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People can smell thousands—perhaps even millions—of different scents. Yet scientists know that in the nose, there are only about 400 different types of odor receptors—proteins that capture scented molecules so that smells can be identified. Thus, there isn’t, obviously, one type of receptor that responds to a rose, while another jumps for jasmine.
So how can we smell so much, with so few types of receptors?
The answer is that cells mix and match. Each nerve cell in the nose can sense more than one odor, but picks up the smell to a different degree. An odor's unique signature depends on which cells respond to it, and how intensely.
What happens when you inhale a rose is that a group of cells is stimulated, and that group sends a combination of signals to the olfactory bulb—the site at the very front of the brain where smell perception takes place. This unique combination of signals tells the brain the odor is the smell of a rose.
How the olfactory bulb interprets this signal as the smell of a rose is not well understood, but scientists do have some insights into the process. For example, many researchers have used brain scans to see what areas of the brain become active after people are exposed to different smells. It is unclear, however, whether that activation corresponds to recognizing a given smell. And other factors—for example our past experiences with certain smells, and whether we liked them or were turned off—also play a role in how we perceive them.
In other words, a rose is a rose is a rose, as Gertrude Stein wrote, but not always to our nose.