What is smell?

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One of the five special senses; chemicals interact with receptor sites in specialized structures of the nasal cavity, and the resulting nerve impulses are classified as certain kinds of odor
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Structure and Functions

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: camphoraceous, musky, floral, minty, ethereal, pungent, and putrid. Among other categories that have been suggested are woody, spicy, citrus, 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 between “left-handed” or “right-handed” molecules. In other words, the olfactory system can identify mirror-image molecules, in which one molecule is the spatial reverse of the other. An example is the substance carvone: when a person sniffs one form of carvone, the smell is spearmint; a sniff of the other form of carvone smells of caraway seed.

While studies of the anatomy and physiology of olfaction have not been finalized, the most accepted model of olfaction depends on the concept of odoriferous molecules attaching to olfactory receptor sites; the size and shape of both the odor molecules and the receptor sites are essential elements in currently adopted theories and descriptions of the mechanics of olfaction. Molecules of an odor-emitting substance chemically interact at receptor sites within specialized structures of the nasal cavity. The olfactory dendrites that are in contact with the external environment are directly linked to the neural centers of the brain. The olfactory tract connects these nerves directly to the hypothalamus region of the brain, which is associated with basic instinctual responses including fight-or-flight cues, food intake, or sexual curiosity and drive. This direct link to the brain causes a rapid and powerful response in animals to odor stimuli.

Because there is no physical barrier to protect olfactory receptors from the outside world, these nerve endings have a certain vulnerability to harm or damage. Olfactory nerves constantly regenerate in about a one- to three-month cycle; these and taste buds are the only nerves that are capable of readily regenerating themselves and returning to full function. Nevertheless, it is estimated that about 1 percent of all olfactory receptors in an individual die each year because of externally induced damage and general wear. Therefore, the sense of smell becomes less sensitive in older adults, which can minimize or repress the desire to eat and result in malnutrition.

The nasal cavity has two roles, one associated with respiration and the other with the sense of smell. The region that is dedicated to olfaction is small and evenly divided between the two septa of the nose. Within each nostril are folds called "conchae." Humans have three conchae pairs—lower, middle, and upper. The specialized sensing structures for olfaction are found inside the middle and upper conchae. It is currently believed that olfactory receptors are regions of specialized molecular architecture located on cilia-like fibers called "olfactory hairs." Olfactory hairs are extensions, or branches, of the olfactory sensing cells. The hairs, which are actually made of microfilaments, are dispersed over the surface of the olfactory sensing region of the nasal cavity.

While the size of the olfactory region of the nasal cavity is small (about the size of a dime), its surface area is comparatively large because of the many olfactory hairs coating the surface, or epithelium, of the olfactory cleft. It is estimated that humans have about five million olfactory receptors. The number of receptor hairs varies among species; some dogs have billions of olfactory receptors and are therefore far more sensitive to specific odors than are humans. This feature explains why dogs can be trained for hunting game, finding missing persons, and locating drugs or explosives. A person can smell a pot of soup cooking and know it is soup, but a dog smelling the same pot of soup smells each ingredient of the soup, not the scent of the mixture. Nevertheless, among the senses, the sense of smell in humans is second only to vision in terms of number of receptors per unit of surface area.

The olfactory hairs are surrounded by a thick, brown-colored mucus and are partially covered and partially exposed. Thus, the mucosal lining on the epithelium of the olfactory region is a thin and poor protective barrier for the specialized structures that it coats. The mucus on the olfactory epithelium has three structures of origin: Bowman’s glands, the goblet cells of the respiratory regions of the nose, and the supporting cells of the olfactory epithelium. Most of the mucus around the olfactory hairs is secreted from Bowman’s glands, and only the Bowman’s secretions contain the brown pigment that colors the mucus. It is known that, in other species, pigment is connected to olfactory ability; for example, albino pigs, which are lacking all pigments, are unable to smell toxic plants and often die from ingesting native plants that are poisonous to their species. The true significance of the brown pigment in humans, however, remains unclear.

The significance of the olfactory mucus itself is not in question. Odoriferous molecules must be trapped by the mucus so that they can travel to chemoreceptor sites on the olfactory hairs. The interaction between the odor molecules and the chemoreceptors of the olfactory hairs requires a mutual attraction, originating from small electrostatic forces. The shape, size, and polarity or nonpolarity of molecules causing odor are important factors in the creation of a smell stimulus.

