is referred to as microsmatic.

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, and bloodhounds, 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 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.

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.

The sense of smell is less developed in the catarrhine primates, and nonexistent in cetaceans, which compensate with a well-developed sense of taste.[41] In some strepsirrhines, such as the red-bellied lemur, scent glands occur atop the head. In many species, smell 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.[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.

Human smell abilities

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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 80 million 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]

Physiological basis in vertebrates

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Main olfactory system

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Main article: Olfactory system

In humans and other 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.[20] The proportion of olfactory epithelium 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 of flavor.[18] Often, land organisms will have separate olfaction systems for smell and taste (orthonasal smell and retronasal smell), but water-dwelling organisms usually have only one system.[49]

Molecules of odorants passing through the superior nasal concha of the nasal passages dissolve in the mucus that lines 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 ten 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

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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 depolarizing 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.

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 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 electroantennography or calcium imaging within the olfactory bulb.

Olfactory bulb projections

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A simple diagram showing small triangles and rectangles that represent various chemical compounds rising up to a few lines that represent cells that can absorb them and then send electrical signals to the brain to interpret
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 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 areas.[56] 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 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 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. Neuromodulators like acetylcholine, serotonin and norepinephrine all send axons to the olfactory bulb and have been implicated in gain modulation,[57] pattern separation,[58] and memory functions,[59] respectively.

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) 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 the medial 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, 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 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 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 two nostrils 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 of binocular rivalry.[61]

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.

Coding and perception

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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 the olfactory 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 the mitral cells, the output neurons from the olfactory bulb. Olfactory bulb sends this pattern to the olfactory 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]

Accessory olfactory system

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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 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 brain's 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 aggression and mating behavior.

In insects

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Main article: Insect olfaction

Insect olfaction refers to the function of chemical receptors that enable insects to detect and identify volatile compounds for foraging, predator avoidance, finding mating partners (via pheromones) and locating oviposition 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 of wasps, including Polybia sericea.

The two organs insects primarily use for detecting odors are the antennae 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 of olfactory receptor neurons typically reside in the antenna. These neurons can be very abundant, for example Drosophila flies have 2,600 olfactory sensory neurons.[76]

Insects are capable of smelling and differentiating between thousands of volatile compounds both sensitively 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 chain carboxylic acids, aldehydes and low molecular weight nitrogenous compounds.[78] Some insects, such as the moth Deilephila elpenor, use smell as a means to find food sources.

In plants

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See also: Plant perception (physiology) § Senses in plants

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 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 moving tannin compounds to their foliage.

Machine-based smelling

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Main article: Machine olfaction

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, 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 US 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.

Culture

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In western cultures, the amount of value traditionally bestowed on the sense of smell has derived from how it places within the mind–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. describing vanilla). This value system contrasts with that of Japan, where more value is placed on the sense of smell, and where Kōdō, the art of appreciating incense, is practiced.[80]

Aroma is understood to stimulate recall, a characteristic emphasized by Proust in In 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]

Classification

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Systems of classifying odors include:

  • Crocker-Henderson system, which rates smells on a 0-8 scale for each of four "primary" smells: fragrant, acid, burnt, and caprylic.[81]
  • Henning's prism[82]
  • Zwaardemaker smell system (invented by Hendrik Zwaardemaker)

Disorders

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Specific terms are used to describe disorders associated with smelling:

Viruses can also infect the olfactory epithelium leading to a loss of the sense of olfaction. About 50% of patients with SARS-CoV-2 (causing COVID-19) experience some type of disorder associated with their sense of smell, including anosmia and parosmia. SARS-CoV-1, MERS-CoV and even the flu (influenza virus) can also disrupt olfaction.[87]

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Sense of smell

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from Grokipedia
The sense of smell, also known as olfaction, is the chemosensory detection of volatile odorant molecules in the environment, enabling organisms to perceive and discriminate among thousands of distinct scents through specialized neural pathways. In humans, olfaction is primarily mediated by the (cranial nerve I), which transmits signals from the to the brain, playing essential roles in survival by identifying , environmental hazards, and . Anatomically, the olfactory system begins in the , a specialized tissue located at the roof of the near the of the , covering an area of approximately 10 cm² in adults and containing millions of olfactory sensory neurons. These bipolar neurons feature cilia extending into a layer that traps odorants, with their axons bundling to form about 20 olfactory fila passing through the to in the , which contains approximately 5,500 glomeruli per bulb for initial odor processing. Nasal turbinates and dynamics further enhance odorant delivery to this region, while the (cranial nerve ) contributes to sensations like irritation from pungent smells. Physiologically, odorants dissolve in the nasal mucus and bind to G-protein-coupled receptors on the cilia of olfactory neurons, activating a signaling cascade involving , cyclic AMP (cAMP), and calcium influx that depolarizes the neuron and generates action potentials. These signals converge in the olfactory bulb's glomeruli, where they are relayed via mitral and tufted cells through the to primary cortical targets like the , as well as limbic structures such as the and , bypassing the for direct emotional and memory integration. Humans possess approximately 400 functional genes, encoding proteins that allow detection of a vast array of odorants at low concentrations, though sensitivity varies and declines with age. Beyond basic detection, olfaction profoundly influences and , enhancing flavor in combination with , evoking memories and emotions through limbic connections, and serving as an early indicator of neurodegenerative diseases like Alzheimer's and Parkinson's, where up to 90% of patients experience impairment. Notably, the has emerged as a major cause of olfactory dysfunction. Olfactory dysfunction, or , affects about 24.5% of adults over 53 and 62.5% over 80 as of a 2002 study, often due to viral infections, trauma, or sinonasal issues, underscoring its vulnerability and clinical significance.

