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The olfactory epithelium is made up of at least six morphologically and biochemically different cell types. This may occur by diffusion or by the binding of the odorant to odorant-binding proteins. The mucus overlying the epithelium contains mucopolysaccharides , salts, enzymes , and antibodies these are highly important, as the olfactory neurons provide a direct passage for infection to pass to the brain.
This mucus acts as a solvent for odor molecules, flows constantly, and is replaced approximately every ten minutes. In insects , smells are sensed by olfactory sensory neurons in the chemosensory sensilla , which are present in insect antenna, palps, and tarsa, but also on other parts of the insect body. Odorants penetrate into the cuticle pores of chemosensory sensilla and get in contact with insect odorant-binding proteins OBPs or Chemosensory proteins CSPs , before activating the sensory neurons.
Receptor neuron[ edit ] The binding of the ligand odor molecule or odorant to the receptor leads to an action potential in the receptor neuron, via a second messenger pathway, depending on the organism. A calcium- calmodulin complex also acts to inhibit the binding of cAMP to the cAMP-dependent channel, thus contributing to olfactory adaptation. This mechanism of transduction is somewhat unusual, in that cAMP works by directly binding to the ion channel rather than through activation of protein kinase A.
It is similar to the transduction mechanism for photoreceptors , in which the second messenger cGMP works by directly binding to ion channels, suggesting that maybe one of these receptors was evolutionarily adapted into the other.
There are also considerable similarities in the immediate processing of stimuli by lateral inhibition. Averaged activity of the receptor neurons can be measured in several ways. In vertebrates, responses to an odor can be measured by an electro-olfactogram or through calcium imaging of receptor neuron terminals in the olfactory bulb.
In insects, one can perform electroantennography or calcium imaging within the olfactory bulb. These nerve fibers, lacking myelin sheaths, pass to the olfactory bulb of the brain through perforations in the cribriform plate , which in turn projects olfactory information to the olfactory cortex and other areas. Mitral cells , located in the inner layer of the olfactory bulb, form synapses with the axons of the sensory neurons within glomeruli and send the information about the odor to other parts of the olfactory system, where multiple signals may be processed to form a synthesized olfactory perception.
A large degree of convergence occurs, with 25, axons synapsing on 25 or so mitral cells, and with each of these mitral cells projecting to multiple glomeruli. Mitral cells also project to periglomerular cells and granular cells that inhibit the mitral cells surrounding it lateral inhibition.
Granular cells also mediate inhibition and excitation of mitral cells through pathways from centrifugal fibers and the anterior olfactory nuclei. Neuromodulators like acetylcholine , serotonin and norepinephrine all send axons to the olfactory bulb and have been implicated in gain modulation,  pattern separation,  and memory functions,  respectively. The mitral cells leave the olfactory bulb in the lateral olfactory tract , which synapses on five major regions of the cerebrum: The anterior olfactory nucleus projects, via the anterior commissure , to the contralateral olfactory bulb, inhibiting it.
The piriform cortex has two major divisions with anatomically distinct organizations and functions. The anterior piriform cortex APC appears to be better at determining the chemical structure of the odorant molecules, and the posterior piriform cortex PPC has a strong role in categorizing odors and assessing similarities between odors e. The orbitofrontal cortex mediates conscious perception of the odor.
The three-layered piriform cortex projects to a number of thalamic and hypothalamic nuclei, the hippocampus and amygdala and the orbitofrontal cortex, but its function is largely unknown.
The entorhinal cortex projects to the amygdala and is involved in emotional and autonomic responses to odor. It also projects to the hippocampus and is involved in motivation and memory.
Odor information is stored in long-term memory and has strong connections to emotional memory. Since any one receptor is responsive to various odorants, and there is a great deal of convergence at the level of the olfactory bulb, it may seem strange that human beings are able to distinguish so many different odors.