From the olfactory hairs in the membrane structure, the odor molecules travel through the cribriform plate and into the olfactory tract, which leads directly to the hypothalamus in the brain. The olfactory hairs extend from the olfactory knobs, unexposed sensory cell endings completely covered by brown mucus. Five to eight hairs extend from each knob; electron micrographs show that the hairs are actually dendrites extending from the cell body into the external environment, while the axons of the cells carry nerve impulses toward the brain.

The sensory cells are found about midway in the olfactory epithelium. Although only one odor receptor is expressed by each sensory cell, odor molecules may excite responses from multiple receptors. Other cells in the epithelium are the supporting cells and the basal cells. The supporting cells provide a scaffolding for the sensory cells and also contribute fluids to the mucus layer. Basal cells are able to assist in the replenishment of receptor cells.

The cribriform plate, often described as a wafer-thin structure, is an important separation point between unmyelinated sensory nerves, which are in contact with the environment, and the myelinated nerves that direct the tiny electrical impulses of smell to the brain. Myelinated olfactory nerves are large and function with great speed and efficiency in comparison to the unmyelinated sensory cells. Thus, the original nerve signal prompted by an odoriferous molecule is slow, but this time is more than recovered in the myelinated nerve fibers. Working together, the unmyelinated and myelinated nerve bundles detect, transmit, and deliver nerve impulses of smell in fractions of a second, aided by the relatively short path between the nasal cavity and the hypothalamus.

The hypothalamus is located under the thalamus in the brain. It is the center into which nerve impulses originating at the sensory organs of sound, taste, smell, and the somatic senses are delivered. The activity of this portion of the brain is closely linked to the activity of the pituitary gland. This association is important in the sense of smell and sexual maturation, which is triggered by hormones released by the hypothalamus. In addition to these attributes, the hypothalamus is essential to regulating the autonomic nervous system, body temperature, and food intake.

Disorders and Diseases

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. Smell disorders can also accompany serious conditions such as hypertension, malnutrition, obesity, or diabetes. In diagnosing possible causes of anosmia or dysosmia, a physician must obtain a complete medical history and perform a thorough physical exam. Special attention is given to the nasal cavity, the head and neck area, and, perhaps surprisingly, to genital maturation and function. Smell function is measured by passing vials containing increasing concentrations of an odoriferous chemical under a patient’s nose until a scent can be detected. It is also important to assess whether the patient can properly identify the smell; if not, further studies must be done.

Olfactory problems can originate in one of the three structures that are involved in olfaction: in the sensory receptors, which convert chemical signals arising from odor molecules into electrical impulses; in the sensory nerve cells, which transmit these electrical impulses to the brain; and in the brain, which interprets the incoming electrical signals.

Abnormalities in the nasal cavity that can modify or destroy olfaction may include nasal polyps, a tumor located in an olfactory bulb, or allergic rhinitis (irritated and swollen membranes of the nose). Other olfactory maladies may be of an indirect origin, such as nutritional abnormalities, radiation exposure, or the presence of a toxic trace metal. Endocrine imbalances can be particularly pertinent in olfaction function.

An example of an endocrine imbalance that can influence the olfactory sense is seen in the cooperative workings of the hypothalamus and the pituitary gland. The hypothalamus receives impulses from olfactory nerves. It also lies just above the major endocrine gland, the pituitary gland. The pituitary gland receives regulating chemicals from the hypothalamus that either stimulate or inhibit the anterior portion of the gland. The anterior region of the pituitary gland monitors the levels of steroid hormones circulating in the body. Steroid hormones are essential to complete sexual maturation in both males and females. A congenital defect that affects both the nose and sexual maturation is Kallmann syndrome (also known as "olfactogenital dysplasia" or "idiopathic hypogonadotropic hypogonadism"). Individuals with this malady are anosmic as a result of underdeveloped olfactory lobes. Low levels of gonadotropic hormones are also found in affected persons, resulting in undeveloped ovaries or testes.

Olfaction is believed to play a role in the timely onset of sexual maturation in puberty. It seems that both male and female pheromones, oily scents that subconsciously cause sexual excitement in a species, can assist in or accelerate the events of sexual maturation. Although researchers are still exploring the role of pheromones in the human species, it appears that smell is relevant to the onset of menstruation in pubescent girls. Scents also contribute to sexual arousal in males.

