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The Science of Olfaction

by Grant Grayson

We humans can recognize and remember about 10,000 different odors. How we go about this has long been a mystery, that is until the discoveries of Richard Axel and Linda Buck, Nobel Laureates in Physiology or Medicine. In a series of pioneering studies, these two researchers discovered a large gene family, comprised of some 1,000 different genes (about 3% of our total genes) that give rise to an equivalent number of olfactory receptors in our brains.

The Sophisticated Science of Smell

Each olfactory receptor cell possesses only one type of odorant receptor, and each receptor can detect a limited number of odorant substances. This means that our olfactory receptor cells are highly specialized for only a few odors. The cells send tiny nerve impulses directly to distinct micro domains, called glomeruli, in the olfactory bulb, the primary olfactory area of the brain.

Receptor cells carrying the same type of receptor send their nerve processes to the same glomerulus. From these micro domains in the olfactory bulb the information is relayed further to other parts of the brain, where the information from several olfactory receptors is combined, forming a pattern. Therefore, we can consciously experience the smell of a lilac flower in the spring and recall this olfactory memory at other times.

A Good Sense of Smell Makes For A Good Quality of Life

When something tastes good to us, it is the olfactory system which is detecting the qualities we regard as positive. A loaf of homemade bread or a bouquet of roses activates a whole array of odorant receptors, helping us to perceive the different odorant molecules.

A unique odor can trigger distinct memories from our childhood or from a particular emotional moment – whether they’re positive or negative. A bit of fish that is not fresh might cause us to pause and leave a memory that stays with us for years. This in turn might prevent us from ingesting any dish, however delicious, with that type of fish in it. To lose the sense of smell is a serious handicap, in that we no longer perceive the different qualities of food and we cannot detect warning signals, for example smoke from a fire.

Smelling Right Is Important For Survival

All living creatures have the ability to detect and identify chemical substances in their environment. Obviously, this is of great value to survival to be able to identify suitable nourishment and to avoid food gone bad. It’s interesting to note that mice have about 1,000 odorant receptors and fish have about 100. Possibly through a lack of use, humans have a somewhat smaller number than mice.

For infant mammals, the sense of smell is essential in finding the teats of its mother and obtaining milk. Without this sense, newborns would not survive unaided. Olfaction is also of tremendous importance for many adult animals, since they observe and interpret their environments largely by their sense of smell. In dogs, for example, the olfactory epithelium is about forty times larger than in humans.

A Large Number of Odorant Receptors

Axel and Buck showed that three per cent of our genes are used to code for the different odorant receptors on the membrane of the olfactory receptor cells. When an odorant receptor is activated by an odorous substance, an electric signal is triggered in the olfactory receptor cell and sent to the brain via nerve processes. Additionally, Axel and Buck showed that every single olfactory receptor cell expresses one and only one of the odorant receptor genes. Thus, there are as many types of olfactory receptor cells as there are odorant receptors. It was possible to show, by registering the electrical signals coming from single olfactory receptor cells, that each cell does not react only to one odorous substance, but to several related molecules with varying intensity.

Most odors are composed of multiple odorant molecules, and each odorant molecule activates several odorant receptors. This leads to forming an odorant code or pattern that is somewhat like the varying colors in a stained-glass window or in a mosaic – which is the basis for our ability to recognize and form memories of approximately 10,000 different odors.


Buck, L. and Axel, R. (1991) Cell, vol. 65, 175-187.

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