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The sense of smell, or olfaction, is the
special sense In medicine Medicine is the Art (skill), art, science, and Praxis (process) , practice of caring for a patient and managing the diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment or Palliative care , palliation of their in ...
through which smells (or odors) are perceived. The sense of smell has many functions, including detecting hazards, and
pheromone A pheromone (from Ancient Greek ' "to bear" and hormone) is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the s ...

pheromone
s, and plays a role in
taste The gustatory system or sense of taste is the sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory re ...

taste
. It occurs when an
odor An odor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States. Cur ...
binds to a receptor within the
nasal cavity The nasal cavity is a large, air-filled space above and behind the human nose, nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. ...

nasal cavity
, transmitting a signal through the
olfactory system The olfactory system, or sense of smell, is the sensory nervous system, sensory system used for smelling (olfaction). Olfaction is one of the special senses, that have directly associated specific organs. Most mammals and reptiles have a main ol ...

olfactory system
.
Glomeruli ''Glomerulus'' () is a common term used in anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργαν ...
aggregate signals from these receptors and transmit them to the
olfactory bulb The olfactory bulb (Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republi ...
, where the sensory input will start to interact with parts of the brain responsible for smell identification,
memory Memory is the faculty of the by which or is , stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If s could not be remembered, it would be impossible for language, r ...

memory
, and
emotion Emotions are psychological state A mental state is a state of mind that an agent is in. Most simplistically, a mental state is a mental condition. It is a relation that connects the agent with a proposition. Several of these states are a comb ...

emotion
. There are many different causes for alteration, lack, or disturbance to a normal sense of smell, and can include damage to the
nose A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for Respiration (physiology), respiration alongside the mouth. Behind the nose are the olfactory mucosa and the Paranasal sinus, sinuses. Be ...

nose
or smell receptors, or central problems affecting the brain. Some causes include
upper respiratory infections An upper respiratory tract infection (URTI) is an illness caused by an acute infection, which involves the upper respiratory tract, including the human nose, nose, Paranasal sinus, sinuses, pharynx, or larynx. This commonly includes nasal obstruct ...
,
traumatic brain injury A traumatic brain injury (TBI), also known as an intracranial injury, is an injury to the brain caused by an external force. TBI can be classified based on severity (ranging from mild traumatic brain injury TBI/concussionto severe traumatic ...
, and
neurodegenerative disease A neurodegenerative disease is caused by the progressive loss of structure or function of neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells ...
.


History of study

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. As the Epicurean and atomistic Roman philosopher
Lucretius Titus Lucretius Carus ( , ; 99 – c. 55 BC) was a Ancient Rome, Roman Roman literature, poet and Ancient Roman philosophy, philosopher. His only known work is the philosophical poem ''De rerum natura'', a didactic work about the tenets and ...
(1stcentury BCE) speculated, different odors are attributed to different shapes and sizes of "atoms" (odor molecules in the modern understanding) that stimulate the olfactory organ. A modern demonstration of that theory was the cloning of olfactory receptor proteins by Linda B. Buck and
Richard Axel Richard Axel (born July 2, 1946) is an American molecular biologist and university professor in the Department of Neuroscience at Columbia University Columbia University (also known as Columbia, and officially as Columbia University in the C ...

Richard Axel
(who were awarded the
Nobel Prize The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel Alfred Bernhard Nobel ( , ; 21 October 1833 – 10 December 1896) was a Swedish chemist, engineer, inventor, busines ...
in 2004), and subsequent pairing of odor molecules to specific receptor proteins. Each odor receptor molecule recognizes only a particular molecular feature or class of odor molecules.
Mammals Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the R ...
have about a thousand
gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

gene
s that code for odor reception. 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. In mammals, each
olfactory receptor neuron An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electric ...
expresses only one functional odor receptor. Odor receptor nerve cells function like a key–lock system: if the airborne molecules of a certain chemical can fit into the lock, the nerve cell will respond. 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 A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecule
. The weak-shape theory, known as the
odotope theoryOdotope theory, also known as weak shape theory, is a theory of how olfactory receptors bind to odor molecules. The theory proposes that a combination of shape factors determine the coupling. The word itself is an analogy to epitopes. See also ...
, 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). According to a new study, researchers have found that a functional relationship exists between molecular volume of odorants and the olfactory neural response. An alternative theory, the vibration theory proposed by Luca Turin, posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by
quantum tunnelling Quantum tunnelling or tunneling (US) is the quantum mechanical Quantum mechanics is a fundamental theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The proce ...
. However, the behavioral predictions of this theory have been called into question. There is no theory yet that explains olfactory perception completely.


