'Olfaction' (also known as 'olfactics') is the
sense of smell driven by the detection of volatile or, in the case of the
accessory olfactory system, fluid-phase chemicals. This, along with
taste, is a form of
chemoreception. The chemicals themselves, generally at very low concentrations, are called
odors.
History
As discovered by
Linda B. Buck and
Richard Axel (who were awarded the
Nobel Prize in 2004),
mammals have about a thousand
genes expressing for
odor reception.
[1] Of these genes, only a portion are functional odor receptors. Humans have far fewer active odor receptor
genes than other mammals and primates
[2]
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
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)
[3]. An alternative theory, the
vibration theory proposed by
Luca Turin[4][5], 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.
[6] As of yet, there is no theory that explains olfactory perception completely.
Olfactory System
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 sixteen 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 odorants 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).
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 Ca
2+ in turn opens a Ca
2+ activated
chloride channel leading to efflux of
Cl-, further depolarising the cell and triggering an
action potential. Ca
2+ 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.
Olfactory Bulb Projections
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 odor. 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 odor. It also projects to the hippocampus and is involved in motivation and memory. Odor 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 odors. 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 odors, half the neurons in the orbitofrontal cortex are responsive only to one odor and the rest to only a few. It has been shown through microelectrode studies that each individual odor 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 odors. However, temporal coding must 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.
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.
Pheromonal olfaction
Many animals, including most mammals and reptiles, have two distinct and segregated olfactory systems: a main olfactory system, which detects volatile stimuli, and an
accessory olfactory system, which detects fluid-phase stimuli. 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 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.
[7]
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.
[8][9]
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.
Disorders of olfaction
The following are disorders of olfaction:
[10]
★
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
★
Hyperosmia - an abnormally acute sense of smell
Quantifying Olfaction in Industry
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
Shoreline Amphitheatre in
Mountain View, California; and the
IT Corporation waste ponds,
Martinez, California.
Olfaction in non-human 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 odor. 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.
References
1. Buck, Linda and Richard Axel. (1991). A Novel Multigene Family May Encode Odorant Receptors: A Molecular Basis for Odor Recognition. ''Cell'' 65:175-183.
2. Gilad Y, Man O, Pääbo S, Lancet D (2003) Human specific loss of olfactory receptor genes. Proc Natl Acad Sci U S A 100:3324–3327.
3. need citation!
4. Turin, Luca. (1996). A spectroscopic mechanism for primary olfactory reception. Chemical Senses, 21, 773-791.
5. Turin, Luca. (2002). A method for the calculation of odor character from molecular structure. Journal of Theoretical Biology, 216, 367-385.
6. 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.
7. 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
8. Boehm T, Zufall F. "MHC peptides and the sensory evaluation of genotype." ''Trends Neurosci.'' 2006 Feb;29(2):100-7. PMID 16337283
9. 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.
10. Hirsch, Alan R. (2003) Life's a Smelling Success
See also
★
Accessory olfactory system
★
Electronic nose
★
Machine olfaction
★
Olfactory fatigue
★
Limbic system
External links
★
Smells and Odours - How Smell Works at thenakedscientists.com
★
Olfaction at cf.ac.uk
★
The importance of smell, and pheromones, to Humans and other Animals at thenakedscientists.com
★
Structure-odor relations: a modern perspective at flexitral.com (PDF)
★
Olfactory network dynamics and the coding of multidimensional signals at caltech.edu (PDF)
★
Olfaction at leffingwell.com
★
Chirality & Odour Perception at leffingwell.com
★
ScienceDaily Artille 08/03/2006, Quick -- What's That Smell? Time Needed To Identify Odors Reveals Much About Olfaction at sciencedaily.com
★
Scents and Emotions Linked by Learning, Brown Study Shows at brown.edu.com
★
Sense of Smell Institute at senseofsmell.org. Research arm of international fragrance industry's
The Fragrance Foundation
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