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Smell

Smell

The sense that perceives odor by means of the nose and olfactory nerve.

Olfaction is one of the two chemical senses: smell and taste . Both arise from interaction between chemical and receptor cells. In olfaction, the chemical is volatile, or airborne. Breathed in through the nostrils or taken in via the throat by chewing and swallowing, it passes through either the nose or an opening in the palate at the back of the mouth, and moves toward receptor cells located in the lining of the nasal passage. As the chemical moves past the receptor cells, part of it is absorbed into the uppermost surface of the nasal passages called the olfactory epithelium, located at the top of the nasal cavity. There, two one-inch-square patches of tissue covered with mucus dissolve the chemical, stimulating the receptors, which lie under the mucus. The chemical molecules bind to the receptors, triggering impulses that travel to the brain . There are thousands of different receptors in the cells of the nasal cavity that can detect as many as 10,000 different odors. Each receptor contains hair-like structures, or cilia, which are probably the initial point of contact with olfactory stimuli. Research suggests that the sensitivity of the olfactory system is related to the number of both receptors and cilia. For example, a dog has 20 times as many receptor cells as a human and over 10 times as many cilia per receptor.

The cribriform plate forms the roof of the nasal cavity. The olfactory nerve passes through openings in this bone and ends in the olfactory bulb, a neural structure at the base of the brain. From there, olfactory signals are diffused throughout the brain to areas including the amygdala, hippocampus, pyriform cortex (located at the base of the temporal lobe), and the hypothalamus . Olfaction is the only sense that does not involve the thalamus . Olfaction messages are especially intensive in the amygdala, a part of the brain responsible for emotions, which may help the unusual power of certain smells to trigger emotions and recollections based on memories from the past. Further, a person's reaction to smell is mediated by context. For example, the same smell present in body odor is responsible for the flavor of cheese. In the first case, the smell is perceived as negative, in the second, it is positive. In humans, olfaction intensifies the taste of food, warns of potentially dangerous food, as well as other dangers (such as fire), and triggers associations involving memory and emotion . Olfaction is an especially important sense in many animals. A predator may use it to detect prey, while prey may use it to avoid predators. It also has a role in the mating process through chemicals called pheromones, which can cause ovulation in females or signal a male that a female is in a sexually receptive state. Although the existence of human pheromones has not been verified, olfaction still plays a role in human sexual attraction, as well as in parenting. Mothers can usually identify their newborn infants by smell, and breast-feeding babies can distinguish between the smell of their mothers and that of other breast-feeding women. Researchers have also found that children are able to recognize their siblings by smell and parents can use smell to distinguish among their own children. However, as people age the sense of smell diminishes, especially for men. By age 80, many men have almost no ability to detect odors. The intensity of a particular odor is strongly affected by adaptation . Odors may become undetectable after only a brief period of exposure. The sense of smell also plays an important role in the discrimination of flavors, a fact demonstrated by the reduced sense of taste in people with colds. The enjoyment of food actually comes more from odors detected by the olfactory system than from the functioning of the taste system. The olfactory and gustatory (taste) pathways are known to converge in parts of the brain, although it is not known exactly how the two systems work together. While an aversion to certain flavors (such as bitter flavors) is innate, associations with odors are learned.

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Smell

Smell

Smell, called olfaction, is the ability of an organism to sense and identify a substance by detecting tiny amounts of the substance that evaporate and produce an odor. Smell is the most important sense for most organisms. Many species use their sense of smell to locate prey, navigate, recognize and communicate with others of their species, and mark territory.

The sense of smell differs from most other senses (sight, hearing, taste, and touch) in its directness. We actually smell microscopic bits of a substance that have evaporated and made their way to the olfactory epithelium, a section of the mucous membrane in the roof of the nasal

cavity of the nose. The olfactory epithelium contains millions of odor-sensitive olfactory nerve cells that are connected to the olfactory nerves. Hairlike fibers on the end of each olfactory cell react to an odor by stimulating the olfactory cells to send a signal along the olfactory nerve to the brain, which interprets the signal as a specific smell.

Human versus animal smell

There is no doubt that many animals have a sense of smell far superior to that of humans. Most vertebrates (animals with backbones) have many more olfactory nerve cells than humans. This probably gives them much more sensitivity to odors. Also, the structure in the brain that processes odors (called the olfactory bulb) takes up a much larger part of the brain in animals than in humans. Thus, animals have a greater ability to process and analyze different odors. This is why humans use dogs to find lost persons, hidden drugs, and explosivesalthough research on "artificial noses" that can detect scent even more reliably than dogs continues.

Still, the human nose is capable of detecting over 10,000 different odors, even some that occur in extremely minute amounts in the air. Many researchers are considering whether smell does not play a greater role in human behavior and biology than has been previously thought. For instance, research has shown that human mothers can smell the difference between clothes worn by their baby and those worn by another baby only days after the child's birth.

Words to Know

Olfactory bulb: The primitive part of the brain that first processes olfactory information.

Olfactory epithelium: The patch of mucous membrane at the top of the nasal cavity that contains the olfactory nerve cells.

Olfactory nerve cell: A cell in the olfactory epithelium that detects odors and transmits the information to the brain.

Pheromone: Scent molecules released by animals that affect the behavior of organisms of the same species.

