How Does Our Sense of Smell Work?

Whether for finding food, avoiding predators or choosing a mate, the sense of smell is critical for the existence of almost all creatures. Humans are able to distinguish over 10,000 different odor molecules, utilizing our sense of smell for a multitude of activities from enjoying the aroma of freshly brewed coffee to deciding whom not to sit next to on the bus.
 
In the last 20 years, scientists have made great advances to our understanding of how our nose detects odor molecules and our brain processes the resulting information that gives rise to the sensation of smell.
 
Every time we inhale, currents of air swirl up through the nostrils, over the bony turbinates, to a “sheet” about the size of a small postage stamp that contains millions of olfactory receptor neurons. This is the olfactory epithelium.

Each of the millions of olfactory neurons has minuscule filaments (cilia) extending from its knob. This knob is located at the tip of the olfactory neuron and the cilia project from the knob directly into the atmosphere. This is the only part of the brain that projects into the atmosphere. The cilia contain olfactory receptors, specialized proteins that bind low molecular weight molecules (odorants).

​One of the big breakthroughs of the past 15 years was the discovery by L. Buck and R. Axel of a large multi-gene family that encode for these olfactory receptors. Each receptor has a pocket (binding site) that is just the right shape to bind either a specific molecule or a group of structurally similar molecules. The interaction of the right molecule with the right receptor causes the receptor to change its shape (structural conformation). This conformational change gives rise to an electrical signal that goes first to the olfactory bulb and then to the areas of the brain that convert the electrical signal to a smell.
 
In 1996, Peter Mombaerts found that olfactory neurons containing the same olfactory receptor, while randomly scattered within one of four spatial zones of the olfactory epithelium, project to only two specific areas (glomeruli) in the olfactory bulb. These findings suggest that the bulb transfers information that is broadly distributed in the olfactory epithelium into a highly organized information map that is in essence a map of the information provided by the different olfactory receptors.
 
Systematic studies have shown that different odorants are represented by distinct spatial activity patterns in the glomerular layer of the olfactory bulb. These results in turn suggest a combinatorial mechanism for olfactory coding wherein the responses of olfactory receptors to odorants produce spatial patterns of olfactory bulb activity that are characteristic for a given odorant or blend of odorants, e.g. a perfume.
Thus, it appears that these spatial patterns of activity create the information that leads to recognition of odor quality and intensity and discrimination between odors. This information is processed at higher levels of the olfactory system and in the brain giving rise to the perception of smell.