Sense of smell

Source: Wikipedia, the free encyclopedia.

Smell
chemicals in the environment that are used to form the sense of smell
Identifiers
MeSHD012903
Anatomical terminology]

The sense of smell, or olfaction,

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

neurodegenerative disease.[5][6]

History of study

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

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.

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

Taste

Flavor perception is an aggregation 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,

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

Smell and

flavor
results from interactions between smell and taste, a smound may result from interactions between smell and sound.

Inbreeding avoidance

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]

Humans can detect blood relatives from olfaction.[25] 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.[26] 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.[27]

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.[28]

Guiding movement

Some animals use

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.[29][30]

Genetics

Main article: Evolution of olfaction

Different people smell different odors, and most of these differences are caused by genetic differences.

cilantro (coriander) has been linked to the olfactory receptor OR6A2.[34]

Variability amongst vertebrates

The importance and sensitivity of smell varies among different organisms; most

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.[36] 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 microsmotic.

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.[37] 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,[38] 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

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

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.[39] 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.[40] 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."[41] 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."[42][43] The logic of his paper has in turn been criticized by the authors of the original paper.[44]

Physiological basis in vertebrates

Main olfactory system

Main article: Olfactory system

In humans and other

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.[46]

Molecules of odorants passing through the

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

sensilla, which are present in insect antenna, palps, and tarsa, but also on other parts of the insect body. Odorants penetrate into the cuticle pores of chemosensory sensilla and get in contact with insect odorant-binding proteins (OBPs) or Chemosensory proteins
(CSPs), before activating the sensory neurons.

Receptor neuron

The binding of the

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.[47] Other subpopulations, such as those that express the receptor guanylyl cyclase GC-D (Gucy2d)[48] or the soluble guanylyl cyclase Gucy1b2,[49] use a cGMP cascade to transduce their odorant ligands.[50][51][52] 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

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

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,[54] pattern separation,[55] and memory functions,[56]
respectively.

The mitral cells leave the olfactory bulb in the

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.[58]

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

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.[59] 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.[60]

After binding the odorant, the receptor is activated and will send a signal to the glomeruli

olfactory cortex. Olfactory cortex is thought to have associative memories,[63] so that it resonates to this bulbar pattern when the odor object is recognized.[64] The cortex sends centrifugal feedback to the bulb.[65] 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.[64][66] During odor search, feedback could also be used to enhance odor detection.[67][64] The distributed code allows the brain to detect specific odors in mixtures of many background odors.[68]

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.[69] While chemotopy remains a highly controversial concept,[70] evidence exists for perceptual information implemented in the spatial dimensions of olfactory networks.[69]

Accessory olfactory system

Many animals, including most mammals and reptiles, but not humans,

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

bed nucleus of the stria terminalis, and from there to the hypothalamus
, where they may influence aggression and mating behavior.

In insects

Main article: Insect olfaction

Insect olfaction refers to the function 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.[74] However, a recent study has demonstrated the olfactory role of ovipositor in fig wasps.[75] 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.[74]

Insects are capable of smelling and differentiating between thousands of

sensitively and selectively.[72][76] 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.[76] Some insects, such as the moth Deilephila elpenor
, use smell as a means to find food sources.

In plants

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

The tendrils of plants are especially sensitive to airborne

dodder make use of this in locating their preferred hosts and locking on to them.[77] 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

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 encountered incidents where proximity of an industrial source or landfill produced adverse reactions among nearby residents regarding airborne odor. The basic theory of odor analysis is to measure what extent of dilution with "pure" air is required before the sample in question is rendered indistinguishable from the "pure" or reference standard. Since each person perceives odor differently, an "odor panel" composed of several different people is assembled, each sniffing the same sample of diluted specimen air. A field olfactometer can be utilized to determine the magnitude of an odor.

Many air management districts in the 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.

Classification

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.[78]
  • Henning's prism[79]
  • Zwaardemaker smell system (invented by Hendrik Zwaardemaker)

Disorders

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

olfaction.[81]