Scientists are only beginning to understand the role that smell plays in animaland humanbehavior. For example, animals release chemicals called pheromones to communicate danger, defend themselves against predators, mark territory, and attract mates. Some researchers have suggested that humans also may release pheromones that play a role in sexual attraction and matingalthough this hypothesis has not been proven.

Current research

Olfactory nerve cells are the only nerve cells arising from the central nervous system that can regenerate (be formed again). Some researchers hope that studying regeneration in olfactory nerve cells or even transplanting them elsewhere in the body can lead to treatments for spine and brain damage that is currently irreversible.

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smell

smell / smel/ • n. the faculty or power of perceiving odors or scents by means of the organs in the nose: a highly developed sense of smell dogs locate the bait by smell. ∎  a quality in something that is perceived by this faculty; an odor or scent: lingering kitchen smells a smell of coffee. ∎  an unpleasant odor: twenty-seven cats lived there—you can imagine the smell! ∎  [in sing.] an act of inhaling in order to ascertain an odor or scent: have a smell of this. • v. (past and past part. smelled or smelt / smelt/ ) 1. [tr.] perceive or detect the odor or scent of (something): I think I can smell something burning. ∎  sniff at (something) in order to perceive or detect its odor or scent: the dogs smell each other. ∎  [intr.] have or use a sense of smell: becoming deaf or blind or unable to smell. ∎  (smell something out) detect or discover something by the faculty of smell: his nose can smell out an animal from ten miles away. ∎  detect or suspect (something) by means of instinct or intuition: he can smell trouble long before it gets serious he can smell out weakness in others. 2. [intr.] emit an odor or scent of a specified kind: it smelled like cough medicine| the food smelled and tasted good | [as adj. , in comb.] (-smelling) pungent-smelling food. ∎  have a strong or unpleasant odor: if I don't get a bath soon I'll start to smell it smells in here. ∎  appear in a certain way; be suggestive of something: it smells like a hoax to me. PHRASES: smell blood discern weakness or vulnerability in an opponent. smell a rat inf. suspect trickery or deception. smell the roses inf. enjoy or appreciate what is often ignored. smell something up permeate an area with a bad smell: he smelled up the whole house.DERIVATIVES: smell·a·ble adj. smell·er n.

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smell

smell, sense that enables an organism to perceive and distinguish the odors of various substances, also known as olfaction. In humans, the organ of smell is situated in the mucous membrane of the upper portion of the nasal cavity near the septum. It is made up of the olfactory cells, which are actually nerve cells that function as receptors for the sense of smell. The free ends of the cells project outward from the epithelial tissue in the form of numerous hairlike processes. These fibers are buried in the mucus that coats the inner surface of the nasal cavity and are stimulated by various odors. Nerve fibers extend from the olfactory cells to an area of the brain called the olfactory bulb. Any disturbance of the nasal cavity—such as the common cold—in which the olfactory hairs are covered with excess mucus or other material, interferes with the sense of smell. Most physiologists agree that although a substance must be volatile to be sniffed by the nose, it must subsequently be dissolved in the mucous lining of the nasal cavity to be smelled. It is also believed that there are only a few basic odors (perhaps about seven), and that all other odors are a combination of these. Attempts at classifying the so-called primary sensations of smell have not yet been successful. The sense of smell is not as strongly developed in humans as in many other vertebrates, particularly carnivores which employ olfactory organs to locate food and detect dangerous predators. To many invertebrates (especially insects) as well, smell is a highly developed sensory mechanism, necessary in obtaining food, in finding mating partners, and in recognizing other animals.

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smell

smell smell a rat begin to suspect trickery or deception.
smell blood discern weakness or vulnerability in an opponent.
smell of the lamp show signs of laborious study and effort; the reference is to an oil-lamp, and according to Plutarch the criticism was once made of the work of Demosthenes, ‘His impromptus smell of the lamp’, meaning that his speeches were written rather than spoken orations.
smell the roses in North American usage, enjoy or appreciate what is often ignored.

See also fish and guests smell after three days, money has no smell, wake up and smell the coffee.

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smell

smell pt., pp. smelled, smelt
A. perceive by the sense of which the nose is the organ;

B. have an odour. XII. ME. smelle, also smülle, smille, pointing to OE. *smiellan, *smyllan, of which no cogns. are known.

Hence sb. XII; superseding stink and stench in the neutral application of sense B.

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smell

smell (olfaction) Sense that responds to airborne molecules. The olfactory receptors in the nose can detect even a few molecules per million parts of air.

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smell

smell See organoleptic.

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smell

smell See olfaction.

