How olfaction works

Receptors

As discovered by Linda B. Buck and Richard Axel (who were awarded the Nobel Prize in 2004), mammals have about 1000 genes for odor reception. 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 ethnic groups, and does vary among individuals. For example, not all people can smell androstenone, a component of male and female sweat.

Receptor neuron

The process of how the binding of the ligand (odor molecule) to the receptor leads to an action potential in the receptor neuron is via second messenger pathway depending on the organism. In mammals the odorants stimulate Adenyl cyclase to synthesize cAMP via a G protein. cAMP which is the second messenger here opens a Cyclic nucleotide-gated ion channel (CNG) which produces an influx of Ca++ in to the cell slightly depolarising it. These Ca++ inturn opens a Ca++ activated chloride channel leading to efflux of Cl- and thus further depolarises it and triggers an action potential.

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.

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 do 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.

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.

• Buck, Linda and Richard Axel. (1991). A Novel Multigene Family May Encode Odorant Receptors: A Molecular Basis for Odor Recognition. Cell 65:175-183.
• 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.
• 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.
• 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.
• Chandler Burr. (2003). The Emperor of Scent : A Story of Perfume, Obsession, and the Last Mystery of the Senses. ISBN 0-375-50797-3