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The sense of smellor olfactionis the special sense through which smells or odors are perceived. It occurs when an odor binds to a receptor within dissertation avoidance complex nasal cavitytransmitting a signal through the olfactory system. There are many different causes for alteration, dissertation avoidance complex, 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 infectionstraumatic brain injuryand neurodegenerative disease, dissertation avoidance complex.
Early scientific study of the sense of smell includes the extensive doctoral dissertation of Eleanor Gamblepublished inwhich compared olfactory to other stimulus modalitiesdissertation avoidance complex, 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 inand subsequent pairing of odor molecules to specific receptor proteins. 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 theoryeach receptor detects a feature of the odor molecule. The weak-shape theory, known as the odotope theorysuggests that different receptors detect only small pieces of molecules, dissertation avoidance complex, 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 dissertation avoidance complex volume of odorants and the olfactory neural response.
However, dissertation avoidance complex, the behavioral predictions of this theory have been called into question. Flavor perception is an aggregation of auditorytastehapticand smell sensory information.
During the process of masticationthe tongue manipulates food to release odorants. These odorants enter the nasal cavity during exhalation. Smell, tasteand trigeminal receptors also called chemesthesis together contribute to flavor. The human tongue can distinguish only among five distinct qualities of taste, dissertation avoidance complex, 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. Smell and sound information has been shown to converge in the olfactory tubercles of rodents. 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. Dissertation avoidance complex can detect blood relatives from olfaction. 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. 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. Different people smell different odors, and most of these differences are caused by genetic differences. For instance, the odorant receptor OR5A1 and its genetic variants alleles are responsible for our ability or failure to smell β- iononea key aroma in foods and beverages. The importance and sensitivity of smell varies among different organisms; most mammals have a good dissertation avoidance complex of smell, whereas most birds do not, except the tubenoses e.
Also, birds have hundreds of olfactory receptors. 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, dissertation avoidance complex. That is, the brain's smell-recognizing centers must react to the stimulus dissertation avoidance complex 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. Scenthounds as a group can smell one- to ten-million times dissertation avoidance complex acutely than a human, and bloodhoundswhich have the keenest sense of smell of any dogs, [35] 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 Houndwhich was bred to track and hunt rabbits and other small animals.
Grizzly bears have a sense dissertation avoidance complex 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 primatesand nonexistent in cetaceanswhich compensate with a well-developed sense of taste.
In many species, smell is highly dissertation avoidance complex 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. 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.
Although conventional wisdom and lay literature, based on impressionistic findings in the s, have long presented human smell as capable of distinguishing between roughly 10, unique odors, recent research has suggested that the average individual is capable of distinguishing over one trillion unique odors. 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.
In November 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. In humans and other vertebratessmells 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. Humans have about 10 cm 2 1. A dog's olfactory epithelium is also considerably more densely innervated, with a hundred times more receptors per square centimeter.
Molecules of odorants passing through the superior nasal concha of dissertation avoidance complex nasal passages dissolve in the mucus that lines the superior portion dissertation avoidance complex the cavity and are detected by olfactory receptors on the dendrites of the olfactory sensory neurons.
This may occur by diffusion or by the binding dissertation avoidance complex the odorant to odorant-binding proteins. The mucus overlying the epithelium contains mucopolysaccharidessalts, enzymesand 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, dissertation avoidance complex, flows constantly, and is replaced approximately every ten minutes. In insectssmells are sensed by olfactory sensory neurons in the chemosensory sensillawhich are present in insect antenna, palps, and dissertation avoidance complex, but also on other dissertation avoidance complex 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 CSPsbefore activating the sensory neurons. 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. In mammals, dissertation avoidance complex odorants stimulate adenylate cyclase to synthesize cAMP via a G protein called G olf.
A calcium- calmodulin complex also acts to inhibit the binding of cAMP to dissertation avoidance complex cAMP-dependent channel, thus contributing to olfactory adaptation. The main olfactory system of some mammals also contains small subpopulations of olfactory sensory neurons that detect and transduce odors somewhat differently. Olfactory sensory neurons that use trace amine-associated receptors TAARs to detect odors use the same second messenger signaling cascade as do the canonical olfactory sensory neurons.
This mechanism of transduction is somewhat unusual, dissertation avoidance complex, in that cAMP works by directly binding to the ion channel rather than through activation of protein dissertation avoidance complex A, dissertation avoidance complex. It is similar to the transduction mechanism for photoreceptorsdissertation avoidance complex, 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, dissertation avoidance complex, one can perform electroantennography or calcium imaging within the olfactory bulb. Olfactory sensory neurons project axons to the brain within the olfactory nervecranial nerve I. These nerve fibers, lacking myelin sheaths, pass to the olfactory bulb of the brain through perforations in the cribriform platewhich in turn projects olfactory information to the olfactory cortex and other areas.
Mitral cellslocated 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 acetylcholinedissertation avoidance complex, serotonin and norepinephrine all send axons to the olfactory bulb and have been implicated in gain modulation, [51] pattern separation, [52] and memory functions, [53] respectively.
The mitral cells leave the olfactory bulb in the lateral olfactory tractwhich synapses on five major regions of the cerebrum: the anterior olfactory nucleusthe olfactory tuberclethe amygdalathe piriform cortexand the entorhinal cortex. The anterior olfactory nucleus projects, via the anterior commissureto 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.
minty, woody, and citrus are odors that can, despite being highly variant chemicals, be distinguished via the PPC in a concentration-independent manner. The orbitofrontal cortex mediates dissertation avoidance complex perception of the odor. 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.
This is possibly due to the olfactory system's close anatomical ties to the limbic system and hippocampus, areas of the brain that have long been known to be involved in emotion and place memory, respectively. 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.
In insectsdissertation avoidance complex, smells are sensed by sensilla located on the antenna and maxillary palp and first processed by the antennal lobe analogous to the olfactory bulband next by the mushroom bodies and lateral horn.
The process by which olfactory information is coded in the brain to allow for proper perception is still being researched, and is not completely understood. When an odorant is detected by receptors, they in a sense break the odorant down, dissertation avoidance complex, and then the brain puts the odorant back together for identification and perception.
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