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smell

smellAdele, Aix-la-Chapelle, aquarelle, artel, au naturel, bagatelle, béchamel, befell, bell, belle, boatel, Brunel, Cadell, carousel, cartel, cell, Chanel, chanterelle, clientele, Clonmel, compel, Cornell, crime passionnel, dell, demoiselle, dispel, dwell, el, ell, Estelle, excel, expel, farewell, fell, Fidel, fontanelle, foretell, Gabrielle, gazelle, gel, Giselle, hell, hotel, impel, knell, lapel, mademoiselle, maître d'hôtel, Manuel, marcel, matériel, mesdemoiselles, Michel, Michelle, Miguel, misspell, morel, moschatel, Moselle, motel, muscatel, nacelle, Nell, Nobel, Noel, organelle, outsell, Parnell, pell-mell, personnel, propel, quell, quenelle, rappel, Raquel, Ravel, rebel, repel, Rochelle, Sahel, sardelle, sell, shell, show-and-tell, smell, Snell, spell, spinel, swell, tell, undersell, vielle, villanelle, well, yell •Buñuel • Pachelbel • handbell •barbell • harebell • decibel • doorbell •cowbell • bluebell • Annabel •mirabelle • Christabel • Jezebel •Isabel, Isobel •nutshell • infidel • asphodel •zinfandel • Grenfell • Hillel • parallel •Cozumel • caramel • Fresnel •pimpernel • pipistrelle • Tricel •filoselle

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Smell

Smell

Definition

Smell is the ability of an organism to sense and identify a substance by detecting trace amounts of the substance that evaporate. Researchers have noted similarities in the sense of smell between widely differing species that reveal some of the details of how the chemical signal of an odor is detected and processed.

Description

The sense of smell has been a topic of debate from humankind's earliest days. The Greek philosopher Democritus of Abdera (460–360 b. c.), speculated that humans smell "atoms" of different size and shape that come from objects. His countryman Aristotle (384–322 b. c.), on the other hand, guessed that odors are detected when the "cold" sense of smell meets "hot" smoke or steam from the object being smelled. It was not until the late eighteenth century that most scientists and philosophers reached agreement that Democritus was basically right: the smell of an object is due to volatile, or easily evaporated, molecules that emanate from it.

In 1821, the French anatomist Hippolyte Cloquet (1787–1840) rightly noted the importance of smell for animal survival and reproduction; but his theorizing about the role of smell in human sex, as well as mental disorders, proved controversial. Many theories of the nineteenth century seem irrational or even malignant today. Many European scientists of that period fell into the trap of an essentially circular argument, that held that non-Europeans were more primitive, and therefore had a more developed sense of smell. The first half of the twentieth century saw progress in making the study of smell more rational. A Spanish neuroanatomist traced the architecture of the nerves leading from the nose to and through the brain. Other scientists carried out the first methodical investigations of how the nose detects scent molecules, the sensitivity of the human nose, and the differences between human and animal olfaction. But the most recent progress in studying the sense of smell and how it affects humans was made with the application of molecular science to the odor-sensitive cells of the nasal cavity.

The sense of smell is the most important sense for most organisms. A wide variety of species use their sense of smell to locate prey, navigate, recognize and perhaps communicate with kin, and mark territory. In a broad sense, the workings of smell in animals as different as mammals, reptiles, fish, and even insects are remarkably similar.

The sense of smell differs from most other senses in its directness; humans and other mammals actually smell microscopic bits of a substance that have evaporated and made their way to the olfactory epithelium, a section of the mucus membrane in the roof of the olfactory cavity. The olfactory epithelium contains the smell-sensitive ending of the olfactory nerve cells, also known as the olfactory epithelial cells. These cells detect odors through receptor proteins on the cell surface that bind to odor-carrying molecules. A specific odorant docks with an olfactory receptor protein in much the same way as a key fits in a lock; this in turn excites the nerve cell, causing it to send a signal to the brain. This is known as the stereospecific theory of smell.

Recently, molecular scientists have cloned the genes for the human olfactory receptor proteins. Although there are perhaps tens of thousands or more of odor-carrying molecules in the world, there are only hundreds, or at most about 1,000, kinds of specific receptors in any species of animal, including humans. Because of this, scientists do not believe that each receptor recognizes a unique odorant; rather, similar odorants can all bind to the same receptor. It appears that a few loose-fitting odorant "keys" of broadly similar shape can turn the same receptor "lock." Researchers do not yet know how many specific receptor proteins each olfactory nerve cell carries, but recent work suggests that the cells specialize just as the receptors do, and any one olfactory nerve cell has only one or a few receptors rather than many.

Function

It is the combined pattern of receptors that are tweaked by an odorant that allow the brain to identify it, much as yellow and red light together are interpreted by the brain as orange. (In fact, just as people can be color-blind to red or green, some can be "odorblind" to certain simple molecules because they lack the receptor for that molecule.) In addition, real objects produce multiple odor-carrying molecules, so that the brain must analyze a complex mixture of odorants to recognize a smell.

Just as the sense of smell is direct in detecting fragments of the objects, it is also direct in the way the signals are transmitted to the brain. In most senses, such as vision this task is accomplished in several steps: a receptor cell detects light and passes the signal to a nerve cell, which passes it on to another nerve cell in the central nervous system, which then relays it to the visual center of the brain. But in olfaction, all these jobs are performed by the olfactory nerve cell. In a very real sense, the olfactory epithelium is a direct outgrowth of the brain.

Role in human health

In humans, the olfactory nerve cell takes the scent message directly to the nerve cells of the olfactory bulb of the brain. There multiple signals from different olfactory cells with different odor sensitivities are organized and processed. The signal then goes to the brain's olfactory cortex, where higher functions such as memory and emotion are coordinated with the sense of smell.

There is no doubt that many animals have a sense of smell far superior than humans. This is why, even today, humans use dogs to find lost persons, hidden drugs, and explosives although research on "artificial noses" that can detect scent even more reliably than dogs continues.