Historical Perspectives

Early Discoveries and Ancient Views

In , the emphasized the sense of smell as a diagnostic tool, noting that the of a patient's breath or bodily excretions could indicate underlying diseases, such as foul smells associated with internal imbalances or infections. Hippocratic texts also advocated aromatic substances like herbs and fumigations to restore health by counteracting harmful vapors, reflecting a belief in olfaction's role in maintaining humoral equilibrium. Aristotle, in his work De Anima, described smell as a sense analogous to , perceiving odors through air as a medium, and classified them into categories like sweet and bitter, mirroring flavor distinctions, while viewing pleasant and unpleasant odors as subjective responses rather than nourishing qualities. He positioned olfaction as an intermediate sense between distant perception (like sight) and contact senses (like touch), influencing early philosophical understandings of sensory . The Roman physician advanced anatomical insights into olfaction, observing that the nasal passages connected directly to the brain via perforations in the , allowing odors carried by inhaled air to reach sensitive neural structures for . proposed that odoriferous particles traveled from the to the brain's olfactory regions, distinguishing this pathway from mucus drainage, and integrated these ideas with humoral theory to explain smell's effects on and vitality. Cultural practices in ancient civilizations highlighted olfaction's ritual significance, as burned like and in temple ceremonies to purify spaces, invoke deities, and facilitate spiritual connections, believing these scents bridged the human and divine realms. In Roman rituals, similar use of aromatic resins in sacrifices and public ceremonies evoked sensory experiences tied to and , with odors symbolizing offerings' acceptability to the gods. During medieval Islamic scholarship, (Ibn Sina) synthesized Greek precedents in his , detailing the olfactory system's anatomy as originating in specialized protrusions within the brain's divided regions, accessible via nasal channels that processed scent vapors. He expanded on Galenic views by theorizing that smells influenced the soul through these pathways, contributing to a comprehensive doctrine of the senses that emphasized olfaction's role in cognition and health. These ideas persisted into the , where anatomists like Berengario da Carpi refined descriptions of olfactory structures through dissections, providing detailed accounts of the and including the .

Modern Scientific Milestones

In the late , the foundations of were established through systematic quantitative studies of thresholds and intensity scaling, pioneered by Hendrik Zwaardemaker, who developed methods to detection limits and categorize scents into descriptive classes, building on earlier sensory research traditions. These efforts marked a shift from qualitative observations to experimental precision, enabling the identification of just-noticeable differences in concentrations and influencing subsequent sensory measurement standards. A pivotal breakthrough occurred in 1991 when Linda Buck and identified a large family of genes encoding odorant receptors in mammals, revealing that approximately 1,000 such receptors enable the detection of diverse volatile molecules through G-protein-coupled mechanisms. Their work demonstrated that each expresses a single receptor type, with axons converging to form glomeruli in the , thus elucidating the molecular basis of . For these discoveries, Buck and Axel shared the 2004 in or , highlighting the genetic architecture underlying olfactory coding. Advancements in electrophysiological techniques further propelled olfactory research in the late , with the patch-clamp method—developed by Erwin Neher and Bert Sakmann—applied to isolated neurons by the mid-1980s to record responses to odorants. This approach revealed cyclic nucleotide-gated channels activated by odor-induced second messengers, providing direct evidence of transduction currents in mammalian neurons. By the 2010s, emerged as a transformative tool for olfaction, allowing precise light-mediated activation of channelrhodopsin-expressing olfactory neurons to dissect circuit dynamics and behavioral responses without chemical confounds. Studies using this technique, such as those mapping modulation in the , demonstrated how specific neuronal ensembles drive odor-guided behaviors like attraction or aversion. In the 2020s, single-cell sequencing has unveiled extensive transcriptional diversity among olfactory sensory neurons, identifying distinct subtypes beyond traditional odorant receptor classifications, including non-canonical populations like Cd36-expressing cells with roles in lipid-derived detection. This technology, applied to and tissues, has mapped regulatory networks governing neuronal specification and plasticity, revealing how epigenetic factors orchestrate subtype differentiation during development. Concurrently, has advanced mapping by constructing principal odor maps via graph neural networks trained on perceptual datasets, enabling prediction of molecular scents for over 500,000 untested compounds and unifying tasks like similarity and quality description. These AI models, validated against psychophysical data, bridge to sensory , with applications emerging by 2023 in fragrance and olfactory diagnostics. More recent work as of 2024 has shown that olfaction can detect rapid changes in odors within milliseconds, rivaling the speed of vision, while 2025 studies introduced 3D models of nasal tissue to better understand function.

