Olfaction

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Young boy smelling a flower

Olfaction, which is also known as Olfactics is the sense of smell, and the detection of chemicals dissolved in air. The chemicals themselves, generally at very low concentrations, are called odors.

1 How olfaction works
2 Quantifying olfaction
3 Olfaction in other animals
4 References
5 See also
6 External links

[edit] How olfaction works

As discovered by Linda B. Buck and Richard Axel (who were awarded the Nobel Prize in 2004), mammals have about a thousand genes for odor reception.^[1] Of these genes, only a portion are functional odor receptors. Humans have 347 functional odor receptor genes; the other genes have nonsense mutations. This number was determined by analyzing the genome in the Human Genome Project; the number may vary among strains, and does vary among individual humans (see anosmia). For example, not all humans can smell androstenone, a component of male sweat (and minor component of female sweat).

Each olfactory receptor neuron in the nose 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. According to shape theory, each receptor detects a feature of the odor molecule. Weak-shape theory, known as 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). An alternative theory, the vibration theory proposed by Luca Turin^[2]^[3], posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by electron tunnelling. However, the behavioral predictions of this theory have been called into question.^[4] As of yet, there is no theory that explains human olfactory perception completely.

[edit] Olfactory epithelium

In vertebrates smells are sensed by olfactory sensory neurons in the olfactory epithelium. The proportion of olfactory epithelium compared to respiratory epithelium (not innervated) gives an indication of the animals olfactory sensitivity. Humans have about 2*4 cm² of olfactory epithelium, whereas some dogs have 150 cm². A dog's olfactory epithelium is also considerably more densely innervated, with a hundred times more receptors per square centimetre.

Molecules of odorands passing through the superior nasal concha of the nasal passages dissolve in the mucus lining the superior portion of 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 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).

[edit] Receptor neuron

The process of how the binding of the ligand (odor molecule or odorant) to the receptor leads to an action potential in the receptor neuron is via a second messenger pathway depending on the organism. In mammals the odorants stimulate adenylate cyclase to synthesize cAMP via a G protein called G_olf. cAMP, which is the second messenger here, opens a cyclic nucleotide-gated ion channel (CNG) producing an influx of cations (largely Ca²⁺ with some Na⁺) into the cell, slightly depolarising it. The Ca²⁺ in turn opens a Ca²⁺ activated chloride channel leading to efflux of Cl^-, further depolarising the cell and triggering an action potential. Ca²⁺ is then extruded through a sodium-calcium exchanger. 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 unique, 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 neuron to an odor can be measured by an electroolfactogram in vertebrates or an electroantenogram in insects.

[edit] In the brain

Olfactory sensory neurons project axons to the brain within the olfactory nerve, (cranial nerve I). These axons pass to the olfactory bulb through the cribriform plate, which in-turn projects olfactory information to the olfactory cortex and other areas. The axons from the olfactory receptors converge in the olfactory bulb within small (~50 micrometers in diameter) structures called glomeruli. Mitral cells in the olfactory bulb form synapses with the axons within glomeruli and send the information about the odor to multiple other parts of the olfactory system in the brain where multiple signals may be processed to form a synthesized olfactory perception. There is a large degree of convergence here, with twenty-five thousand axons synapsing on one hundred 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 fibres and the anterior olfactory nuclei.

The mitral cells leave the olfactory bulb in the lateral olfactory tract, which synapses on five major regions of the olfactory cortex: the anterior olfactory nucleus, the olfactory tubercle, the orbitofrontal cortex, the pyriform cortex and the enterorhinal cortex. The anterior olfactory nucleus projects, via the anterior comissure, to the contralateral olfactory bulb, inhibiting it. The olfactory tubercle projects to the medial dorsal nucleus of the thalamus, which then projects to the orbitofrontal cortex. The orbitofrontal cortex mediates conscious perception of the odour. The 3-layered pyriform 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 enterorhinal cortex projects to the amygdala and is involved in emotional and autonomic responses to odour. It also projects to the hippocampus and is involved in motivation and memory. Odour information is easily 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 seems strange that we are able to distinguish so many different odours. There must be a highly complex form of processing occurring, however, as it can be shown that whilst many neurons in the olfactory bulb (and even the pyriform cortex and amygdala) are responsive to many different odours, half the neurons in the orbitofrontal cortex are responsive only to one odour and the rest to only a few. It has been shown through microelectrode studies that each individual odour gives a particular specific spatial map of excitation in the olfactory bulb. It is possible that through spatial encoding, the brain is able to distinguish specific odours. However, temporal coding must be taken into account. Over time, the spatial maps change, even for one particular odour, and the brain must be able to process these details as well.

In insects smells are sensed by sensilla located on the antenna and first processed by the antennal lobe (analogous to the olfactory bulb), and next by the mushroom bodies.