Because of their humble abilities of olfaction, humans are called microsmatic, rather than macrosmatic. Still, the human nose is capable of detecting over 10,000 different odors, some in the range of parts per trillion of air; and many researchers suspect that smell plays a greater role in human behavior and biology than has been previously thought. For instance, research has shown that human mothers can smell the difference between a vest worn by their baby and one worn by another baby only days after the child's birth.

Yet some olfactory abilities of animals are probably beyond humans. Most vertebrates have many more olfactory nerve cells in a proportionately larger olfactory epithelium than humans, which probably gives them much more sensitivity to odors. The olfactory bulb in these animals takes up a much larger portion of the brain than it does in humans, giving the animal more ability to process and analyze olfactory information. In addition, most land vertebrates have a specialized scent organ in the roof of the mouth called vomeronasal organ. This organ, believed to be vestigial in humans, is a pit lined by a layer of cells with a similar structure to the olfactory epithelium, which feeds into its own processing part of the brain, called accessory olfactory bulb, an area of the brain that is absent in humans.

Researchers have learned a lot about how the olfactory nerve cells detect odorants. However, they have not yet learned how this information is coded by the olfactory cell. Scientists are only beginning to understand the role that smell plays in animal and human behavior. The vomeronasal sense of animals is still largely not understood and some researchers have even suggested that the human vomeronasal organ might retain some function, and that humans may have pheromones that play a role in sexual attraction and mating. However, this hypothesis is still very controversial.

KEY TERMS

Anosmia— A disorder in which one is able to detect no odors.

Olfactory bulb— The primitive part of the brain that first processes olfactory information.

Olfactory cortex— The cerebral cortex that makes use of information from the olfactory bulb.

Olfactory epithelium— The patch of mucus membrane at the top of the nasal cavity that is sensitive to odor.

Olfactory nerve cell— The cell in the olfactory epithelium that detects odor and transmits the information to the olfactory bulb of the brain.

Pheromones— Scent molecules made by the body that attract a mate and help initiate mating behaviors.

Receptor protein— A protein in a cell that sticks to a specific odorant or other signal molecule.

Stereospecific theory— The theory that the nose recognizes odorants when they bind to receptor proteins that recognize the odorants' molecular shape.

Volatile— Easily evaporated.

Vomeronasal— A pit on the roof of the mouth in most vertebrates that serves to detect odor molecules that are not as volatile as those detected by the nose.

Detailed study of the biology of the olfactory system may yield gains in other fields. For instance, olfactory nerve cells are the only nerve cells that are derived from the central nervous system that can regenerate, possibly because the stress of their exposure to the outside world gives them a limited life span. Some researchers hope that studying regeneration in olfactory nerve cells or even transplanting them elsewhere in the body can lead to treatments for as yet irreversible damage to the spine and brain.

Common diseases and disorders

The most common complaint registered by patients is the loss of the sense of smell (anosmia). Smell disorders usually develop after an illness or an injury. Loss of the sense of smell is commonly caused by upper respiratory illnesses or a head injury. It can result from polyps in the nose or nasal cavity, sinus infections, hormonal fluctuations, or dental problems.

Resources

BOOKS

Schiffman, Harvey. Sensation and Perception: An Integrated Approach. New York: Wiley and Sons, 2001.

Watson, Lyall. Jacobson's Organ: And the Remarkable Nature of Smell. New York: W. W. Norton, 2000.

PERIODICALS

Dajer, Tony. "How the Nose Knows." Discover, Jan. 1992.

Farbman, Albert I. "The Cellular Basis of Olfaction." Endeavor, 18, no. 1 (1994).

Kreiter, Marcella S. "Brain Smells Out Signals." July 25, 2001. 〈http://www.nlm.nih.gov/medlineplus/news/fullstory_2928.html〉.

OTHER

"Smell—Impaired." Medical Encyclopedia. Medline. 2001. 〈http://www.nlm.nih.gov/medlineplus/ency/article/003052.htm)〉.

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Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

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  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Smell

Smell

A controversial history

A direct sense

Human vs. animal smell

Unknown territory

Resources

Smell is the ability of an organism to sense and identify a substance by detecting trace amounts of the substance that evaporates. Researchers have noted

similarities in the sense of smell between widely differing species that reveal some of the details of how the chemical signal of an odor is detected and processed.

A controversial history

The sense of smell has been a topic of debate from humankinds earliest days. Greek philosopher Democritus of Abdera (460360 BC) speculated that humans smell atoms of different size and shape that come from objects. His compatriot Aristotle (384322 BC), on the other hand, guessed that odors are detected when the cold sense of smell meets hot smoke or steam from the object being smelled. It was not until the late eighteenth century that most scientists and philosophers reached agreement that Democritus was basically right: the smell of an object is due to volatile, or easily evaporated, molecules that emanate from it.