Fundamental Mechanisms

Olfactory Detection Process

The , a pseudostratified neuro located in the superior of vertebrates, serves as the primary site for odor detection. It consists of several cell types, including olfactory sensory neurons (OSNs) with apical cilia extending into the layer, sustentacular (supporting) cells that provide and metabolic assistance to OSNs, and basal cells that act as progenitors for neuronal replacement. Bowman's glands, embedded in the underlying , secrete a seromucous that forms a protective layer over the , facilitating odorant solubility and transport to receptor sites. Odorants, volatile molecules dissolved in the nasal mucus, diffuse to the ciliary surface of OSNs where they bind to specific G-protein-coupled receptors (GPCRs), known as odorant receptors (ORs), embedded in the plasma membrane. This binding induces a conformational change in the OR, activating the associated heterotrimeric G-protein, specifically G_olf in vertebrates, which dissociates into Gα_olf and Gβγ subunits. The activated Gα_olf then stimulates , catalyzing the conversion of ATP to cyclic AMP (cAMP), thereby increasing intracellular cAMP levels. Elevated cAMP binds to and opens cyclic nucleotide-gated (CNG) ion channels, predominantly composed of CNGA2 subunits, on the ciliary membrane, permitting influx of Na⁺ and Ca²⁺ ions. This cation influx causes membrane , which further amplifies the signal through Ca²⁺-activated chloride channels (e.g., ANO2), leading to Cl⁻ efflux and additional . The pathway can be summarized as: odorant binding to receptor → G_olf activation → stimulation → cAMP production → CNG channel opening → Ca²⁺ influx and . These electrical changes generate action potentials in the axon that project to the . To prevent overstimulation and enable dynamic odor detection, adaptation mechanisms rapidly modulate the transduction process. Receptor desensitization occurs primarily through phosphorylation of the OR by G-protein receptor kinases (GRKs), such as GRK3, which recruits β-arrestin to uncouple the receptor from G_olf, thereby attenuating downstream signaling. Additionally, elevated Ca²⁺ levels activate , which binds to and closes CNG channels, contributing to short-term .

Neural Pathways and Processing

Olfactory sensory neurons (OSNs) transmit signals from the olfactory epithelium to the olfactory bulb, the primary central relay station for olfaction, where axons converge in a structured glomerular layer. Each OSN expresses a single type of odorant receptor, and axons from OSNs sharing the same receptor type project to one or a few specific glomeruli, ensuring that odorant information is organized by receptor identity from the outset of central processing. This convergence creates discrete functional units, with the human olfactory bulb containing an average of approximately 5,500 glomeruli, far exceeding initial estimates and reflecting a high degree of spatial precision in odor mapping. Within these glomeruli, excitatory inputs from OSNs synapse onto the primary dendrites of second-order neurons, primarily mitral and tufted cells, which integrate and relay the signals forward. Mitral and tufted cells serve as the principal output neurons of the , projecting their axons via the lateral to multiple cortical and subcortical targets that form the core of the olfactory cortex. The primary recipient is the , a paleocortical structure dedicated to initial representation and discrimination, followed by connections to the , which links olfaction to memory and spatial navigation via hippocampal pathways. Further projections reach the , a higher-order association area that integrates olfactory inputs with other sensory modalities and is critical for conscious and hedonic . These pathways lack a thalamic relay, allowing direct cortico-cortical transmission unique among sensory systems, which facilitates rapid and flexible processing of olfactory information. Olfactory signals are encoded through a combination of spatial and temporal mechanisms that preserve odor identity across brain regions. Spatial coding arises from the topographic glomerular maps, where distinct odors activate unique combinations of glomeruli, creating a distributed representation that mitral and tufted cells convey to downstream areas. Temporal coding complements this by modulating firing patterns, such as oscillatory rhythms and latency differences, which refine discrimination and concentration encoding. A key principle underlying this efficiency is sparse coding, in which only a small subset of neurons in the or cortex activates per stimulus, enabling robust pattern separation with minimal metabolic cost and high information capacity. In addition to the main olfactory system, many vertebrates, particularly non-human species, possess an accessory olfactory system specialized for detecting pheromones and , primarily via the (VNO). VNO sensory neurons express vomeronasal receptors and project axons to the accessory olfactory bulb, where they converge into glomeruli analogous to the main system but with broader receptive fields. Output from this bulb travels via mitral and tufted-like cells to the and then directly to the , bypassing the to drive instinctive behaviors like and without conscious awareness. This segregated pathway underscores the dual architecture of olfaction, with the accessory route emphasizing rapid, subcortical modulation of endocrine and autonomic responses.

