23.10: Smell: receptors in the nose detect airborne chemicals.

Our sense of smell works in almost exactly the same way as our sense of taste. Neurons that can detect smells have dendrites modified with tiny, hair-like projections. These dendrites—densely packed within the nasal cavity—are covered with chemoreceptors. Airborne chemicals move through mucus in the nasal cavity and bind to the smell receptors, triggering action potentials that shoot down the axon, all the way into the smell center of the brain, where the signal is perceived as a particular odor. More than a thousand different types of receptors—modified neurons—are present in a human nose, each capable of detecting a different scent (FIGURE 23-20). (These are the only neurons in the body that are in direct contact with the outside world. As a result, they’re frequently damaged and so are constantly being made and replaced.)

In humans, the senses of smell and taste are closely connected, because the air in the mouth, throat, and nasal passages circulates around all these areas. You may have noticed that when you have a cold and your nose is stuffed up, you taste little beyond the basic salt, sweet, bitter, and sour tastes on your tongue, and you can’t smell anything at all. Your senses of taste and smell are dulled because increased mucus in your nasal passages reduces the rate at which airborne chemicals can reach the smell receptors and taste receptors on dendrites in your nose and on your tongue.

Animals vary greatly in their sensitivity to smells, and humans don’t fare too well in the competition. Recent evidence even suggests that fewer and fewer of the genes coding for the different smell receptors in humans function properly any more. Interestingly, human females are significantly better than males at detecting, distinguishing, and identifying odors, a sensitivity that is even greater during the days surrounding ovulation.

Dogs are among the most smell-sensitive vertebrates, having as many as 40 times more smell receptors than a human—hence their tremendous proficiency at detecting drugs or explosives (FIGURE 23-21). Some moths, such as gypsy moths and silkworm moths, put even dogs to shame and may be the champions of chemoreception. The tiny hairs on the males’ antennae can detect just a few molecules of the sex attractant released by females. By moving in the direction of increasing concentration of the airborne molecules, a male can track down a female two miles away. Snakes, as they stick their tongue out, are actually catching the odor molecules that enable them to “smell” their environment.