sense of smell sense of smell the sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. the sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. smellpainting of a woman smelling a carnation. olfaction uses chemoreceptors that create signals processed in the brain that form the sense of smell.detailssystemolfactory systemfunctionsense chemicals in the environment that are used to form the sense of smellidentifiersmeshd012903anatomical terminology in humans, it occurs when an odor binds to a receptor within the nasal cavity, transmitting a signal through the olfactory system. glomeruli aggregate signals from these receptors and transmit them to the olfactory bulb, where the sensory input will start to interact with parts of the brain responsible for smell identification, memory, and emotion. there are many different causes for alteration, lack, or disturbance to a normal sense of smell, and can include damage to the nose or smell receptors, or central problems affecting the brain. some causes include upper respiratory infections, traumatic brain injury, and neurodegenerative disease. history of study the lady and the unicorn, a flemish tapestry depicting the sense of smell, 1484–1500. musée national du moyen âge, paris. early scientific study of the sense of smell includes the extensive doctoral dissertation of eleanor gamble, published in 1898, which compared olfactory to other stimulus modalities, and implied that smell had a lower intensity discrimination. as the epicurean and atomistic roman philosopher lucretius (1st century bce) speculated, different odors are attributed to different shapes and sizes of "atoms" (odor molecules in the modern understanding) that stimulate the olfactory organ. a modern demonstration of that theory was the cloning of olfactory receptor proteins by linda b. buck and richard axel (who were awarded the nobel prize in 2004), and subsequent pairing of odor molecules to specific receptor proteins. each odor receptor molecule recognizes only a particular molecular feature or class of odor molecules. mammals have about a thousand genes that code for odor reception. of the genes that code for odor receptors, only a portion are functional. humans have far fewer active odor receptor genes than other primates and other mammals. in mammals, each olfactory receptor neuron expresses only one functional odor receptor. odor receptor nerve cells function like a key–lock system: if the airborne molecules of a certain chemical can fit into the lock, the nerve cell will respond. there are, at present, a number of competing theories regarding the mechanism of odor coding and perception. according to the shape theory, each receptor detects a feature of the odor molecule. the weak-shape theory, known as the odotope theory, suggests that different receptors detect only small pieces of molecules, and these minimal inputs are combined to form a larger olfactory perception (similar to the way visual perception is built up of smaller, information-poor sensations, combined and refined to create a detailed overall perception). according to a new study, researchers have found that a functional relationship exists between molecular volume of odorants and the olfactory neural response. an alternative theory, the vibration theory proposed by luca turin, posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by quantum tunnelling. however, the behavioral predictions of this theory have been called into question. there is no theory yet that explains olfactory perception completely. functions taste flavor perception is an aggregation of auditory, taste, haptic, and smell sensory information. retronasal smell plays the biggest role in the sensation of flavor. during the process of mastication, the tongue manipulates food to release odorants. these odorants enter the nasal cavity during exhalation. the smell of food has the sensation of being in the mouth because of co-activation of the motor cortex and olfactory epithelium during mastication. smell, taste, and trigeminal receptors (also called chemesthesis) together contribute to flavor. the human tongue can distinguish only among five distinct qualities of taste, while the nose can distinguish among hundreds of substances, even in minute quantities. it is during exhalation that the smell's contribution to flavor occurs, in contrast to that of proper smell, which occurs during the inhalation phase of breathing. the olfactory system is the only human sense that bypasses the thalamus and connects directly to the forebrain. hearing smell and sound information has been shown to converge in the olfactory tubercles of rodents. this neural convergence is proposed to give rise to a perception termed smound. whereas a flavor results from interactions between smell and taste, a smound may result from interactions between smell and sound. inbreeding avoidance the mhc genes (known as hla in humans) are a group of genes present in many animals and important for the immune system; in general, offspring from parents with differing mhc genes have a stronger immune system. fish, mice, and female humans are able to smell some aspect of the mhc genes of potential sex partners and prefer partners with mhc genes different from their own. humans can detect blood relatives from olfaction. mothers can identify by body odor their biological children but not their stepchildren. pre-adolescent children can olfactorily detect their full siblings but not half-siblings or step siblings, and this might explain incest avoidance and the westermarck effect. functional imaging shows that this olfactory kinship detection process involves the frontal-temporal junction, the insula, and the dorsomedial prefrontal cortex, but not the primary or secondary olfactory cortices, or the related piriform cortex or orbitofrontal cortex. since inbreeding is detrimental, it tends to be avoided. in the house mouse, the major urinary protein (mup) gene cluster provides a highly polymorphic scent signal of genetic identity that appears to underlie kin recognition and inbreeding avoidance. thus, there are fewer matings between mice sharing mup haplotypes than would be expected if there were random mating. guiding movement some animals use scent trails to guide movement, for example social insects may lay down a trail to a food source, or a tracking dog may follow the scent of its target. a number of scent-tracking strategies have been studied in different species, including gradient search or chemotaxis, anemotaxis, klinotaxis, and tropotaxis. their