Functions and Roles

Sensory Integration and Perception

The sense of smell profoundly influences flavor perception through retronasal olfaction, where odorants released during travel from the mouth to the via the nasopharynx, integrating with gustatory signals to form a multisensory . This process accounts for approximately 80% of what is perceived as flavor, as demonstrated in psychophysical studies where blocking retronasal significantly diminishes intensity ratings for sweet and bitter stimuli. Retronasal olfaction thus enhances the salience of food-related odors, creating a unified percept that distinguishes flavor from mere . Olfaction also interacts with vision to guide attentional processes, such as when congruent odors direct gaze toward matching visual objects, improving detection speed and accuracy in cluttered scenes. For instance, the scent of strawberries can bias visual search toward images of strawberries, reflecting cross-modal facilitation at early perceptual stages. Interactions with audition are evident in rare cases of auditory-olfactory synesthesia, where sounds involuntarily evoke specific odors, or in broader cross-modal effects where olfactory stimuli modulate auditory processing in the olfactory bulb. These integrations highlight olfaction's role in multisensory binding, potentially rooted in shared neural coding mechanisms in higher cortical areas. Perceptual phenomena in olfaction include illusions like "olfactory white," where mixtures of 30 or more equi-intense odorants spanning physicochemical space converge to a neutral, indistinct quality analogous to white noise in audition. Hedonic valence—judgments of pleasantness or unpleasantness—is a primary dimension of odor perception, with activation in the orbitofrontal cortex (OFC) differentiating positive and negative odors; for example, pleasant scents engage medial OFC regions, while unpleasant ones activate lateral areas, influencing emotional responses. Human olfactory thresholds vary widely but can reach extreme sensitivity, as seen with mercaptans like ethyl mercaptan, detectable at concentrations as low as 0.4 parts per billion (ppb), enabling early warning for hazards like gas leaks. Discrimination abilities are similarly acute, with just-noticeable differences (JNDs) in odor intensity influenced by trigeminal components; odors with stronger irritant properties yield lower JNDs, allowing finer perceptual resolution. A 2014 study estimated that humans can discriminate among more than one trillion distinct olfactory mixtures, though this figure has been challenged by subsequent analyses suggesting a substantially lower capacity, underscoring the system's capacity for nuanced perception.

Behavioral and Survival Applications

The sense of smell plays a crucial role in detection and , enabling animals to identify nutritious sources and avoid harmful substances. Many frugivorous , such as , rely on olfactory cues to assess ripeness by detecting volatile organic compounds emitted during maturation, which signal higher sugar content and palatability. For instance, African elephants can distinguish between ripe and unripe fruits based solely on scent, optimizing their energy intake in resource-scarce environments. Similarly, spider monkeys use chemical signatures from fruit volatiles to select ripe specimens, associating specific odors with nutritional quality during . In addition to attraction to beneficial foods, olfaction aids in toxin avoidance; bitter almonds, containing the cyanogenic glycoside , produce a characteristic almond-like odor primarily from released during , serving as a warning against ingestion since amygdalin can yield toxic , which is lethal in high doses. This sensory mechanism integrates with to enhance overall flavor perception and safety in feeding behaviors. Olfaction contributes to inbreeding avoidance by allowing individuals to detect genetic compatibility through (MHC) odor cues, which influence mate selection in various . In , such as mice, MHC-disparate individuals produce distinct body odors that females prefer, promoting and reducing the risk of deleterious recessive traits from close kin . This preference is mediated by volatile peptides from MHC proteins, enabling olfactory discrimination of kin versus non-kin. Evidence in humans similarly shows that women rate the body odors of MHC-dissimilar men as more pleasant, suggesting an evolutionary adaptation for outbreeding, though this effect may be modulated by factors like oral contraceptives. Such MHC-dependent olfactory signaling underscores smell's role in reproductive fitness by facilitating choices that enhance viability. Pheromonal communication via olfaction is essential for mating, territory marking, and , coordinating social and reproductive behaviors in and mammals. In social like , trail pheromones—volatile hydrocarbons deposited during —guide colony members to food sources and reinforce territorial boundaries, with alarm pheromones triggering defensive aggregation. Mammals, including , employ pheromones from glands to mark territories, signaling ownership and deterring intruders while attracting potential mates during estrus. For , social and mammals use colony-specific odor signatures, derived from cuticular hydrocarbons or urinary volatiles, to distinguish relatives and allocate cooperative behaviors like . These chemical signals ensure by synchronizing mating and maintaining social cohesion. Olfactory navigation supports survival through scent-based orientation and homing, as seen in and . Ants follow trails laid by foragers, using volatile cues to navigate complex terrains and return to nests efficiently, with trail strength modulating path choices based on food profitability. In , olfactory imprinting during early life stages allows juveniles to memorize natal stream odors, enabling precise homing for spawning years later via detection of specific waterborne chemicals. This mechanism integrates with geomagnetic cues in some species but relies primarily on smell for fine-scale navigation, ensuring reproductive site fidelity despite long migrations.

