Structure and Functions (Magill’s Medical Guide, Sixth Edition)
Smell, one of the five special senses, plays an important role in both conscious and subconscious thought. While the loss of smell (anosmia) is troublesome, in isolation it is not a life-threatening problem. Nevertheless, anosmia is frequently an indication of an underlying pathology in either the olfactory or related organs; some of these pathologies may be life-threatening.
Of the special senses—gustation (taste), sight, olfaction (smell), audition (hearing), and equilibrium (balance and direction)—smell is the most primitive. As such, the organs that compose the olfactory system in humans are essentially identical to those found in other animals, including lampreys, cats, or dogs. The olfactory region of the nose is a very discriminating organ. Humans are able to classify smells according to at least seven agreed upon, although vague, classifications of primary odors: camphorlike, musky, floral, minty, ethereal, pungent, and putrid. Other categories that have been suggested are woody, spicy, and burned.
Within each odor category, the olfactory nerves and the brain are able to identify specific aromas with precision. For example, within the category of pungent, smells of onion, garlic, or skunk spray are easily discerned as similar, yet different, odors. Within the floral category, the human mind can readily distinguish among rose, lavender, and gardenia. The human olfactory sense can even distinguish...
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Disorders and Diseases (Magill’s Medical Guide, Sixth Edition)
Loss of the ability to sense all smells is called anosmia. Hyposmia (a decrease of smell function), dysosmia (an altered sense of smell), and anosmia can manifest themselves in numerous ways.
Most people are well acquainted with the inability to smell during a heavy cold. This condition is a temporary one caused by the presence of excessive mucus. The presence of a cold virus causes the respiratory region of the nose to respond by producing excessive volumes of cleansing mucus from goblet cells. Unfortunately, there tends to be so much mucus that the olfactory region becomes flooded; instead of swimming in mucus, the olfactory hairs are drowning. A thick coating over the hairs prevents odoriferous molecules from reaching the chemoreceptors. The sense of smell is lost until partial recovery decreases the mucus levels and once again allows the olfactory hairs to be partially exposed to the exterior world. The ability to sense odors fully returns once recovery from the head cold is complete.
Because it is uncommon to lose the ability to smell all odors, true anosmia is a rare condition. Furthermore, anosmia is seldom a problem found in isolation. Often there are simultaneously occurring symptoms such as a loss of taste (gustatory) function, undeveloped ovaries and testes, or head injury. In diagnosing possible causes of anosmia or dysosmia, a physician must obtain a complete medical history and perform a thorough...
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Perspective and Prospects (Magill’s Medical Guide, Sixth Edition)
The sense of smell has been recognized as one of the most primitive attributes of the human species, and it once held a high position in the hierarchy of skills required for species survival. Olfaction has long been a topic of intrigue in intellectual circles. The Greek philosopher Democritus of Abdera (384-322 b.c.e.) proposed his theory of the atom in a time when modern science and scientific methods did not exist. Democritus incorporated his description of atoms into an explanation of olfaction. He presumed that the sense of smell in humans resulted from some kind of connection that formed when atoms of odor-emitting substances entered the nose. Different odor sensations, he proposed, would result from differences in the texture and shape of these atoms. The anatomy of the nose and brain was not considered in his philosophy. Democritus’s idea of atoms was largely rejected in Greek circles of thought, however, and the concept of atoms combining to form molecules would not appear for hundreds of centuries. Modern understanding of the sense of smell is largely a more advanced, more informed, and more technical description of the very ideas imagined by this great Greek philosopher.
Another Greek contemplating the subject of smell was the physician Galen of Pergamum (129-c. 199 c.e.), who proposed an insightful description of the neuroanatomy of olfaction which also proved to be validated, with some alterations,...
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For Further Information: (Magill’s Medical Guide, Sixth Edition)
Engen, Trygg. The Perception of Odors. New York: Academic Press, 1982. Designed for the beginner, this text describes the anatomy, physiology, and psychology behind olfaction. Provides detailed explanations of the classification of odors, odor theories, and the tests that are used to assess olfactory function.
Finger, Thomas E., Wayne L. Silver, and Diego Restrepo, eds. Neurobiology of Taste and Smell. 2d ed. New York: Wiley-Liss, 2000. Introduces the study of taste and smell, focusing on the way chemical senses work, with coverage ranging from microorganisms to humans and from genetics to behavior.
Møller, Aage R. Sensory Systems: Anatomy, Physiology, and Pathophysiology. Boston: Academic Press, 2003. An excellent text that describes how human sensory systems function, with comparisons of the five senses and detailed descriptions of the functions of each of them. Also covers how sensory information is processed in the brain to provide the basis for communication and for the perception of one’s surroundings.
Schmidt, Robert F., ed. Fundamentals of Sensory Physiology. Translated by Marguerite A. Biedermann-Thorson. Rev. 3d ed. Berlin: Springer, 1986. Chapter 9, “Physiology of Olfaction,” addresses the organization of the olfactory system and its relevant connections in the brain.
Shier, David N., Jackie L. Butler, and Ricki Lewis. Hole’s Essentials...
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Smell (Encyclopedia of Science)
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
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Smell (Encyclopedia of Psychology)
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...
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Smell (Encyclopedia of Nursing & Allied Health)
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.
The sense of smell has been a topic of debate from humankind's earliest days. The Greek philosopher Democritus of Abdera (46060B.C.), speculated that humans smell "atoms" of different size and shape that come from objects. His countryman Aristotle (38422B.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 (1787840) 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.
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 "odor-blind" 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 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" than 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.
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 (anonosmia). 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.
Anosmia disorder in which one is able to detect no odors.
Olfactory bulbhe primitive part of the brain that first processes olfactory information.
Olfactory cortexhe cerebral cortex that makes use of information from the olfactory bulb.
Olfactory epitheliumhe patch of mucus membrane at the top of the nasal cavity that is sensitive to odor.
Olfactory nerve cellhe cell in the olfactory epithelium that detects odor and transmits the information to the olfactory bulb of the brain.
Pheromonescent molecules made by the body that attract a mate and help initiate mating behaviors.
Receptor protein protein in a cell that sticks to a specific odorant or other signal molecule.
Stereospecific theoryhe theory that the nose recognizes odorants when they bind to receptor proteins that recognize the odorants' molecular shape.
Vomeronasal 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.
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. W.W. Norton, 2000.
Dajer, Tony. "How the Nose Knows." Discover, Jan. 1992.
Farbman, Albert I. "The Cellular Basis of Olfaction." Endeavor, 18, no. 1 (1994).
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Peggy Elaine Browning