Smell is a complex sense
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Two US scientists have been awarded the Nobel Prize for Medicine for uncovering the secrets of the human sense of smell.
The way the brain recognises and remembers thousands of different odours has long baffled scientists.
Professor Richard Axel, of Columbia University, and Professor Linda Buck, of Fred Hutchinson Cancer Research Center in Seattle, cracked the problem.
The Nobel Prize, the most prestigious in medicine, is worth $1.3m.
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Until Axel and Buck's studies the sense of smell was a mystery.
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The scientists discovered a large gene family, made up of 1,000 different genes that control production of specialised protein receptors.
These receptors are found on cells which line a small area of the upper part of the nose and detect odour molecules when they are breathed in.
However, each cell possesses only one type of receptor, and each receptor can detect only a limited number of substances. Therefore each cell is highly specialised for a few odours.
The cells each send signals along tiny strands of nerve tissue directly into the area of the brain that controls the sense of smell - the olfactory bulb. However, each type of cell connects to a different area - or glomerulus - within this tissue.
From here the information is relayed to other parts of the brain, where the information from several olfactory receptors is combined, forming a pattern which is recognised as a distinct odour.
Professor Axel and Buck first published a joint paper identifying the key genes in 1991.
Since then they have worked on a number of studies to pin down the organisation of the olfactory system from molecular to cellular level.
Professor Buck, during a lecture to the Karolinksa Institute, which decides who should get the Nobel Prize, said it was thought that humans could differentiate between up to 10,000 different odours.
She said: "The discriminatory power of the olfactory system is immense. Even closely related molecules have different smells."
Well deserved
Professor Sten Grillner, one of the panel of experts who judged the prize, said: "Until Axel and Buck's studies the sense of smell was a mystery."
Dr Peter Brennan, an expert in smell and behaviour at the University of Cambridge, UK, said: "The discovery of this large family of genes has revolutionised our understanding of this major sense.
"Although this work is not directly related to any major human diseases, it has opened new windows on the way the brain interprets the world around us and how this effects behaviour.
"Smell is different from the other senses in that the sensory cells are continually dying and being replaced by new cells that have to be wired up correctly in the brain.
"Their work, and that of their co-workers, has increased our knowledge of how the complex patterns of connections in the brain are formed during development."