Genetic Foundations

Olfactory Receptor Genes

The (OR) genes constitute the largest multigene family in the , encoding G protein-coupled receptors that detect odorant molecules. In humans, approximately 400 of these genes are functional, out of a total of around 800 OR genes including . These genes are organized in clusters distributed across nearly all chromosomes, with major clusters located on chromosomes 1, 6, and 11. The pseudogene ratio is notably higher in humans compared to other mammals, such as dogs, where only about 18% of OR genes are pseudogenes, reflecting differences in olfactory reliance. Vertebrate OR genes are phylogenetically divided into two main classes: class I, which are fish-like and primarily expressed in zone 1 of the , and class II, which are terrestrial-specific and expressed in zones 2 through 4. This zonal expression pattern ensures in the olfactory sensory neurons, contributing to the specificity of detection. Gene regulation involves dedicated promoters proximal to each OR gene and multiple enhancers, such as the Greek islands and elements, that drive transcription in specific epithelial zones. A key feature is monoallelic expression, where each olfactory neuron selects and expresses only one allele of a single OR gene, ensuring singular receptor choice per cell. Evolutionary pseudogenization has significantly reduced the functional OR repertoire in humans, with about 60% of OR genes classified as pseudogenes, compared to only 18% in mice. This high rate of pseudogenization is associated with a diminished reliance on olfaction in , possibly due to enhanced visual capabilities and . These genetic changes underlie the foundational role of OR genes in the olfactory detection process, where expressed receptors bind odorants to initiate sensory signaling.

Genetic Diversity and Evolution

Genetic diversity in (OR) genes contributes significantly to individual differences in among humans. Single polymorphisms (SNPs) and deletions in OR genes can lead to specific s, where individuals are unable to detect particular odors. A well-documented example is the OR7D4 gene, which encodes a receptor sensitive to , a steroidal compound found in sweat and ; variants such as the RT/WM in OR7D4 reduce or eliminate of this in approximately 30% of the population, leading to specific anosmia. These polymorphisms highlight how subtle genetic variations can alter olfactory sensitivity without affecting overall smell function, influencing experiences like food preferences or derived from body odors. Evolutionary pressures have shaped the size and composition of OR gene repertoires across vertebrate lineages, reflecting adaptations to environmental demands. In aquatic vertebrates, such as teleost fish, the OR gene family has undergone significant expansion, with over 100 functional genes in some species dedicated to detecting water-soluble odorants like , which are crucial for and in aquatic habitats. Conversely, in , including humans, there has been a notable contraction of the OR —down to about 400 functional genes from an ancestral mammalian estimate of around 1,000—correlated with the evolutionary shift toward enhanced visual reliance and reduced dependence on olfaction for survival.31060-0) This illustrates how sensory priorities drive genetic changes, with pseudogenization of OR genes in hominids potentially linked to adaptations in social and dietary behaviors. Twin studies provide evidence for a substantial genetic basis underlying variations in olfactory sensitivity. Analyses of monozygotic and dizygotic twins indicate that accounts for 40-60% of the variance in identification, intensity , and threshold detection, with the remainder attributed to environmental factors. These estimates underscore the interplay between and experience in shaping olfactory abilities, as shared environments explain only a minor portion of differences. Recent advancements in have explored the potential to address defects causing olfactory dysfunction in animal models of , offering insights into therapeutic restoration of olfaction. A 2021 study in a model of the ciliopathy Bardet-Biedl syndrome demonstrated that adenoviral-mediated delivery of wild-type BBS1 can restore olfactory cilia structure and partially recover olfactory signaling and behavioral responses to odors. Such findings highlight the feasibility of precision interventions for genetic olfactory disorders, though challenges remain in translating these to humans.