In 1821, French anatomist Hippolyte Cloquet (17871840) rightly noted the importance of smell for animal survival and reproduction; but his theorizing about the role of smell in human sex, as well as mental disorders, proved controversial. Many theories of the nineteenth century seem irrational or even malignant today. Many European scientists of that period fell into the trap of an essentially circular argument, which held that non-Europeans were more primitive and, therefore, had a more developed sense of smell, and therefore were more primitive. However, other thinkersFrench anatomist Hippolyte Cloquet (17871840) for onenoted that an unhealthy fixation on the sense of smell seemed much more common in civilized Europeans than to primitives. The first half of the twentieth century saw real progress in making the study of smell more rational. Spanish neuroanatomist Santiago Ramon y Cajal (1852 1934) traced the architecture of the nerves leading from the nose to and through the brain. Other scientists carried out the first methodical investigations of how the nose detects scent molecules, the sensitivity of the human nose, and the differences between human and animal olfaction. However, much real progress on the workings of this remarkable sense has had to wait upon the recent application of molecular science to the odor-sensitive cells of the nasal cavity.

A direct sense

Smell is the most important sense for most organisms. A wide variety of species use their sense of smell to locate prey, navigate, recognize and perhaps communicate with kin, and mark territory. Perhaps because the task of olfaction is so similar between species, in a broad sense the workings of smell in animals as different as mammals, reptiles, fish, and even insects are remarkably similar.

The sense of smell differs from most other senses in its directness: humans actually smell microscopic bits of a substance that have evaporated and made their way to the olfactory epithelium, a section of the mucus membrane in the roof of the olfactory cavity. The olfactory epithelium contains the smell-sensitive endings of the olfactory nerve cells, also known as the olfactory epithelial cells. These cells detect odors through receptor proteins on the cell surface that bind to odor-carrying molecules. A specific odorant docks with an olfactory receptor protein in much the same way as a key fits in a lock; this in turn excites the nerve cell, causing it to send a signal to the brain. This is known as the stereospecific theory of smell.

Molecular scientists have cloned the genes for the human olfactory receptor proteins. Although there are perhaps tens of thousands (or more) of odor-carrying molecules in the world, there are only hundreds, or at most about 1,000 kinds of specific receptors in any species of animal. Because of this, scientists do not believe that each receptor recognizes a unique odorant; rather, similar odorants can all bind to the same receptor. In other words, a few loose-fitting odorant keys of broadly similar shape can turn the same receptor lock. Researchers do not know how many specific receptor proteins each olfactory nerve cell carries, but recent work suggests that the cells specialize just as the receptors do, and any one olfactory nerve cell has only one or a few receptors rather than many.

It is the combined pattern of receptors that are tweaked by an odorant that allow the brain to identify it, much as yellow and red light together are interpreted by the brain as orange. (In fact, just as people can be color-blind to red or green, they can be odor-blind to certain simple molecules because they lack the receptor for that molecule.) In addition, real objects that we smell produce multiple odor-carrying molecules, so that the brain must analyze a complex mixture of odorants to recognize a smell.

Just as the sense of smell is direct in detecting fragments of the objects, it is also direct in the way the signal is transmitted to the brain. In most senses, such as vision, this task is accomplished in several steps: a receptor cell detects light and passes the signal to a nerve cell, which passes it on to another nerve cell in the central nervous system, which then relays it to the visual center of the brain. But in olfaction, all these jobs are performed by the olfactory nerve cell: in a very real sense, the olfactory epithelium is a direct outgrowth of the brain.

The olfactory nerve cell takes the scent message directly to the nerve cells of the olfactory bulb of the brain (or, in insects and other invertebrates that lack true brains, the olfactory ganglia), where multiple signals from different olfactory cells with different odor sensitivities are organized and processed. In higher species the signal then goes to the brains olfactory cortex, where higher functions such as memory and emotion are coordinated with the sense of smell.

Human vs. animal smell

There is no doubt that many animals have a sense of smell far superior than humans. This is why, even today, humans use dogs to find lost persons, hidden drugs, and explosives, although research on artificial noses that can detect scent even more reliably than dogs continues. Humans are called microsmatic, rather than macrosmatic, because of their humble abilities of olfaction.

Still, the human nose is capable of detecting over 10,000 different odors, some in the range of parts per trillion of air; and many researchers are beginning to wonder whether smell does not play a greater role in human behavior and biology than has been thought. For instance, research has shown that human mothers can smell the difference between a vest worn by their baby and one worn by another baby only days after the childs birth.

Yet some olfactory abilities of animals are probably beyond humans. Most vertebrates have many more olfactory nerve cells in a proportionately larger olfactory epithelium than humans, which probably gives them much more sensitivity to odors. The olfactory bulb in these animals takes up a much larger proportion of the brain than humans, giving them more ability to process and analyze olfactory information.

In addition, most land vertebrates have a specialized scent organ in the roof of their mouth called the vomeronasal organ (also known as the Jacobsons organ or the accessory olfactory organ). This organ, believed to be vestigial in humans, is a pit lined by a layer of cells with a similar structure to the olfactory epithelium, which feeds into its own processing part of the brain, called the accessory olfactory bulb (an area of the brain absent in humans).

The vomeronasal sense appears to be sensitive to odor molecules with a less volatile, possibly more complex molecular structure than the odorants to which humans are sensitive. This sense is important in reproduction, allowing many animals to sense sexual attractant odors, or pheromones, thus governing mating behavior. It is also used by reptilian and mammalian predators in tracking prey.