It seems that a highly complex form of processing must be occurring; however, as it can be shown that, while many neurons in the olfactory bulb and even the pyriform cortex and amygdala are responsive to many different odors, half the neurons in the orbitofrontal cortex are responsive to only one odor, and the rest to only a few. It has been shown through microelectrode studies that each individual odor gives a particular spatial map of excitation in the olfactory bulb.
It is possible that the brain is able to distinguish specific odors through spatial encoding, but temporal coding must also be taken into account. Over time, the spatial maps change, even for one particular odor, and the brain must be able to process these details as well. Inputs from the two nostrils have separate inputs to the brain, with the result that, when each nostril takes up a different odorant, a person may experience perceptual rivalry in the olfactory sense akin to that of binocular rivalry.
Accessory olfactory system[ edit ] Many animals, including most mammals and reptiles, but not humans, have two distinct and segregated olfactory systems: Behavioral evidence suggests that these fluid-phase stimuli often function as pheromones , although pheromones can also be detected by the main olfactory system.
In the accessory olfactory system, stimuli are detected by the vomeronasal organ , located in the vomer, between the nose and the mouth. Snakes use it to smell prey, sticking their tongue out and touching it to the organ. Some mammals make a facial expression called flehmen to direct stimuli to this organ. The sensory receptors of the accessory olfactory system are located in the vomeronasal organ.
As in the main olfactory system, the axons of these sensory neurons project from the vomeronasal organ to the accessory olfactory bulb , which in the mouse is located on the dorsal-posterior portion of the main olfactory bulb. Human incest avoidance[ edit ] See also: Body odor 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. 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.
When an odorant is detected by receptors, they in a sense break the odorant down, and then the brain puts the odorant back together for identification and perception. Because several receptor types are activated due to the different chemical features of the odorant, several glomeruli are activated as well. All of the signals from the glomeruli are then sent to the brain, where the combination of glomeruli activation encodes the different chemical features of the odorant.
The brain then essentially puts the pieces of the activation pattern back together in order to identify and perceive the odorant. 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. During the process of mastication , the tongue manipulates food to release odorants. These odorants enter the nasal cavity during exhalation.
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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 olfaction contribution to flavor occurs, in contrast to that of proper smell, which occurs during the inhalation phase of breathing.
The following are disorders of olfaction: Since the s industrial countries have encountered incidents where proximity of an industrial source or landfill produced adverse reactions among nearby residents regarding airborne odor.
The basic theory of odor analysis is to measure what extent of dilution with "pure" air is required before the sample in question is rendered indistinguishable from the "pure" or reference standard.
Since each person perceives odor differently, an "odor panel" composed of several different people is assembled, each sniffing the same sample of diluted specimen air.
A field olfactometer can be utilized to determine the magnitude of an odor. Many air management districts in the US have numerical standards of acceptability for the intensity of odor that is allowed to cross into a residential property. For example, the Bay Area Air Quality Management District has applied its standard in regulating numerous industries, landfills, and sewage treatment plants. In plants and animals[ edit ] The tendrils of plants are especially sensitive to airborne volatile organic compounds.
Parasites such as dodder make use of this in locating their preferred hosts and locking on to them. Threatened plants are then able to take defensive chemical measures, such as moving tannin compounds to their foliage.
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. 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.
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. 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. Bears , such as the Silvertip Grizzly found in parts of North America, 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.
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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 many species, olfaction 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. Insect olfactory system[ edit ] Inbreeding avoidance[ edit ] 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.
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Early scientific study of olfaction includes the extensive doctoral dissertation of Eleanor Gamble , published in , which compared olfactory to other stimulus modalities , and implied that smell had a lower intensity discrimination. 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 , 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.
Humans have far fewer active odor receptor genes than other primates and other mammals. 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 [ citation needed ].
According to a new study, researchers have found that a functional relationship exists between molecular volume of odorants and the olfactory neural response. However, the behavioral predictions of this theory have been called into question.