Functions


Taste

Flavor perception is an aggregation of
auditory
auditory
,
taste The gustatory system or sense of taste is the sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory re ...

taste
, haptic, and smell sensory information.
Retronasal smellRetronasal smell, retronasal olfaction Olfaction, or the sense of smell, is the special sense through which smells (or odors) are perceived. It occurs when an odor binds to a olfactory receptor, receptor within the nasal cavity, transmitting a si ...
plays the biggest role in the sensation of flavor. During the process of
mastication Chewing or mastication is the process by which food Food is any substance consumed to provide nutritional support for an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any indiv ...
, the tongue manipulates food to release odorants. These odorants enter the nasal cavity during exhalation. 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. Smell,
taste The gustatory system or sense of taste is the sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory re ...

taste
, and
trigeminal The trigeminal nerve (the fifth cranial nerve, or simply CN V) is a nerve responsible for sensation in the face and motor functions such as biting and chewing; it is the most complex of the cranial nerves Cranial nerves are the nerve A ner ...

trigeminal
receptors (also called
chemesthesis Chemesthesis is the chemical sensitivity of the skin and mucous membranes. Chemesthetic sensations arise when chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular enti ...
) together contribute to
flavor Flavor, or flavour, is the perceptual Perception (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area arou ...

flavor
. The human
tongue The tongue is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Brita ...

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 Exhalation (or expiration) is the flow of the breath Breathing (or ventilation) is the process of moving air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represent ...
that the smell's contribution to flavor occurs, in contrast to that of proper smell, which occurs during the
inhalation Inhalation happens when air or other gases enter the lungs. Inhalation of air Inhalation of air, as part of the cycle of breathing File:X-ray video of a female American alligator (Alligator mississippiensis) while breathing - pone.0004497. ...

inhalation
phase of breathing. The olfactory system is the only human sense that bypasses the thalamus and connects directly to the forebrain.


Hearing

Smell and
sound In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...

sound
information has been shown to converge in the olfactory tubercles of
rodents Rodents (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Ro ...

rodents
. This neural convergence is proposed to give rise to a perception termed
smound Smound is a perception or sense experience created from the convergence of and in the brain. The word is a of smell and sound. Research by Wesson and Wilson The smound concept is based on a study done by Daniel Wesson, PhD and Donald Wilson, PhD ...
. Whereas a
flavor Flavor, or flavour, is the perceptual Perception (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area arou ...

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 The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as Tumor immunology, cancer cells and objects such ...
; 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. Humans can detect blood relatives from olfaction. 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 The Westermarck effect, also known as reverse sexual imprinting, is a psychological hypothesis that people who live in close domestic proximity during the first few years of their lives become desensitized to sexual attraction Sexual attract ...
. Functional imaging shows that this olfactory kinship detection process involves the frontal-temporal junction, the , and the dorsomedial
prefrontal cortex In mammalian brain A brain is an organ that serves as the center of the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, senso ...

prefrontal cortex
, but not the primary or secondary olfactory cortices, or the related
piriform cortex The piriform cortex, or pyriform cortex, is a region in the brain A brain is an organ (anatomy), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to th ...
or
orbitofrontal cortex The orbitofrontal cortex (OFC) is a prefrontal cortex In mammalian brain anatomy, the prefrontal cortex (PFC) is the cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum ...

orbitofrontal cortex
. Since
inbreeding Inbreeding is the production of offspring In biology, offspring are the young born of living organism, organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as ...
is detrimental, it tends to be avoided. In the house mouse, the
major urinary protein Major urinary proteins (Mups), also known as α2u-globulins, are a protein subfamily, subfamily of proteins found in abundance in the urine and other secretions of many animals. Mups provide a small range of identifying information about the donor ...
(MUP) gene cluster provides a highly polymorphic scent signal of genetic identity that appears to underlie
kin recognitionKin recognition, also called kin detection, is an organism's ability to distinguish between close genetic kin and non-kin. In evolutionary biology Evolutionary biology is the subfield of biology that studies the evolution, evolutionary processes ...
and inbreeding avoidance. Thus, there are fewer matings between mice sharing MUP haplotypes than would be expected if there were random mating.


