December 8 – Smell of success

Today’s factismal: The chemical responsible for a smell is called an oderant; the sensation of smelling that chemical is called an odor.

As we’ve seen before, scientists can be pretty finicky about their language. But the language of the senses is particularly tricky, and no part of it is more exacting than the description of how our sense of smell works. Part of that is because our sense of smell is particularly acute; where we can describe most tastes using just five different descriptors, more than 140 different distinct smells have been defined. And part of that is because the process of detecting a new oderant is particularly complex.

So how do we detect these chemical signals? It all starts with the oderants. These chemical bundles are small enough to be carried along on air currents and volatile enough to be thrown off of the smelly thing in large enough numbers to be detected. Once those puppies are in the air, they diffuse out; to get an idea of how this works, plop one drop of food coloring into a big bowl of water and watch. The smell of that nice new scented candle you just bought fills the room the same way that the food coloring slowly fills the bowl of water.

Once you have oderants in the air, you need to have something to detect them; for people, that’s a nose. More specifically, it is the olfactory epithelium (“smelling {skin}”) that lies within our noses. The nose starts by filtering out dust and bring the temperature of the incoming air to within one degree of the body temperature. The air is then passed over two turbinates (one on each side) that cause the air to swirl about so that the oderants in it are given every opportunity to get stuck in mucus. That’s right – that icky stuff that creates runny noses is also the stuff that is responsible for our ability to smell!

The oderants are trapped within the mucus, but it also traps dust and germs. Fortunately, the mucus contains antibodies that kill off the germs; if this weren’t the case, then infections could spread from our noses directly into our brains. (This is why doctors frown on nose piercing; it can provide a channel for infection that skips the mucus.) The oderants filter through the mucus and are finally presented to the 1.6 square inches of olfactory epithelium that a human uses to smell. (Don’t be too smug about the size of your olfactory epithelium; a dog has one that is about seventeen times larger and that has 100 times the nerve density that you do.)

As you might guess, with a system this complex, there are a lot of things that can go wrong. Rapid changes in temperature can cause the blood vessels in the turbinate to change size, creating “brain freeze”. Nerve cells die off with old age, leading to a general decrease in sensitivity. Exposure to cigarette smoke and other pollutants can kill off nerve cells, again causing loss of the sense of smell. And people who suffer from brain cancer or other diseases frequently experience changes in their sense of smell. And that last is something that medical researchers desperately want to know more about. At the Smell Experience Project, researchers are asking for stories from people who have had a change in their sense of smell. The stories will be used in medical publications and teaching. If you’d like to participate, then go to:
http://psych-institute.med.nyu.edu/research/submit-story

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