Comparative Olfaction

Across Vertebrates

The sense of smell in s exhibits remarkable diversity, shaped by evolutionary adaptations to diverse environments, from aquatic to terrestrial habitats. Olfactory systems across vertebrate classes vary in receptor types, neural organization, and sensitivity, reflecting ecological demands such as , predation, and . While all vertebrates share a basic chemosensory architecture involving olfactory epithelia and central processing in the , differences in receptor repertoires and accessory structures underscore specialized functions. In , olfaction primarily detects water-soluble odorants through G-protein-coupled receptor (GPCR) mechanisms, such as vomeronasal-like receptors and trace amine-associated receptors, which are optimized for and other hydrophilic molecules essential for locating prey or mates in aquatic environments. These systems achieve extraordinary sensitivity; for instance, can detect (specifically like ) at dilutions as low as 1 part in 10 million, enabling them to sense injured prey from kilometers away. This high acuity is facilitated by a large and direct neural projections to the , bypassing more complex processing seen in tetrapods. Amphibians and reptiles possess bimodal olfactory systems capable of processing both - and air-borne odorants, an to their semi-aquatic or transitional lifestyles. In amphibians like frogs, the main handles aqueous cues during larval stages, while adults rely more on volatile detection; reptiles extend this versatility with the (VNO), or Jacobson's organ, which in and snakes allows tongue-based sampling of pheromones and environmental scents by delivering them to the organ's sensory lining. This accessory structure enhances chemoreception for social signaling and hunting, with reptiles showing a higher of vomeronasal receptors compared to the main . Birds generally have a reduced olfactory capability relative to other s, with approximately 500 functional (OR) genes, a contraction from the ancestral repertoire, reflecting their emphasis on vision and audition. However, certain species feature macroglomeruli in the —enlarged synaptic structures—for processing food-related s, aiding in ; procellariiform seabirds, for example, use smell to locate prey patches at . In migratory birds like pigeons, olfaction contributes to , with experiments showing they can home using odor cues alone when visual landmarks are obscured. This olfactory role, though secondary, integrates with geomagnetic and visual senses for orientation. Mammals display a broad spectrum of olfactory prowess, with macrosmatic species—those relying heavily on —showing expansions in OR genes; dogs, for instance, possess around 1,100 functional OR genes, enabling discrimination of scents at sensitivities up to 300 times greater than humans, crucial for tracking and detection tasks. This genetic expansion correlates with an enlarged and vomeronasal system, supporting roles in predation, social bonding, and territory marking. In contrast, microsmatic mammals like have fewer OR genes (around 400 in humans), prioritizing other senses, yet retain functional olfaction for flavor and hazard detection.02745-7)

In Invertebrates and Other Animals

Invertebrate olfactory systems exhibit remarkable diversity, adapted to aquatic, terrestrial, and aerial environments, with chemosensory structures often integrated into appendages like antennae or antennules. In insects, olfaction primarily occurs through specialized sensilla on the antennae, which house olfactory receptor neurons that detect volatile odorants. These sensilla contain odorant-binding proteins (OBPs) that solubilize and transport hydrophobic odor molecules across the sensillar lymph to the receptors on the neuronal membrane. Insects utilize two main receptor types: conventional odorant receptors (ORs), which are heteromers of a tuning OR and the co-receptor Orco forming ligand-gated ion channels, and ionotropic receptors (IRs), which are variant ionotropic glutamate receptors functioning as ligand-gated cation channels for detecting a broader range of cues including pheromones and general odorants.01002-6) Pheromone detection in moths, for instance, involves specialized ORNs projecting to the macroglomerular complex (MGC) in the antennal lobe, a male-specific structure where individual glomeruli process specific pheromone components to elicit oriented flight behaviors toward mates. Crustaceans, as aquatic arthropods, rely on antennular aesthetascs—tufts of hair-like sensilla on the antennules—for detecting water-borne odors, enabling behaviors such as , , and predator avoidance. These aesthetascs contain olfactory sensory neurons (s) that express a variety of chemoreceptors, including gustatory receptors (GRs) and ionotropic receptors (IRs), alongside G protein-coupled receptors (GPCRs) resembling ORs in decapods like lobsters. In species such as the Panulirus argus, these OR-like GPCRs are tuned to and peptides in , facilitating prey localization through plume tracking. The olfactory signals are processed in the antennular lobe, analogous to the antennal lobe, where axons converge into glomeruli for initial coding. Cnidarians, basal metazoans lacking a centralized , possess simple chemosensory cells distributed across their that detect dissolved chemical cues for prey capture and environmental navigation. These cells, often nematocyte-associated or free-standing, express G protein-coupled receptors (GPCRs) and other chemoreceptors to sense and peptides, triggering feeding responses in polyps and medusae. Such rudimentary chemosensation represents an evolutionary precursor to the more complex bilaterian olfactory systems, providing insights into the origins of metazoan chemical signaling. Recent advances in have illuminated the fine-scale organization of olfactory circuits, particularly in the fruit fly Drosophila melanogaster. Electron microscopy reconstructions from the 2020s have mapped the antennal lobe's approximately 50 glomeruli, revealing distinct synaptic motifs where broadly tuned glomeruli integrate diverse inputs via extensive local connections, while narrowly tuned ones, such as those for pheromones, exhibit specialized circuitry for precise behavioral outputs. These glomerular modules parallel the functional segregation seen in olfactory bulbs, underscoring conserved principles in coding across phyla.