Genetics

Different people smell different odors, and most of these differences are caused by genetic differences. Although odorant receptor genes make up one of the largest gene families in the human genome, only a handful of genes have been linked conclusively to particular smells. For instance, the odorant receptor
OR5A1 Olfactory receptor 5A1 is a protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including ...
and its genetic variants (alleles) are responsible for our ability (or failure) to smell β-
ionone The ionones are a series of closely related chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday obj ...

ionone
, a key aroma in foods and beverages. Similarly, the odorant receptor OR2J3 is associated with the ability to detect the "grassy" odor, cis-3-hexen-1-ol. The preference (or dislike) of
cilantro Coriander (;
cilantro
(coriander) has been linked to the olfactory receptor
OR6A2 Olfactory receptor 6A2 is a that in humans is encoded by the ''OR6A2'' . Function s interact with odorant molecules in the nose, to initiate a neuronal response that triggers the perception of a smell. The olfactory receptor proteins are membe ...
.


Variability amongst vertebrates

The importance and sensitivity of smell varies among different organisms; most
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s have a good sense of smell, whereas most
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s do not, except the tubenoses (e.g.,
petrel Petrels are tube-nosed seabirds in the bird order Procellariiformes. Description The common name does not indicate relationship beyond that point, as "petrels" occur in three of the four families within that group (all except the albatross fa ...

petrel
s and
albatross Albatrosses are very large Seabird, seabirds in the family (biology), family Diomedeidae. They range widely in the Southern Ocean and the North Pacific Ocean, Pacific. They are absent from the North Atlantic Ocean, Atlantic, although fossil rem ...

albatross
es), certain species of new world
vultures A vulture is a bird of prey that scavenges on carrion. The Old World vultures include 15 living species native to Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (no ...

vultures
, and the
kiwi KIWI (102.9 FM, "Radio Lobo") is a commercial radio station , Sweden , Norway Radio broadcasting is transmission of audio signal, audio (sound), sometimes with related metadata, by radio waves intended to reach a wide audience. In terrestria ...
s. Also, birds have hundreds of olfactory receptors. Although, recent analysis of the chemical composition of
volatile organic compounds Volatile organic compounds (VOC) are organic chemicals , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's abilit ...

volatile organic compounds
(VOCs) from
king penguin The king penguin (''Aptenodytes patagonicus'') is the second largest species of penguin, smaller, but somewhat similar in appearance to the emperor penguin. There are two subspecies: ''A. p. patagonicus'' and ''A. p. halli''; ''patagonicus'' ...

king penguin
feathers suggest that VOCs may provide olfactory cues, used by the penguins to locate their colony and recognise individuals. Among mammals, it is well developed in the
carnivore A carnivore , meaning "meat Meat is animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume orga ...
s and
ungulate Ungulates ( ) are members of the diverse clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evoluti ...
s, 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''. 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. 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.
Scenthound: long ears, large nasal passages, and a sturdy body for enduranceImage:Baying.jpg, A baying houndimage:Franz Rudolf Frisching.jpg, Franz Rudolf Frisching in the uniform of an officer of the Bernese Huntsmen Corps with his Berner Laufhund, painted by ...
s as a group can smell one- to ten-million times more acutely than a human, and
bloodhound The Bloodhound is a large scent hound: long ears, large nasal passages, and a sturdy body for endurance Image:Baying.jpg, A baying hound Franz Rudolf Frisching in the uniform of an officer of the Bernese Huntsmen Corps with his Berner Laufhund, ...