See also

References

  1. Latin: olfactus, past participle of olfactere, "to smell", itself from olere "to emit a smell" and facere "to make".[1]
  1. ^ Harper, Douglas. "olfaction". Online Etymology Dictionary.
  2. ISBN 978-0-87893-572-7
    .
  3. doi:10.1002/ffj.3249
    .
  4. ISBN 978-1-4292-3719-2
    .
  5. PMID 25788559
    .
  6. S2CID 5121325. Archived
    from the original on 1 December 2021. Retrieved 10 October 2019.
  7. ^ Kimble, GA; Schlesinger, K (1985). Topics in the History of Psychology, Volume 1. L. Erlbaum Associates.
  8. ^ Holtsmark, E (1978). "Lucretius, the biochemistry of smell, and scientific discovery". Euphrosyne: Revista de Filologia Clássica. 9: 7–18. Archived from the original on 8 May 2022. Retrieved 14 August 2020 – via academia.edu.
  9. S2CID 218627484. Archived
    (PDF) from the original on 5 November 2020. Retrieved 17 February 2021.
  10. PMID 1840504
    .
  11. PMID 12612342
    .
  12. ISBN 0-205-42651-4
    .
  13. PMID 17603536
    .
  14. PMID 27112241
    .
  15. PMID 8985605
    .
  16. PMID 12183125
    .
  17. S2CID 1073550
    . See also the editorial on p. 315.
  18. ^
    OCLC 882238865
    .
  19. ^
    S2CID 2671577
    .
  20. ^
    PMID 11148312
    .
  21. PMID 20181598
    .
  22. ^ Peeples, L (23 February 2010). "Making scents of sounds: noises may alter how we perceive odors". Scientific American. Archived from the original on 19 March 2011. Retrieved 30 December 2012.
  23. S2CID 15621496
    .
  24. S2CID 8568275
    .
  25. S2CID 42316168
    .
  26. PMID 12810039
    .
  27. PMID 19067327
    .
  28. PMID 17997307
    .
  29. doi:10.1146/knowable-030623-4
    . Retrieved 13 March 2023.
  30. S2CID 243966350. Archived
    from the original on 20 June 2022. Retrieved 13 March 2023.
  31. ^ Howgego, J (1 August 2013). "Sense for scents traced down to genes". Nature News. Archived from the original on 8 October 2017. Retrieved 7 September 2013.
  32. PMID 23910657
    .
  33. PMID 22714804
    .
  34. ^ Callaway, E (12 September 2012). "Soapy taste of coriander linked to genetic variants". Nature News. Archived from the original on 29 March 2021. Retrieved 7 September 2013.
  35. PMID 34180521
    .
  36. doi:10.1111/ibi.12544. Archived
    (PDF) from the original on 26 April 2019. Retrieved 21 July 2019.
  37. ^ Tyson, P (4 October 2012). "Dogs' dazzling sense of smell: what lies behind their exceptional gift of sniff?". PBS. Archived from the original on 7 November 2016. Retrieved 2 November 2016.
  38. ^ a b Padodara, RJ; Ninan, J (2014). "Olfactory sense in different animals". The Indian Journal of Veterinary Science. 2 (1). Retrieved 14 August 2020.
  39. ^ Briggs, H (20 March 2014). "Nose can detect one trillion odours". BBC News. Archived from the original on 2 March 2018. Retrieved 20 June 2018.
  40. ^ Williams, SCP (20 March 2014). "Human nose can detect a trillion smells". Science Now/AAAS News. Archived from the original on 15 October 2021. Retrieved 23 March 2014.
  41. PMID 24653035
    .
  42. PMID 26151672
    .
  43. PMID 26151672
    .
  44. bioRxiv 10.1101/022103
    .
  45. ^ Bear, M; Connors, B; Paradiso, M (2007). Neuroscience: Exploring the Brain. USA: Lippincott Williams & Wilkins. pp. 265–275.
  46. ^ Boroditsky, L (1999). "Taste, Smell, and Touch: Lecture Notes" (PDF). p. 1. Archived (PDF) from the original on 30 November 2012. Retrieved 29 May 2012.
  47. S2CID 2864195
    .
  48. PMID 7724600
    .
  49. PMID 25701815
    .
  50. PMID 17724338
    .
  51. PMID 20637621
    .
  52. PMID 27916458
    .
  53. ^ Morris, H; Schaeffer, JP (1953). Morris' Human Anatomy: A Complete Systematic Treatise (11 ed.). New York: Blakiston. pp. 1218–1219.
  54. PMID 24672011
    .
  55. PMID 26752161
    .
  56. PMID 18923046
    .
  57. S2CID 28094328
    .
  58. PMID 19699095
    .
  59. PMID 11418503
    .
  60. ^
    S2CID 22505357
    .
  61. PMID 77765
    .
  62. S2CID 7932310
    .
  63. S2CID 3947494
    .
  64. ^
    S2CID 27989941. Archived
    from the original on 14 August 2022. Retrieved 14 August 2022.
  65. ^ "Olfactory Cortex - an overview | ScienceDirect Topics". www.sciencedirect.com. Archived from the original on 14 August 2022. Retrieved 14 August 2022.
  66. PMID 28641388
    .
  67. S2CID 43129982
    .
  68. PMID 25086608. Archived
    (PDF) from the original on 4 November 2018. Retrieved 4 November 2018.
  69. ^
    S2CID 207090474. Archived
    from the original on 6 March 2023. Retrieved 19 February 2019.
  70. S2CID 30978362. Archived
    (PDF) from the original on 18 July 2018. Retrieved 1 December 2018.
  71. S2CID 19372867
    .
  72. ^
    PMID 26416146
    .
  73. S2CID 207568844
    .
  74. ^
    PMID 25649823
    .
  75. PMID 28468812
    .
  76. ^
    PMID 29588018
    .
  77. ^ Fountain, H (3 October 2006). "This plant has the sense of smell (loves tomatoes, hates wheat)". The New York Times. Archived from the original on 1 July 2017. Retrieved 22 February 2017.
  78. ^ Berenstein, N (25 August 2014). "Is there a dewey decimal system for the library of smells?". Nadia Berenstein. Archived from the original on 17 April 2021. Retrieved 15 August 2020.
  79. ISBN 9783750176669
    .
  80. ^
    ISBN 978-0-7506-7287-0
    .
  81. PMID 33600902
    .

External links

Media related to Smell at Wikimedia Commons