Olfaction Beyond Animals

Volatile Detection in Plants

Plants perceive airborne volatile organic compounds (VOCs) through specialized receptor proteins that initiate signaling cascades, enabling responses to environmental cues without a centralized . Unlike animal olfaction, which relies on G-protein-coupled receptors in olfactory neurons, plant detection involves membrane-bound receptors such as those for the gaseous , including ETR1 and ERS1, which bind directly to modulate downstream via histidine kinase signaling. For derivatives like the volatile (), perception occurs through the COI1-JAZ co-receptor complex, which ubiquitinates repressor proteins to activate defense-related transcription factors. Emerging research also implicates KARRIKIN INSENSITIVE2 (KAI2) as a key receptor in a broader pathway for detecting stress-induced VOCs, facilitating inter-plant communication by integrating signals from diverse airborne molecules. Upon detecting these volatiles, plants trigger rapid physiological responses, often involving induced defenses against herbivores or pathogens. For instance, when tomato plants (Solanum lycopersicum) are infested by herbivores like the (Spodoptera exigua), they release herbivore-induced plant volatiles (HIPVs) such as (E)-β-ocimene and (3E)-4,8-dimethyl-1,3,7-nonatriene, which attract predatory wasps (Cotesia marginiventris) to parasitize the herbivores, thereby reducing damage to the plant. These responses are mediated by signaling, leading to transcriptional reprogramming that enhances volatile emission within hours of perception. Volatile detection also underpins , including and attraction. In , receiver plants exposed to VOCs from damaged relatives exhibit enhanced defense responses compared to those from non-kin; for example, in (), volatiles from clipped kin neighbors reduce subsequent herbivory on receivers by up to 42% more effectively than those from strangers, indicating kin-specific communication for mutual protection. Floral scents, composed of VOC blends like monoterpenes and benzenoids, serve to attract specific ; for example, petunia flowers (Petunia hybrida) emit volatile to draw nocturnal moths, with emission rhythms synchronized to pollinator activity for optimal . Evolutionarily, plants have adapted volatile detection as a sessile strategy for , relying on decentralized signaling rather than neural processing. Perception of GLVs, for instance, induces cytosolic calcium waves that propagate systemically, activating calcium-dependent protein kinases and altering for defense priming without requiring a . This calcium-mediated transduction, coupled with changes in and levels, allows rapid acclimation to threats, highlighting the sophistication of chemical sensing despite the absence of specialized sensory organs.

Engineered and Artificial Systems

Engineered and artificial systems aim to replicate the olfactory capabilities of biological noses through arrays and computational , enabling machines to detect and identify volatile compounds. Electronic noses (e-noses) typically consist of an array of gas s, such as metal-oxide semiconductor (MOS) s, that respond to volatile organic compounds (VOCs) by generating electrical signals based on changes in resistance or conductance. These signals form a pattern that is analyzed using algorithms, like (PCA) or artificial neural networks (ANNs), to classify odors without identifying individual molecules. For instance, in control, e-noses have been employed to monitor freshness and detect spoilage in products like and dairy by recognizing patterns indicative of bacterial contamination or oxidation. Bio-inspired designs draw from biological olfaction to enhance performance, incorporating synthetic G-protein-coupled receptors (GPCRs) or that mimic olfactory receptors (ORs) for improved selectivity. These systems often integrate , such as carbon nanotubes functionalized with peptides derived from OR binding sites, to simulate the molecular recognition in biological noses. In the 2020s, advancements in AI have further refined these designs; models, including convolutional neural networks (CNNs), have achieved classification accuracies exceeding 90% for complex odor mixtures by processing sensor data as images or time-series inputs. For example, hybrid CNN-linear discriminant analysis (LDA) models have demonstrated 93% accuracy in distinguishing environmental pollutants. Such bio-mimetic approaches provide a brief nod to biological transduction mechanisms, where odorants bind to receptors to trigger signaling cascades. Applications of these systems span security, healthcare, and . In , e-noses equipped with or MOS sensors identify trace vapors from nitroaromatic compounds, offering portable alternatives to canine detection with response times under 10 seconds. For diagnostics, breath analysis via e-noses detects biomarkers, such as VOCs associated with , enabling non-invasive screening with sensitivities comparable to traditional methods. In , drone-mounted e-noses facilitate real-time , mapping odor plumes from plants or industrial emissions to localize sources over large areas. Despite progress, engineered olfactory systems face significant challenges compared to their biological counterparts, particularly in sensor specificity and long-term stability. MOS sensors often suffer from cross-sensitivity to and interfering gases, leading to reduced selectivity for target odors, whereas biological noses achieve high specificity through diverse receptor tuning. Additionally, sensor drift—gradual shifts in baseline response over time due to environmental factors or material degradation—necessitates frequent recalibration, limiting reliability in field applications and contrasting with the adaptive of living olfactory epithelia. Ongoing research focuses on drift compensation algorithms and hybrid bio-electronic interfaces to bridge these gaps.

