How Do Humans Distinguish The Tens Of Thousands Of Odors Which Assault Them?

By Author: tearsjoong
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Eight years ago, Linda Buck and Richard Axel, then at Columbia University, explained part of the mystery by describing a class of proteins, called olfactory (nH^itftj) receptors, which are the body's front line odour detectors. Now Dr Buck and her colleagues at Harvard Medical School have gone on to explain how these receptors act together to distinguish different scents.

Olfactory receptors are found on millions of nerve cells lining the nose. Dr Buck's earlier research with mice showed that they have roughly 1000 different kinds of these receptors, but that each olfactory nerve cell has just a single kind on its surface. (Human noses are similar.) Receptors are thought to sense scents by binding to particular atomic structures on specific odorants—small yet smelly carbon-containing molecules.

But how can a thousand receptors, each dedicated to recognizing only a single structure, distinguish many thousands of different odors? To uncover such patterns, Dr Buck, Bettina Malnic and their colleagues at the Life Electronic Research Center in Amagasaki, Japan , wafted 30 different odorants over 600-old olfactory ...
... nerve cells
taken from the noses of mice. The cells contained a special sort of dye to indicate when a receptor had been triggered. Each respond-ing cell had its RNA analyzed to identify which of the thousand or so olfactory proteins is produced, enabling the researchers to work out which receptors had been triggered by which odorants.1

The Harvard team found that a simple odor molecule, like non-anol (3r!S￡) triggered not one but five different olfactory receptors, while structurally similar—but more pungent — relative, the cheesy-smell nonanoic acid, activated the same five receptors plus three extra or ones. This shows that a single odorant can activate more than one receptor. Furthermore, the researchers found that some receptors were triggered by high concentrations of odorant but not at lower levels, which may explain why odorous molecules can have very different smells at different doses.2

So it would seem to be the combination of receptors, recognizing different bits of various odor molecules, that enables humans distinguish roses from goats, at least at nose level.3 But how the brain interprets olfactory signals and distinguishes bad smells from sweet ones is still unknown. Dr Buck and other researchers are trying to piece together the path of neural connections from the olfactory receptors to the areas of the brain involved in emotion, memory and other high functions.4