How does your nose turn a molecule into a smell?

Your nose smells by catching molecules. Whatever gives off a scent, a rose, coffee, a wet dog, releases tiny molecules into the air, and you breathe them in. At the top of your nasal cavity they dissolve in a layer of mucus, where proteins called olfactory receptors grab them. When a receptor is activated, its nerve cell fires and sends an impulse to the brain. The brain turns that signal into what you experience as a smell.

For a long time nobody knew what those receptors were. Linda Buck was a scientist who joined Richard Axel's laboratory at Columbia University in 1982, and she set out to trace odours through the cells of the nose to the brain at the molecular level. In 1991 she and Axel announced something remarkable: working with rat genes, they had found a family of more than a thousand genes that code for odour receptors. Each receptor is a protein that changes shape when the right molecule attaches to it, which is what sends the electrical signal. Their discovery earned them the 2004 Nobel Prize in Physiology or Medicine.

Humans have far fewer of these genes than rats and mice, around 400 against 1400 in mice, and yet we can recognise a huge range of smells. The trick is that a receptor is not a lock that fits one key. A single odour molecule can fit several receptors, each to a different degree, and a single receptor responds to many molecules. So a smell is not one signal but a pattern: a particular combination of receptors switching on, like a chord on a piano rather than a single note.