Along with taste
, the sense of smell
is one of the two "chemical senses" of the human body, gathering information information from the chemicals present in a given environment
. It is also the much more powerful of the two: in one study a strong taste became perceptible at 1 * 10-6 mole/liter
, whereas a known smell needed only 7 * 10-13 mole
). Olfaction is also the sense that works over the longest distance, and in evolution can function as an early warning
for (and thus way to seek out or avoid) food
s, and potential mate
For a chemical to have a scent, it must be compatible with the olfactory system, be able to be absorbed by it and match receptors in it. Generally, this means it must have some degree of both water and fat solubility, and must be volatile enough to evaporate from a liquid state. Generally speaking, any molecule fitting this description will be compatible with the nerve terminal sensors, and thus have its own distinct scent.
Air inhaled through the nose is carried past the olfactory epithelium, in the shared roof of the nasal cavities just below and posterior to the eyes. This area is roughly 2.5 square centimeters, and contains roughly 50 million sensory receptor cells, all of which are regenerated every 40 days. These cells project ten to twenty cilia one direction into a mucous barrier generated by the Bowman's glands, and axons the other direction through a bone wall into the olfactory bulb of the brain. It is the cells of the epithelium which produce new sensory neurons by mitotic division, and also contain a pigment that varies based on the animal's olfactory sensitivity. In the olfactory bulb, the axons synapse with olfactory nerves (via nervous bundles called glomeruli) at a ratio of roughly 1000:1. This concentrates the signal, which will be sent on to the brain's limbic system for emotional and other subconscious processing, and to the thalamus which will relay it to the cerebral cortex for conscious processing.
There has been a pretty remarkable amount of arguing in biology about exactly how receptors which must be constructed by a discrete section of DNA can generate the practically infinite variety of scents perceived in different chemicals. Each possibility could easily warrant its own five paragraphs of explanation, so I'll stick to an outline of the latest and most plausible one here. It seems that there is an "alphabet" of possibilities which combine together in different patterns when presented with different chemicals, and thus give a unique overall response for each one. In other words, olfaction seems to be a combinatoric property, again much like the sense of taste. Just as food for thought, there would only need to be eight unique receptors to give 8! = 40320 possible unique scents.