bloodhound
s, which have the keenest sense of smell of any dogs, 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 Trail pheromones are semiochemical A semiochemical, from the Greek σημεῖον (''semeion''), meaning "signal", is a chemical substance or mixture released by an organism that affects the behaviors of other individuals. Semiochemical communica ...
a few days old. The second-most-sensitive nose is possessed by the , which was bred to track and hunt rabbits and other small animals.
Grizzly bear The grizzly bear (''Ursus arctos horribilis''), also known as the North American brown bear or simply grizzly, is a population Population typically refers the number of people in a single area whether it be a city or town, region, countr ...
s 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 The parvorder Catarrhini , catarrhine monkeys or Old World anthropoids are the sister group to the New World monkeys, the Platyrrhini. The Platyrrhini emerged within "monkeys" by migration to South America from Afro-Arabia (the Old World), likel ...

catarrhine
primate A primate ( ) (from Latin , from 'prime, first rank') is a eutherian mammal constituting the Taxonomy (biology), taxonomic order (biology), order Primates (). Primates arose 85–55 million years ago first from small Terrestrial animal, ...

primate
s, and nonexistent in
cetacea Cetaceans (from la, cetus Cetus () is a constellation, sometimes called 'the whale' in English. The Cetus (mythology), Cetus was a sea monster in Greek mythology which both Perseus and Heracles needed to slay. Cetus is in the region of the ...

cetacea
ns, which compensate with a well-developed sense of
taste The gustatory system or sense of taste is the sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory re ...

taste
. In some
strepsirrhines Strepsirrhini or Strepsirhini (; ) is a suborder In biological classification, the order ( la, wikt:ordo#Latin, ordo) is # a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. The well-known ranks ...

strepsirrhines
, such as the
red-bellied lemur The red-bellied lemur (''Eulemur rubriventer'') is a medium-sized Strepsirrhini, strepsirrhine primate with a luxuriant chestnut brown coat. This lemur is Endemic (ecology), endemic to eastern Madagascar, Madagascan rainforests and is disting ...
, scent glands occur atop the head. In many species, smell is highly tuned to
pheromone A pheromone (from Ancient Greek ' "to bear" and hormone) is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the s ...

pheromone
s; a male
silkworm ''Bombyx mori'', the domestic silk moth, is an insect from the moth Moths are a paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a ...

silkworm
moth, for example, can sense a single molecule of
bombykol Bombykol is a pheromone released by the female silkworm moth to attract mates. It is also the sex pheromone in the wild silk moth (''Bombyx mandarina''). Discovered by Adolf Butenandt in 1959, it was the first pheromone to be characterized chemic ...

bombykol
. Fish, too, have a well-developed sense of smell, even though they inhabit an aquatic environment. Salmon utilize their sense of smell to identify and return to their home stream waters. Catfish use their sense of smell to identify other individual catfish and to maintain a social hierarchy. Many fishes use the sense of smell to identify mating partners or to alert to the presence of food.


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. Researchers in the most recent study, which tested the psychophysical responses to combinations of over 128 unique odor molecules with combinations composed of up to 30 different component molecules, noted that this estimate is "conservative" and that some subjects of their research might be capable of deciphering between a thousand trillion odorants, adding that their worst performer could probably still distinguish between 80million scents. 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." 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." The logic of his paper has in turn been criticized by the authors of the original paper.


Physiological basis in vertebrates


Main olfactory system

In humans and other
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
s, smells are sensed by
olfactory sensory neuron Olfaction, or the sense of smell, is the special sense through which smells (or odors) are perceived. It occurs when an odor binds to a receptor within the nasal cavity, transmitting a signal through the olfactory system. Olfaction has many ...
s in the
olfactory epithelium The olfactory epithelium is a specialized epithelium, epithelial tissue inside the nasal cavity that is involved in olfaction, smell. In humans, it measures 9 cm2 and lies on the roof of the nasal cavity about 7 cm above and behind the n ...
. The olfactory epithelium is made up of at least six morphologically and biochemically different cell types. The proportion of olfactory
epithelium Epithelium is one of the four basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as ...
compared to respiratory epithelium (not innervated, or supplied with nerves) gives an indication of the animal's olfactory sensitivity. Humans have about of olfactory epithelium, whereas some dogs have . A dog's olfactory epithelium is also considerably more densely innervated, with a hundred times more receptors per square centimeter. The sensory olfactory system integrates with other senses to form the perception of
flavor Flavor, or flavour, is the perceptual Perception (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area arou ...