success is influenced by the turbulence of the air plume that is being followed. genetics different people smell different odors, and most of these differences are caused by genetic differences. although odorant receptor genes make up one of the largest gene families in the human genome, only a handful of genes have been linked conclusively to particular smells. for instance, the odorant receptor or5a1 and its genetic variants (alleles) are responsible for our ability (or failure) to smell β-ionone, a key aroma in foods and beverages. similarly, the odorant receptor or2j3 is associated with the ability to detect the "grassy" odor, cis-3-hexen-1-ol. the preference (or dislike) of cilantro (coriander) has been linked to the olfactory receptor or6a2. variability amongst vertebrates the importance and sensitivity of smell varies among different organisms; most mammals have a good sense of smell, whereas most birds do not, except the tubenoses (e.g., petrels and albatrosses), certain species of new world vultures, and the kiwis. also, birds have hundreds of olfactory receptors. although, recent analysis of the chemical composition of volatile organic compounds (vocs) from king penguin feathers suggest that vocs may provide olfactory cues, used by the penguins to locate their colony and recognize individuals. among mammals, it is well developed in the carnivores and ungulates, which must always be aware of each other, and in those that smell for their food, such as moles. having a strong sense of smell is referred to as macrosmatic in contrast to having a weak sense of smell which is referred to as microsmotic. figures suggesting greater or lesser sensitivity in various species reflect experimental findings from the reactions of animals exposed to aromas in known extreme dilutions. these are, therefore, based on perceptions by these animals, rather than mere nasal function. that is, the brain's smell-recognizing centers must react to the stimulus detected for the animal to be said to show a response to the smell in question. it is estimated that dogs, in general, have an olfactory sense approximately ten thousand to a hundred thousand times more acute than a human's. this does not mean they are overwhelmed by smells our noses can detect; rather, it means they can discern a molecular presence when it is in much greater dilution in the carrier, air. scenthounds as a group can smell one- to ten-million times more acutely than a human, and bloodhounds, which have the keenest sense of smell of any dogs, have noses ten- to one-hundred-million times more sensitive than a human's. they were bred for the specific purpose of tracking humans, and can detect a scent trail a few days old. the second-most-sensitive nose is possessed by the basset hound, which was bred to track and hunt rabbits and other small animals. grizzly bears have a sense of smell seven times stronger than that of the bloodhound, essential for locating food underground. using their elongated claws, bears dig deep trenches in search of burrowing animals and nests as well as roots, bulbs, and insects. bears can detect the scent of food from up to eighteen miles away; because of their immense size, they often scavenge new kills, driving away the predators (including packs of wolves and human hunters) in the process. the sense of smell is less developed in the catarrhine primates, and nonexistent in cetaceans, which compensate with a well-developed sense of taste. in some strepsirrhines, such as the red-bellied lemur, scent glands occur atop the head. in many species, smell is highly tuned to pheromones; a male silkworm moth, for example, can sense a single molecule of bombykol. fish, too, have a well-developed sense of smell, even though they inhabit an aquatic environment. salmon utilize their sense of smell to identify and return to their home stream waters. catfish use their sense of smell to identify other individual catfish and to maintain a social hierarchy. many fishes use the sense of smell to identify mating partners or to alert to the presence of food. human smell abilities although conventional wisdom and lay literature, based on impressionistic findings in the 1920s, have long presented human smell as capable of distinguishing between roughly 10,000 unique odors, recent research has suggested that the average individual is capable of distinguishing over one trillion unique odors. researchers in the most recent study, which tested the psychophysical responses to combinations of over 128 unique odor molecules with combinations composed of up to 30 different component molecules, noted that this estimate is "conservative" and that some subjects of their research might be capable of deciphering between a thousand trillion odorants, adding that their worst performer could probably still distinguish between 80 million scents. authors of the study concluded, "this is far more than previous estimates of distinguishable olfactory stimuli. it demonstrates that the human olfactory system, with its hundreds of different olfactory receptors, far out performs the other senses in the number of physically different stimuli it can discriminate." however, it was also noted by the authors that the ability to distinguish between smells is not analogous to being able to consistently identify them, and that subjects were not typically capable of identifying individual odor stimulants from within the odors the researchers had prepared from multiple odor molecules. in november 2014 the study was strongly criticized by caltech scientist markus meister, who wrote that the study's "extravagant claims are based on errors of mathematical logic." the logic of his paper has in turn been criticized by the authors of the original paper. physiological basis in vertebrates main olfactory system main article: olfactory system in humans and other vertebrates, smells are sensed by olfactory sensory neurons in the olfactory epithelium. the olfactory epithelium is made up of at least six morphologically and biochemically different cell types. the proportion of olfactory epithelium compared to respiratory epithelium (not innervated, or supplied with nerves) gives an indication of the animal's olfactory sensitivity. humans have about 10 cm2 (1.6 sq in) of olfactory epithelium, whereas some dogs have 170 cm2 (26 sq in). a dog's olfactory epithelium is also considerably more densely innervated, with a hundred times more receptors per square centimeter. the sensory olfactory system integrates with other senses to form the perception