Smell

Smell is the chemical perception of odorants, molecules that float in the air in the gas phase. The structure that makes it possible is the olfactory nerve, which is the first cranial nerve, the first of the nerves that come directly off the brain.

The mechanism starts like a lock being opened. The odor molecule binds to a G protein coupled receptor on the surface of an olfactory sensory neuron, one of those nerve cells that poke through the roof of the nose. That binding sets off a cascade of reactions inside the cell that produces cyclic AMP, an internal messenger. Cyclic AMP opens ion channels and the neuron depolarizes, that is, its voltage changes.

That depolarization triggers an action potential, the electrical impulse that nerves use to talk to each other. The impulse travels along the olfactory nerves, crosses the cribriform plate, a bone perforated like a sieve that separates the nose from the brain, and reaches the glomeruli of the olfactory bulb, clusters of connections that pass the signal along to the brain areas that interpret it.