flavor
. Often, land organisms will have separate olfaction systems for smell and taste (orthonasal smell and
retronasal smellRetronasal smell, retronasal olfaction Olfaction, or the sense of smell, is the special sense through which smells (or odors) are perceived. It occurs when an odor binds to a olfactory receptor, receptor within the nasal cavity, transmitting a si ...
), but water-dwelling organisms usually have only one system. Molecules of odorants passing through the
superior nasal concha The back part of the medial surface of the labyrinth of ethmoid The ethmoidal labyrinth or lateral mass of the ethmoid bone consists of a number of thin-walled cellular cavities, the ethmoid air cells, arranged in three groups, anterior, middle, a ...
of the nasal passages dissolve in the
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists o ...
that lines the superior portion of the cavity and are detected by
olfactory receptor Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is ...
s on the
dendrites Dendrites (from Ancient Greek language, Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the neurotransmission, electrochemical stimulation received from other neural ...

dendrites
of the olfactory sensory neurons. This may occur by diffusion or by the binding of the odorant to
odorant-binding proteinOdorant-binding proteins (OBPs) are small (10 to 30 kDa) soluble protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array ...
s. The mucus overlying the epithelium contains
mucopolysaccharides . For polysaccharide nomenclature see here Here is an adverb that means "in, on, or at this place". It may also refer to: Software * Here Technologies, a mapping company * Here WeGo (formerly Here Maps), a mobile app and map website by Here ...
, salts,
enzymes Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates int ...
, and
antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and Viral disease, viruses. The antibody recognizes a unique mo ...

antibodies
(these are highly important, as the olfactory neurons provide a direct passage for infection to pass to the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tis ...

brain
). This mucus acts as a solvent for odor molecules, flows constantly, and is replaced approximately every ten minutes. In
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

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


Receptor neuron

The binding of the ligand (odor molecule or odorant) to the receptor leads to an action potential in the receptor neuron, via a second messenger pathway, depending on the organism. In mammals, the odorants stimulate adenylate cyclase to synthesize cyclic adenosine monophosphate, 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 Calcium, Ca2+ with some sodium, Na+) into the cell, slightly depolarising it. The Ca2+ in turn opens a Ca2+-activated chloride channel, leading to efflux of chlorine, 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. Other subpopulations, such as those that express the receptor guanylyl cyclase GC-D (Gucy2d) or the soluble guanylyl cyclase Gucy1b2, use a cGMP cascade to transduce their odorant ligands. 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 Photoreceptor cell, photoreceptors, in which the second messenger Cyclic guanosine monophosphate, 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-olfactography, 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

Olfactory sensory neurons project axons to the brain within the olfactory nerve, (cranial nerveI). These nerve fibers, lacking myelin sheaths, pass to the
olfactory bulb The olfactory bulb (Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republi ...
of the brain through perforations in the cribriform plate, which in turn projects olfactory information to the olfactory cortex and other areas. The axons from the
olfactory receptor Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is ...
s converge in the outer layer of the olfactory bulb within small (≈50 micrometer (unit), micrometers in diameter) structures called glomerulus (olfaction), 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 An odor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States. Cur ...
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 granule cell, 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, pattern separation, and
memory Memory is the faculty of the by which or is , stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If s could not be remembered, it would be impossible for language, r ...

memory
functions, 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 The piriform cortex, or pyriform cortex, is a region in the brain A brain is an organ (anatomy), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to th ...
, 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). The piriform cortex projects to the medial dorsal nucleus of the thalamus, which then projects to the orbitofrontal cortex. The orbitofrontal cortex mediates conscious perception of the odor. The 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. In
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s, smells are sensed by sensilla located on the antenna and maxillary palp and first processed by the antennal lobe (analogous to the
olfactory bulb The olfactory bulb (Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republi ...
), and next by the mushroom bodies and lateral horn of insect brain, 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. 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. After binding the odorant, the receptor is activated and will send a signal to the glomeruli. 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. All of the signals from the glomeruli are then sent to the brain, where the combination of glomeruli activation encodes the different chemical features of the odorant. The brain then essentially puts the pieces of the activation pattern back together in order to identify and perceive the odorant. This distributed code allows the brain to detect specific odors in mixtures of many background odors. 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. While chemotopy remains a highly controversial concept, evidence exists for perceptual information implemented in the spatial dimensions of olfactory networks.