Unknown territory

Researchers have learned much about how the olfactory nerve cells detect odorants. However, they have not yet learned how this information is coded by the olfactory cell. Other topics of future research will be how olfactory cell signals are processed in the olfactory bulb, and how this information relates to higher brain functions and the awareness of smell.

Scientists are only beginning to understand the role that smell plays in animal, and human, behavior. The vomeronasal sense of animals is still largely a mystery. Some researchers have even suggested that the human vomeronasal organ might retain some function, and that humans may have pheromones that play a role in sexual attraction and mating although this hypothesis is very controversial.

In addition, detailed study of the biology of the olfactory system might yield gains in other fields. For instance, olfactory nerve cells are the only nerve cells that are derived from the central nervous system that can regenerate, possibly because the stress of their exposure to the outside world gives them a limited life span. Some researchers hope that studying regeneration in olfactory nerve cells or even transplanting them elsewhere in the body can lead to treatments for as yet irreversible damage to the spine and brain.

KEY TERMS

Olfactory bulb The primitive part of the brain that first processes olfactory information; in insects, its function is served by nerve-cell bundles called olfactory ganglia

Olfactory cortex The parts of the cerebral cortex that make use of information from the olfactory bulb.

Olfactory epithelium The patch of mucus membrane at the top of the nasal cavity that is sensitive to odor.

Olfactory nerve cell The cell in the olfactory epithelium that detects odor and transmits the information to the olfactory bulb of the brain.

Pheromones Scent molecules made by the body that attract a mate and help initiate mating behaviors.

Receptor protein A protein in a cell that sticks to a specific odorant or other signal molecule.

Stereospecific theory The theory that the nose recognizes odorants when they bind to receptor proteins that recognize the odorants molecular shape.

Volatile Readily able to form a vapor at a relatively low temperature.

Vomeronasal organ A pit on the roof of the mouth in most vertebrates that serves to detect odor molecules that are not as volatile as those detected by the nose.

In 2004, American neuroscientist Richard Axel (1946), from Columbia University (New York City), and American neuroscientist Linda B. Buck (1947), from the Fred Hutchinson Cancer Research Center (Seattle, Washington), won the Nobel Prize in Physiology or Medicine for their discoveries of odorant receptors and the organization of the olfactory system. They were working in association with the Howard Hughes Medical Institute when they discovered a gene family with about 1,000 different genes that help to clarify how the olfactory system works. The cells provide the same number of olfactory receptor types. Located on the olfactory receptor cells, the various receptors were found in a tiny area within the upper portion of the nasal epithelium.

Resources

BOOKS

Doty, Richard, ed. Handbook of Olfaction and Gustation. New York: Marcel Dekker, 2003.

Moller, Aage R. Sensory Systems: Anatomy and Physiology. New York: Academic Press, 2002.

Rouby, Catherine, ed. Olfaction, Taste, and Cognition. New York: Cambridge University Press, 2002.

Watson, Lyall. Jacobsons Organ and the Remarkable Nature of Smell. New York: Plume, 2001.

Kenneth B. Chiacchia

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Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

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http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Smell

Smell

Smell is the ability of an organism to sense and identify a substance by detecting trace amounts of the substance that evaporate. Researchers have noted similarities in the sense of smell between widely differing species that reveal some of the details of how the chemical signal of an odor is detected and processed.


A controversial history

The sense of smell has been a topic of debate from humankind's earliest days. The Greek philosopher Democritus of Abdera (460-360 b.c.) speculated that we smell "atoms" of different size and shape that come from objects. His countryman Aristotle (384-322 b.c.), on the other hand, guessed that odors are detected when the "cold" sense of smell meets "hot" smoke or steam from the object being smelled. It was not until the late eighteenth century that most scientists and philosophers reached agreement that Democritus was basically right: the smell of an object is due to volatile, or easily evaporated, molecules that emanate from it.

In 1821 the French anatomist Hippolyte Cloquet (1787-1840) rightly noted the importance of smell foranimal survival and reproduction; but his theorizing about the role of smell in human sex, as well as mental disorders, proved controversial. Many theories of the nineteenth century seem irrational or even malignant today. Many European scientists of that period fell into the trap of an essentially circular argument, which held that non-Europeans were more primitive, and therefore had a more developed sense of smell, and therefore were more primitive. However, other thinkers—Cloquet for one—noted that an unhealthy fixation on the sense of smell seemed much more common in "civilized" Europeans than to "primitives." The first half of the twentieth century saw real progress in making the study of smell more rational. The great Spanish neuroanatomist Santiago Ramón y Cajal (1852-1934) traced the architecture of the nerves leading from the nose to and through the brain . Other scientists carried out the first methodical investigations of how the nose detects scent molecules, the sensitivity of the human nose, and the differences between human and animal olfaction. But much real progress on the workings of this remarkable sense has had to wait upon the recent application of molecular science to the odor-sensitive cells of the nasal cavity.


A direct sense

Smell is the most important sense for most organisms. A wide variety of species use their sense of smell to locate prey , navigate, recognize and perhaps communicate with kin, and mark territory. Perhaps because the task of olfaction is so similar between species, in a broad sense the workings of smell in animals as different as mammals , reptiles , fish , and even insects are remarkably similar.