[edit] Pheromonal olfaction

Some pheromones are detected by the olfactory system, although in many vertebrates pheromones are also 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 face called flehmen to direct air to this organ.

In women, the sense of olfaction is strongest around the time of ovulation, significantly stronger than during other phases of the menstrual cycle and also stronger than the sense in males.^[5]

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.^[6]^[7]i likw to eat pie. jesse McCartney is soooooooooooooooooooooooooooooooooooooo cuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuute

[edit] Olfaction and taste

Olfaction, taste and trigeminal receptors together contribute to flavor. The human tongue can only distinguish among seven to eight distinct types of taste, while the nose can distinguish among hundreds of substances, even in minute quantities. Olfaction amplifies the sense of taste, as can be proven by a simple "kitchen" experiment. If peeled pieces of apple are placed in one bowl, and peeled pieces of potato in another, and then the nostrils are held completely closed while a piece from one bowl is sampled, the taste of apple and potato are indistinguishable.

[edit] Disorders of olfaction

The following are disorders of olfaction:^[8]

Anosmia: Lack of ability to smell
Hyposmia: Decreased ability to smell
Phantosmia: "hallucinated smell", often unpleasant in nature
Dysosmia: Things smell differently than they should

[edit] Quantifying olfaction

Scientists have devised methods for quantifying the intensity of odors, particularly for the purpose of analyzing unpleasant or objectionable odors released by an industrial source into a community. Since the 1800s industrial countries have encountered incidents where proximity of an industrial source or landfill produced adverse reactions to 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.

Many air management districts in the USA 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. Example applications this district has engaged are the San Mateo, California wastewater treatment plant; the Bill Graham ampitheatre, Mountain View, California; and the IT Corporation waste ponds, Martinez, California.

[edit] Olfaction in other animals

The importance and sensitivity of smell varies among different organisms; most mammals have a good sense of smell, whereas most birds do not, excepting the tubenoses (e.g., petrels and albatrosses) and the kiwis. Among mammals it is well developed in the carnivores and ungulates, who must always be aware of each other, and in those, such as the moles, who smell for their food.

Dogs in general have a nose approximately a hundred thousand to a million times more sensitive than a human's. Scenthounds as a group can smell one to ten million times more acutely than a human, and Bloodhounds, who have the keenest sense of smell of any dogs, have noses ten to a 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.

The sense of smell is less developed in the catarrhine primates (Catarrhini), and nonexistent in cetaceans, which compensate with a well-developed sense of taste. In some prosimians, such as the Red-bellied Lemur, scent glands occur atop the head. In many species, olfaction is highly tuned to pheromones; a male silkworm moth, for example, can sense a single molecule of bombykol.

Insects primarily use their antennae for olfaction. Sensory neurons in the antenna generate odor-specific electrical signals called spikes in response to odour. They process these signals from the sensory neurons in the antennal lobe followed by the mushroom bodies and lateral horn of the brain. The antennae have the sensory neurons in the sensilla and they have their axons terminating in the antennal lobes where they synapse with other neurons there in semidelineated (with membrane boundaries) called glomeruli. These antennal lobes have two kinds of neurons, projection neurons (excitatory) and local neurons (inhibitory). The projection neurons send their axon terminals to mushroom body and lateral horn (both of which are part of the protocerebrum of the insects) and local neurons have no axons. Recordings from projection neurons show in some insects strong specialization and discrimination for the odors presented (especially for the projection neurons of the macroglomeruli, a specialized complex of glomeruli responsible for the pheromones detection). Processing beyond this level is not exactly known though some preliminary results are available.

[edit] References

^ Buck, Linda and Richard Axel. (1991). A Novel Multigene Family May Encode Odorant Receptors: A Molecular Basis for Odor Recognition. Cell 65:175-183.
^ Turin, Luca. (1996). A spectroscopic mechanism for primary olfactory reception. Chemical Senses, 21, 773-791.
^ Turin, Luca. (2002). A method for the calculation of odor character from molecular structure. Journal of Theoretical Biology, 216, 367-385.
^ Keller, A and Vosshall, LB. (2004). A psychophysical test of the vibration theory of olfaction. Nature Neuroscience 7:337-338. See also the editorial on p. 315.
^ Navarrete-Palacios E, Hudson R, Reyes-Guerrero G, Guevara-Guzman R. "Lower olfactory threshold during the ovulatory phase of the menstrual cycle." Biol Psychol. 2003 Jul;63(3):269-79. PMID 12853171
^ Boehm T, Zufall F. "MHC peptides and the sensory evaluation of genotype." Trends Neurosci. 2006 Feb;29(2):100-7. PMID 16337283
^ Santos PS, Schinemann JA, Gabardo J, Bicalho Mda G. "New evidence that the MHC influences odor perception in humans: a study with 58 Southern Brazilian students." Horm Behav. 2005 Apr;47(4):384-8. PMID 15777804
^ Hirsch, Alan R. (2003) Life's a Smelling Success