Societal and Pathological Dimensions

Cultural Representations of Smell

Across languages, vocabulary remains notably limited compared to other sensory domains, with most cultures possessing only a handful of basic terms for smells. research indicates that while like English typically feature around four primary categories (e.g., "stinky," "fruity," "spicy," "burnt"), some non-industrialized languages exhibit richer lexicons, such as the Maniq foragers of , who use over a dozen dedicated terms to describe specific olfactory qualities. This variation underscores smell's abstract nature in linguistic expression, often relying on metaphors that link odors to moral or social concepts, as in the English "stinking rich," which equates foul smells with excess or corruption. Historically, smell has been integral to cultural and religious practices, particularly through the use of perfumes and incenses. In , —a sacred compound of up to 16 ingredients including resins, , wine, and spices—was burned in temples for rituals, believed to appease gods and promote , reflecting the society's view of aromas as bridges between the earthly and divine realms. During the in , perfumes gained prominence among the elite to counteract body odors resulting from infrequent bathing, with alcohol-based formulations like rosemary-infused "Queen of Hungary Water" marking innovations in scent application for and . Literature has long captured smell's evocative power, as seen in Marcel Proust's In Search of Lost Time (1913–1927), where the aroma of a madeleine cake steeped in lime-blossom tea triggers an involuntary flood of childhood memories for the narrator, illustrating olfaction's unique capacity to bypass conscious recall and access the subconscious. This "Proustian moment" has influenced cultural understandings of smell as a profound mnemonic device. Cultural attitudes toward body odor reveal stark contrasts, with Western societies often imposing taboos that equate natural scents with uncleanliness or social deviance, driving the deodorant industry's growth since the early 20th century. In contrast, some Indigenous groups, such as certain Amazonian communities, view unmasked body odors as neutral or even integral to identity and social bonding, without the stigma prevalent in industrialized contexts. In contemporary arts and technology, smell is increasingly harnessed for immersive experiences. Olfactory installations, like Norwegian artist Sissel Tolaas's scent-based works exploring urban and human odors, have appeared in major venues such as the Institute of Contemporary Art in (2022), challenging viewers to engage with aroma as an artistic medium. By 2025, virtual reality scent technologies have advanced, incorporating olfactory displays that release targeted aromas during simulations—such as in therapeutic VR games for cognitive training—enhancing multisensory immersion and emotional depth.

Disorders and Clinical Impacts

Olfactory disorders include several conditions that disrupt normal smell perception. represents the total inability to detect odors, whereas denotes a diminished capacity to sense smells. involves the distortion of odors, leading to incorrect perceptions of scents, and manifests as the of smells without any external stimulus. These qualitative and quantitative impairments can significantly affect , , and by reducing detection of hazards like or spoiled . Causes of olfactory dysfunction are broadly classified as congenital or acquired. Congenital forms often stem from genetic anomalies, such as , a rare disorder with a prevalence of approximately 1 in 30,000 to 50,000 individuals, occurring more frequently in males (1 in 30,000) than females (1 in 125,000), characterized by or alongside due to . Acquired causes predominate in clinical practice and include viral upper respiratory infections, with leading to temporary olfactory loss in 50-80% of cases through mechanisms like epithelial inflammation and viral persistence in the olfactory mucosa. Long-term studies as of 2025 show that up to 66% of individuals with prior infection but no self-reported change exhibit on testing, highlighting persistent subclinical effects. Other common etiologies encompass head trauma, which damages olfactory nerves or bulbs in up to 15% of severe cases, and neurodegenerative conditions like , where affects over 90% of patients and often emerges years before motor symptoms. Diagnosis relies on objective assessments to differentiate and quantify impairment. Standardized psychophysical tests, such as the Smell Identification Test (UPSIT), evaluate odor identification through a 40-item scratch-and-sniff format, yielding scores that classify (below 20), (20-34), or normosmia (above 34) while accounting for age and gender norms. Structural evaluation via magnetic resonance imaging (MRI) detects atrophy or volume reduction, which correlates with chronic dysfunction and helps rule out tumors or congenital malformations. Management strategies target underlying causes and promote recovery where possible. Olfactory training, involving daily exposure to essential oils like , , , and for 20-30 seconds each over 3-6 months, achieves recovery rates of 30-50% in postviral and posttraumatic by stimulating neural plasticity. Corticosteroids, such as oral or intranasal sprays, address inflammatory causes like postviral , with short courses (1-2 weeks) improving function in 25-40% of select cases, though evidence is mixed for COVID-19-related loss. For congenital genetic disorders like , hormone replacement remains standard, but research into gene therapies targeting mutations in genes such as ANOS1 is ongoing, with recent identification of novel variants as of 2025.

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