Dear Matthew:

I can understand how the odor molecules get to our odor sensors, and I can understand that these sensors send the signals directly to our brain. But I cannot understand what are the processes inside the sensors themselves. Do they distinguish the size of molecules, or do they feel their forms, or perhaps there are some specified chemical reactions in these sensors giving the signals? It is more or less clear how we can hear, a little less clear how we can see, but how do we feel the odor?

Yuri Balega, astronomer from Russia

Yeah, we've figured out how to feel the burn, but we're not so clear on feel the stink. But there's hope. Since two American scientists made a Nobel-winning smell breakthrough back in the '90s, the bio-chem guys have been up our noses with a vengeance. Actually, up rats' noses mostly, since they're a popular lab model for the human schnozz and rats are cham-peen smellers, after dogs. But there are even scientists who managed to coax out grant money to study smell ability in fruit fly antennae. Fruit flies have big old genes, easy to see, but don�t ask me where that work is going.

The Nobel-worthy discovery was a set of genes dedicated to producing individual smell sensors in noses. We have about a thousand of them. Each sensor is different from the other 999 and is configured to detect a very narrow and chemically similar range of odor molecules. Oh, I probably should say that what we call the smell of cow pies is actually formed by a unique combination of basic chemical molecules that produces "Aha! Cow pies!" when recombined in our brains. There's no single cow-pie-smell gene or single cow pie molecule.

So we snort in a good, rich stink, the molecules are grabbed by mucus in the smell-detection area at the backs of our noses, and they lock into the appropriate sensors. Unfortunately, there's your question, and though they're toiling away like little beavers, the science guys don't yet have a solid answer. But it looks like sensors recognize the molecules by their size, shape, and the physical organization of their surfaces.

Anyway, the connection between the sensor and the odor molecule changes the chemistry of the sensor, which generates a signal (unique to each sensor) that passes through a series of smell-dedicated cells. The many signals that make up "cow pie" hit our brains, our brains recognize the "cow pie" pattern, and we look down to see what we've stepped in. The whole system, from nose to brain, is a physical connection of cells. And of course the whole system is much more complicated than I've made it appear.

From our thousand receptors, the science guys figure there are enough signal combinations to allow us to detect about 10,000 odors. Actually, only 500 or 600 of our sensors may actually still work. In the animal world humans are smell wimps, and we've lost some detection abilities through the millennia. We don't have to rely as much on smell these days to keep ourselves safe or find food or mates. (But the love connection thing does have a smell component, even though we are not consciously aware of it.)

Back in the real world, there are big bucks in smells. The emotional and behavioral associations we've have with odors are strong motivators. Get a scent just right and it could make your product a winner, even if it's not a food or a perfume. And personally my nose seems okay, but this whole thing has given me a headache.