Olfaction

Olfactory perception

Olfaction (the sense of smell)is one of the five senses originally described by Aristotle. It is one of two senses that detect chemicals, the other being taste. Olfaction is the detection of chemicals dissolved in air. The chemicals themselves are called odors or odorants. The sense of smell is also important to the perception of flavor.

How olfaction works

Overview of the process of olfaction

A volatile chemical is either carried in the air to the nose or is taken into the mouth and then diffuses around the palate to the nasal receptors,which are located on the cilia in the nasal mucosa(see diagram). The odorant molecules then interact with the odor receptors. This process of give-and-take,whether it is a lock-and-key type of interaction or vibrational tunneling or some other process is still debated hotly among scientists.

Once the odorant has been bound to the matching receptor(s), a neural signal is produced which travels along the receptor axon through the cribiform bony layer to the glomeruli which lie in the olfactory bulb. From the glomeruli a signal is sent to the mitral cells. These cells produce a signal which is sent down the olfactory nerve as part of the olfactory nerve tract to several brain areas: (1)the limbic region and (2)the thalamus and then to the frontal cortex for recognition.

Receptors

Mammals have about 1,000 genes involved in odor reception (Buck et al. 1991). Of these genes, only a small portion make functional polypeptides involved in odor detection. Humans have 347 functional odor receptor genes; the other genes (pseudogenes) are nonsense mutations. This number may vary among ethnic groups and among individuals. For example, not all people can smell androstenone, a component of male and female sweat.

Human beings can detect about 10,000 different odors. Each odorant molecule must be small enough to be volatile. The maximum molecular weight for an odor-inducing molecule is 294.

Each olfactory receptor neuron in the nose expresses only one type of odor. Odor receptor nerve cells may function like a lock and key system: if any portion of an airborne molecules of a certain chemical (a key) can fit into the lock the nerve cell will be triggered and a specific odor will be perceived. 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 create 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(1996, 2002), 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 found lacking (Keller and Vosshall, 2004).

Receptor neuron

In the process of smelling, the binding of the odor molecule to the receptor leads to an action potential in the receptor neuron via a second messenger pathway depending on the organism. In mammals the odorants stimulate adenyl cyclase to synthesize cyclic AMP (cAMP) via a G protein. Cyclic AMP, which is the second messenger, opens a cyclic nucleotide-gated ion channel ( CNG ) which produces an influx of calcium ions (Ca++) into the cell, slightly depolarising it. These Ca++ in turn open a Ca++ activated chloride channel leading to an efflux of chloride ions (Cl-) and thus further depolarises it and triggers an action potential.

Averaged activity of the receptor neuron to an odor can be measured by an electroolfactogram in vertebrates or an electroantenogram in insects.

In the brain

In vertebrates smells are sensed by olfactory sensory neurons in the olfactory epithelium. Molecules passing through the superior nasal concha of the nasal passages mix with the mucus lining the superior portion of the cavity and are detected by olfactory receptors on the dendrites of the olfactory sensory neurons. Olfactory sensory neurons project axons to the brain within the olfactory nerve, (cranial nerve I). These axons target the olfactory bulb, which in-turn projects olfactory information to the olfactory cortex.

The axons from all the thousands of cells expressing the same olfactory receptor 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 other parts of the olfactory system in the brain where multiple features of the odor may be combined to form a synthesized olfactory perception. Since olfactory receptors can detect many chemical features of an odor molecule, the combination of features gives the olfactory system a broad range of odors that it can detect.

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.

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

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.

Olfaction and taste

Olfaction, taste and trigeminal receptors together contribute to flavor. The human tongue can only distinguish among 7-8 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 proved 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

• 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

(Hirsch, 2003)

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.

Olfaction in 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 sense of smell 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 the bloodhound, which has the keenest sense of smell of any dog, has a nose ten to a hundred million times more sensitive than a human's. It was bred for the specific purpose of tracking human beings, 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.

Schematic of the olfactory system of insects

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 body 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 the mushroom body and the lateral horn (both of which are part of the protocerebrum) 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 pheromone detection). Processing beyond this level is not exactly known ,though some preliminary results are available.

References

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• Buck, L.and R. Axel. 1991. A Novel Multigene Family May Encode Odorant Receptors: A Molecular Basis for Odor Recognition. Cell 65:175-183.
• Chandler Burr. (2003). The Emperor of Scent : A Story of Perfume, Obsession, and the Last Mystery of the Senses. ISBN 0-375-50797-3
• Hirsch, Alan R. (2003) Life's a Smelling Success
• 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.
• Stopfer, M, Jayaraman, V, Laurent, G (2003) Intensity versus Identity Coding in an Olfactory System, Neuron 39, 991-1004.
• Stopfer, M. and Laurent, G. (1999). Short-term memory in olfactory network dynamics, Nature 402, 664-668.

• 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.

See also

• Machine olfaction
• Presbyosmia

External links

• http://www.thenakedscientists.com/HTML/Columnists/peterbrennancolumn.htm Smells and Odours - How Smell Works]
• http://www.cf.ac.uk/biosi/staff/jacob/teaching/sensory/olfact1.html Olfaction]
• http://www.thenakedscientists.com/HTML/Columnists/peterbrennancolumn2.htm The importance of smell, and pheromones, to Humans and other Animals]
• Structure-odor relations: a modern perspective (PDF)
• Olfactory network dynamics and the coding of multidimensional signals (PDF)
• Olfaction (Leffingwell)
• Chirality & Odour Perception
• ScienceDaily Artille 08/03/2006, Quick — What's That Smell? Time Needed To Identify Odors Reveals Much About Olfaction

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