Accessory olfactory system

Many animals, including most mammals and reptiles, but not humans, have two distinct and segregated olfactory systems: a main olfactory system, which detects volatile stimuli, and an accessory olfactory system, which detects fluid-phase stimuli. Behavioral evidence suggests that these fluid-phase stimuli often function as
pheromone A pheromone (from Ancient Greek ' "to bear" and hormone) is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the s ...

pheromone
s, 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 A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for Respiration (physiology), respiration alongside the mouth. Behind the nose are the olfactory mucosa and the Paranasal sinus, sinuses. Be ...

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 Olfactory bulb#Accessory olfactory bulb, accessory olfactory bulb, which in the mouse is located on the dorsal-posterior portion of the main
olfactory bulb The olfactory bulb (Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republi ...
. 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

Insect olfaction refers to the function of chemical receptors that enable insects to detect and identify Volatile organic compound, volatile compounds for foraging, predator avoidance, finding mating partners (via
pheromone A pheromone (from Ancient Greek ' "to bear" and hormone) is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the s ...

pheromone
s) and locating oviposition habitats. Thus, it is the most important sensation for insects. Most important insect behaviors must be timed perfectly which is dependent on what they smell and when they smell it. 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 Antenna (biology), antennae and specialized mouth parts called the maxillary palps. However, a recent study has demonstrated the olfactory role of ovipositor in fig wasps. 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 membrane. The majority of
olfactory receptor neuron An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electric ...
s typically reside in the Antenna (biology), antenna. These neurons can be very abundant, for example ''Drosophila'' flies have 2,600 olfactory sensory neurons. Insects are capable of smelling and differentiating between thousands of volatile organic compound, volatile compounds both sensitivity (human), sensitively and selectively. 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. Some insects, such as the moth ''Deilephila elpenor'', use smell as a means to find food sources.


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. 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

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 United States, 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 acid, caprylic. * Henning's prism * Zwaardemaker smell system (invented by Hendrik Zwaardemaker)


Disorders

Specific terms are used to describe disorders associated with smelling: *Anosmia – inability to smell *Hyperosmia – an abnormally acute sense of smell *Hyposmia – decreased ability to smell *Presbyosmia – the natural decline in the sense of smell in old age *Dysosmia – distortion in the sense of smell **Parosmia – distortion in the perception of an odor **Phantosmia – distortion in the absence of an odor, "hallucinated smell" * Heterosmia – inability to distinguish odors *Olfactory reference syndrome – psychological disorder that causes the patient to imagine he or she has strong body odor * Osmophobia – aversion or psychological hypersensitivity to odors Viruses can also infect the olfactory epithelium leading to a loss of the sense of olfaction. About 50% of patients with Impact of COVID-19 on neurological, psychological and other mental health outcomes, SARS-CoV-2 (causing COVID-19) experience some type of Impact of COVID-19 on neurological, psychological and other mental health outcomes, disorder associated with their sense of smell, including anosmia and parosmia. SARS-CoV-1, Middle East respiratory syndrome–related coronavirus, MERS-CoV and even the flu (Orthomyxoviridae, influenza virus) can also disrupt olfaction.


See also

*Electronic nose *Evolution of olfaction *Nasal administration olfactory transfer *Olfactory ensheathing cell *Olfactory fatigue *Perfume (novel), ''Perfume'' (novel) *Scent transfer unit *Sniffing (behavior) *Vibration theory of olfaction


References


External links


Olfaction at cf.ac.ukOlfactory Systems Laboratory at Boston UniversitySmells Database
''Neuroscience Online'' (electronic neuroscience textbook by UT Houston Medical School)
Digital Olfaction Society
{{Authority control Olfaction, Limbic system