A Direct Sense
Smell is the most important sense for most organisms. A wide variety of species use their sense of smell to locate prey, navigate, recognize and perhaps communicate with kin, and mark territory. Perhaps because the task of olfaction is so similar between species, in a broad sense the workings of smell in animals as different as mammals, reptiles, fish, and even insects are remarkably similar.
The sense of smell differs from most other senses in its directness: we actually smell microscopic bits of a substance that have evaporated and made their way to the olfactory epithelium, a section of the mucus membrane in the roof of the olfactory cavity. The olfactory epithelium contains the smell-sensitive endings of the olfactory nerve cells, also known as the olfactory epithelial cells. These cells detect odors through receptor proteins on the cell surface that bind to odor-carrying molecules. A specific odorant docks with an olfactory receptor protein in much the same way as a key fits in a lock; this in turn excites the nerve cell, causing it to send a signal to the brain. This is known as the stereospecific theory of smell.
In the past few years molecular scientists have cloned the genes for the human olfactory receptor proteins. Although there are perhaps tens of thousands (or more) of odor-carrying molecules in the world, there are only hundreds, or at most about 1,000 kinds of specific receptors in any species of animal. Because of this, scientists do not believe that each receptor recognizes a unique odorant; rather, similar odorants can all bind to the same receptor. In other words, a few loose-fitting odorant "keys" of broadly similar shape can turn the same receptor "lock." Researchers do not know how many specific receptor proteins each olfactory nerve cell carries, but recent work suggests that the cells specialize just as the receptors do, and any one olfactory nerve cell has only one or a few receptors rather than many.
It is the combined pattern of receptors that are tweaked by an odorant that allow the brain to identify it, much as yellow and red light together are interpreted by the brain as orange. (In fact, just as people can be color-blind to red or green, they can be "odor-blind" to certain simple molecules because they lack the receptor for that molecule.) In addition, real objects that we smell produce multiple odor-carrying molecules, so that the brain must analyze a complex mixture of odorants to recognize a smell.
Just as the sense of smell is direct in detecting fragments of the objects, it is also direct in the way the signal is transmitted to the brain. In most senses, such as vision, this task is accomplished in several steps: a receptor cell detects light and passes the signal to a nerve cell, which passes it on to another nerve cell in the central nervous system, which then relays it to the visual center of the brain. But in olfaction, all these jobs are performed by the olfactory nerve cell: in a very real sense, the olfactory epithelium is a direct outgrowth of the brain.
The olfactory nerve cell takes the scent message directly to the nerve cells of the olfactory bulb of the brain (or, in insects and other invertebrates that lack true brains, the olfactory ganglia), where multiple signals from different olfactory cells with different odor sensitivities are organized and processed. In higher species the signal then goes to the brain's olfactory cortex, where higher functions such as memory and emotion are coordinated with the sense of smell.