The sense of smell differs from most other senses in its directness: we actually smell microscopic bits of a substance that have evaporated and made their way to the olfactory epithelium, a section of the mucus membrane in the roof of the olfactory cavity. The olfactory epithelium contains the smell-sensitive endings of the olfactory nerve cells, also known as the olfactory epithelial cells. These cells detect odors through receptor proteins on the cell surface that bind to odor-carrying molecules. A specific odorant docks with an olfactory receptor protein in much the same way as a key fits in a lock ; this in turn excites the nerve cell, causing it to send a signal to the brain. This is known as the stereospecific theory of smell.

In the past few years molecular scientists have cloned the genes for the human olfactory receptor proteins. Although there are perhaps tens of thousands (or more) of odor-carrying molecules in the world, there are only hundreds, or at most about 1,000 kinds of specific receptors in any species of animal. Because of this, scientists do not believe that each receptor recognizes a unique odorant; rather, similar odorants can all bind to the same receptor. In other words, a few loose-fitting odorant "keys" of broadly similar shape can turn the same receptor "lock." Researchers do not know how many specific receptor proteins each olfactory nerve cell carries, but recent work suggests that the cells specialize just as the receptors do, and any one olfactory nerve cell has only one or a few receptors rather than many.

It is the combined pattern of receptors that are tweaked by an odorant that allow the brain to identify it, much as yellow and red light together are interpreted by the brain as orange. (In fact, just as people can be color-blind to red or green, they can be "odor-blind" to certain simple molecules because they lack the receptor for that molecule.) In addition, real objects that we smell produce multiple odor-carrying molecules, so that the brain must analyze a complex mixture of odorants to recognize a smell.

Just as the sense of smell is direct in detecting fragments of the objects, it is also direct in the way the signal is transmitted to the brain. In most senses, such as vision , this task is accomplished in several steps: a receptor cell detects light and passes the signal to a nerve cell, which passes it on to another nerve cell in the central nervous system , which then relays it to the visual center of the brain. But in olfaction, all these jobs are performed by the olfactory nerve cell: in a very real sense, the olfactory epithelium is a direct outgrowth of the brain.

The olfactory nerve cell takes the scent message directly to the nerve cells of the olfactory bulb of the brain (or, in insects and other invertebrates that lack true brains, the olfactory ganglia), where multiple signals from different olfactory cells with different odor sensitivities are organized and processed. In higher species the signal then goes to the brain's olfactory cortex, where higher functions such as memory and emotion are coordinated with the sense of smell.


Human vs. animal smell

There is no doubt that many animals have a sense of smell far superior than humans. This is why, even today, humans use dogs to find lost persons, hidden drugs, and explosives , although research on "artificial noses" that can detect scent even more reliably than dogs continues. Humans are called microsmatic, rather than macrosmatic, because of their humble abilities of olfaction.

Still, the human nose is capable of detecting over 10,000 different odors, some in the range of parts per trillion of air; and many researchers are beginning to wonder whether smell does not play a greater role in human behavior and biology than has been thought. For instance, research has shown that human mothers can smell the difference between a vest worn by their baby and one worn by another baby only days after the child's birth .

Yet some olfactory abilities of animals are probably beyond humans. Most vertebrates have many more olfactory nerve cells in a proportionately larger olfactory epithelium than humans, which probably gives them much more sensitivity to odors. The olfactory bulb in these animals takes up a much larger proportion of the brain than humans, giving them more ability to process and analyze olfactory information.

In addition, most land vertebrates have a specialized scent organ in the roof of their mouth called the vomeronasal organ (also known as the Jacobson's organ or the accessory olfactory organ). This organ, believed to be vestigial in humans, is a pit lined by a layer of cells with a similar structure to the olfactory epithelium, which feeds into its own processing part of the brain, called the accessory olfactory bulb (an area of the brain absent in humans).

The vomeronasal sense appears to be sensitive to odor molecules with a less volatile, possibly more complex molecular structure than the odorants to which humans are sensitive. This sense is important in reproduction, allowing many animals to sense sexual attractant odors, or pheromones, thus governing mating behavior. It is also used by reptilian and mammalian predators in tracking prey.

Unknown territory

Researchers have learned a lot about how the olfactory nerve cells detect odorants. However, they have not yet learned how this information is coded by the olfactory cell. Other topics of future research will be how olfactory cell signals are processed in the olfactory bulb, and how this information relates to higher brain functions and our awareness of smell.

Scientists are only beginning to understand the role that smell plays in animal, and human, behavior. The vomeronasal sense of animals is still largely not understood. Some researchers have even suggested that the human vomeronasal organ might retain some function, and that humans may have pheromones that play a role in sexual attraction and mating—although this hypothesis is very controversial.

In addition, detailed study of the biology of the olfactory system might yield gains in other fields. For instance, olfactory nerve cells are the only nerve cells that are derived from the central nervous system that can regenerate, possibly because the stress of their exposure to the outside world gives them a limited life span. Some researchers hope that studying regeneration in olfactory nerve cells or even transplanting them elsewhere in the body can lead to treatments for as yet irreversible damage to the spine and brain.


Resources

books

Getchel, T.V., ed. Smell and Taste in Health and Disease. New York: Raven Press, 1991.

Moller, Aage R. Sensory Systems: Anatomy and Physiology. New York: Academic Press, 2002.

