And a shift in concentration can turn a scent from pleasant to disgusting. Take octanol, for example. Change one atom in the molecule's structure, and it becomes octanoic acid, which is characterized by a rancid, sweaty smell. When concentrated, indole, a substance found in both coal tar and perfumes, just plain stinks. When sufficiently diluted, indole gives off a fragrance like jasmine.
Odors waft up the nasal cavity to a patch of nerve cells above the eyes.
Click here for a high-res tiff image. Odorants are vaporized from gasoline, baking bread, or perfume, and waft up the nasal cavity, where they contact nerve cells. Each cell extends thin hairs, or cilia, and the odorant receptors sit on those hairs. When an odorant binds with receptors, the cell sends a signal to the brain, specifically to the olfactory bulb above the eyes. In the nose, cells that use the same receptor lie scattered over the wall of the nasal cavity.