An altered sense of smell can occur with pregnancy because of the resulting changes in hormone levels. For some pregnant women, formerly pleasant aromas may become repugnant, sometimes contributing to the feeling of nausea that some pregnant women experience. In addition, the increased mucus production that occurs with pregnancy works to block full smell function.

Brain tumors or lesions can sometimes account for anosmia, hyposmia, or dysosmia. Head injury is another possible cause because nerves or the hypothalamus itself can be crushed or otherwise damaged. Tumors, lesions, and neural damage can be detected using positron emission tomography (PET) scanning, computed tomography (CT) scanning, or magnetic resonance imaging (MRI). Some neurological diseases may be considered when diagnosing these olfactory disorders since the sense of smell requires only organs found in the nervous system.

Treatment of olfactory malfunctions varies greatly depending on the origin of the problem. Surgery may be needed to remove tumors or polyps. Allergies may be treated by shots or oral antihistamines; corticosteroids may be used to prevent the inflammation of nasal mucous membranes. Drugs may be administered to either inhibit or activate nerve conduction. Treatment often results in the recovery of olfaction, but not in all cases.

Perspective and Prospects

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 bce) 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 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–ca. 199 ce), who proposed an insightful description of the neuroanatomy of olfaction that also proved to be validated, with some alterations, centuries later. Whether he accepted the notion of atoms or not, Galen believed that particles actually tunneled into what are now called "olfactory bulbs," thereby causing an odor to be detected. Galen also believed that these olfactory bulbs were extensions connected directly to the brain. Living in an era when microscopes and the scientific method did not exist, Galen could only describe what he saw with the unaided eye and reason intuitively. It is fascinating, therefore, to learn that his belief that the olfactory bulbs were extensions of the brain has been proven correct.

As the most primitive, and thus less evolved, of the five special senses, smell is associated with basic instincts, reactions or responses to external stimuli that aid an individual organism. Smell influences instincts of aggression when odors are released in fighting or battle through sweat and perhaps blood. These odors may inspire fight-or-flight responses in the brain and body. Social groups, such as a street gang, a group of soldiers, or a den of lions, can learn to recognize the scent (or the absence of scent) of its members in training or other group activities, helping to identify safe and unsafe groups in darkness or battle when other cues may be masked. Thus, while it is a subtle form of recognition often registered in the subconscious, smell apparently plays a role in modern survival tactics as well.

Another basic instinct that utilizes the sense of smell is the so-called mothering instinct. For example, new mothers are better able to identify their newborns by scent than by sight only hours after delivery. This sense seems adaptively helpful to the exhausted mother—who may have been in labor for days and is likely to be suffering from general exhaustion and diminished energy—in locating and feeding her baby. In a primitive human culture, this ability would help mothers identify babies if a flight from danger or a search for food or water caused a temporary separation of the mother-child pair. In addition, a nursing baby is guided to the mother’s nipple by the scent released from the sweat glands surrounding the nipples.

Mate selection is believed to be linked to scents and olfactory appeal. There is some evidence that even the most heavily perfumed person of modern society emits pheromones that are sexually alluring to some and repulsive to others. This allure or repulsion seems to occur in the subconscious mind, or the limbic region of the brain.

On a more conscious level, the sense of smell is sometimes useful as a warning of a health problem; thus, odors can be helpful in either describing or diagnosing a disease. For example, some people with epilepsy, days or only minutes before the onset of an epileptic seizure, have olfactory hallucinations—they smell odors in the absence of any stimulating molecules. The odor is usually described as either a scent of decay, as at a fish market, or a chemical, such as ether or petroleum. The sweet smell of acetone on a person’s breath can indicate a diabetic who is in danger of coma or who is already in a coma and cannot ask for help. Vincent angina (commonly called "trench mouth") can be suspected if a foul breath odor is present, while diphtheria causes a sweet scent.

Because of the unique regenerative capacity of the olfactory neurons, neurologists and other researchers are actively attempting to understand the mechanisms that allow these nerves to be so efficient and effective in nerve regeneration. Such research may have an impact upon the understanding and treatment of a variety of neurological disorders that are not directly affiliated with olfaction. Other areas of ongoing research include the influence of environmental factors on olfaction, the relationship between diminishing olfaction with age and race or ethnicity, the impact of olfaction on nutrition status, the protection of the brain from infection via the olfactory nerve, and better diagnosis and treatment of olfactory disorders.

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