Jude Stewart writes:

If all our genius lies in our nostrils, as Nietzsche remarked, the nose is an untrained genius, brilliant but erratic. The human nose can detect a dizzying array of smells, with a theoretical upper limit of one trillion smells—yet many of us are incapable of describing these smells in words more precise than smelly and fragrant.

Our auditory and visual receptors offer little mystery—they were mapped and explained by scientists many decades ago—but human olfactory receptors were discovered only in 1991. This might be an indication of the massive complexity of smelling: the human body has only four visual receptors, compared with more than four hundred olfactory receptors. Or perhaps it’s a matter of cultural priority: smells are so often thought of as unwanted intrusions.

Smell begins when odorous molecules—often called “odorants”—are whisked through the air into the nose. The air bumps through the nasal passages, where it is warmed and filtered, and arrives at the olfactory epithelium, a mucous-lined layer of the nasal cavity where olfactory neurons nestle like carrots in earth. These neurons detect the smells, but it’s the olfactory receptor proteins that actually bind to odorants. These receptors, in turn, fire an electrical signal to the olfactory bulbs, two buds that hang from a bundle of nerves connecting to the brain, and which are located at the bridge of the nose, roughly where your eyeglasses would rest.

The olfactory bulbs are thought to be the brain’s primary processing center for smell. The bulbs take in information from the olfactory receptors, encode it into a unique odor signal, and then pass this signal to the olfactory center in the brain’s cortex. Olfactory neurons regenerate every four to eight weeks; over time, they respond to whatever smells they encounter most often. That means you can train your nose to smell more effectively just by practicing.

How a receptor detects a smell remains a deep riddle to scientists. The odorant’s shape appears to determine which olfactory receptors it binds to; beyond that, we have no idea why molecules smell as they do. [Continue reading…]