Whitfield, Philip, and D.M. Stoddart. Hearing, taste and smell: pathways of perception. Tarrytown, NY: Torstar Books, 1984.

periodicals

Dajer, Tony. "How the Nose knows." Discover January 1992.

Farbman, Albert I. "The Cellular Basis of Olfaction." Endeavour vol. 18 no. 1, 1994.

"A Nose by Any Other Name." The Economist September 1991.

Pennisi, Elizabeth. "Nose Nerve Cells Show Transplant Potential." Science News April 1993.


Kenneth B. Chiacchia

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Smell

Smell


Smell is the sense that enables an organism to detect airborne chemicals. It serves as a way of identifying, sorting out, and warning an organism about its environment. As one of the five senses, the sense of smell plays a key role in human development and is also linked to human emotions and memory.

Smell is technically called "olfaction," and it is one of the senses an animal uses to orient itself to its surroundings. Olfaction is sometimes described as a "sensitivity to substances in a gaseous phase." This means that smell allows an odor (which is an airborne chemical or a gas) to be detected. Being able to smell is one sense that the earliest life forms probably used to find food and avoid being eaten themselves.

Smell can be very important to the survival of both humans and other animals. It affects people's quality of life, or how much they enjoy life. Like the sense of taste, smell involves a complex process called chemoreception. Described simply, things can be smelled if they give off molecules (small particles) into the air. Organisms that have the sense of smell have specialized receptors, or cells, that receive these molecules. Substances that give off molecules are called "volatiles."

THE SENSE OF SMELL IN HUMANS

For humans, smell begins with the nose. The nose breathes in air carrying these molecules. This air enters through the nasal cavities, or nostrils (and sometimes the mouth). It passes over the body's "smelling skin," called its olfactory epithelium. This is a small patch of moist, specialized cells located in the upper part of the very rear of the nasal cavities just above the bridge of the nose. Here the body's chemoreceptors are stimulated by the gas molecules. The chemoreceptors are covered with extremely tiny hairs, called cilia, and a fluid. When the gas molecules dissolve in the fluid and touch the cilia, the cell reacts. Scientists still do not know if there is a special receptor for every different type of possible scent, or if there are certain receptors that are triggered in a different sequence by a particular odor. Most think that the latter is probably true.

When a chemoreceptor cell reacts, a nerve impulse is produced. The impulse travels to the brain's olfactory cortex (a layer of gray matter that

covers most of the brain's surface) where the smell is identified. If the smell is recognized from a previous experience it is easily identified. If it is new, it is stored and remembered for the next time. In humans, the olfactory cortex is located deep within the brain's limbic system. This is the part of the brain that is the source of people's emotions. Smell is also linked to the brain's hippocampus and amygdala in the limbic system, which controls memories. It is thought that this link to the parts of the brain that control emotion and memory is the reason why smells can cause people to feel certain emotions or have strong memories. All people have experienced a particular smell bringing back a flood of childhood feelings or making them remember an event. In this way, smell plays a large role in forming life experiences and influencing moods. Odors associated with a pleasant experience instantly bring back fond memories, while those associated with an unpleasant experience trigger negative emotions.

In humans, smell is closely related to taste since they both operate by chemoreception. If food has a foul smell, it becomes unappetizing and someone knows not to eat it. Without a sense of smell, humans' ability to taste would be severely impaired. People experience this when they are badly stuffed up from a head cold and cannot smell. Newborn babies recognize their mother within three days of birth by her smell. People with no sense of smell can hardly taste cheese and pepper. The sense of smell can be damaged or lost as a result of a head injury, infection, a brain tumor, or exposure to toxic chemicals.

THE SENSE OF SMELL IN ANIMALS

While the sense of smell does not play an obvious role in the lives of humans, it is a critically important sense for many animals. Those animals who depend a great deal on their sense of smell usually possess an acute hunting ability, and smelling plays many different roles in their lives. It is known that a dog's area of "smelling skin" in its nose is roughly fifty times larger than a human's olfactory epithelium. Such extreme sensitivity to odor by animals is exemplified by a shark, which can smell a few drops of blood mixed with sea water from great distances.

While many mammals smell with their noses, some animals and insects smell with their tongues, feet, or antennas. For example, a snake picks up chemical odor molecules from the air by flicking its tongue, while a butterfly senses sweetness with its feet and detects the smell of the opposite sex with its antennas. Ants and bees also smell with their antennas, and salmon are guided upstream by their sense of smell. Other animals, like fish and amphibians, have scent-detecting organs all over their bodies.

In the natural world, animals use smell to identify their own kind, find food, and avoid predators. They also use this sense to communicate using pheromones, or "messenger substances." These chemicals indicate when an individual is receptive, or ready, to mate. They also can guide a predator to its prey or warn the prey that a predator is close by. Many mammals use their urine to mark their territory and signal others to keep out. Animals of the same species can smell a number of things about the "marker" of the territory, such as its sex, readiness to mate, and even its general age.

Mammals have the best sense of smell among all vertebrates (animals with a backbone). Within the group of mammals, carnivores (animals that eat other animals) and rodents have the best sense of smell. Primates, like humans and apes, have the poorest sense of smell.

[See alsoOrgan; Sense Organ ]

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