The end result is combinational, with the brain distinguishing scents based on the unique sequence of signals it receives for different compounds.
Whether it’s the orange you’re peeling, the french bread cooking in the oven, a flowering tree you just walked by, or the smelly socks in your hamper, there are compounds residing on the surface of it. Um yeah … duh. Everything is made up of atoms and compounds. So why do we care about these objects and these compounds specifically?
The compounds of these objects have a vapor pressure that enables them to lift off the surface of the object and become suspended in the air. Once suspended, they can be sucked up into your nose. Some lucky compounds will travel all the way to the back of the nose. Once there, they will encounter the olfactory epithelium. A mucus lines the olfactory epithelium and this mucus catches and traps the compounds. We will refer to these compounds as odorants from now on. The odorants dissipate through the mucus. At the end of their journey, they meet olfactory receptors, which bud off of olfactory neurons. The olfactory receptors are simply proteins on the surface of neuron cells and are not independent entities. The human genome contains 800 genes that encode for olfactory receptors, but only about 350 of those genes are actually expressed. As such, there are about 350 unique olfactory receptors each with almost a million copies that reside in the far reaches of the nasal cavity.
Here's where things really get interesting. Odorant molecules will only bind to certain olfactory receptors, constrained by protein-ligand binding principles. To learn more about protein-ligand binding, jaunt on over here! After the odorants are bound to olfactory receptors, a signaling cascade is activated within the neurons. The chemical reactions that ensue within the cells create an electrochemical gradient which triggers an action potential. The action potential travels along the olfactory neurons which ultimately connect to “second-order” neurons residing in the olfactory bulb. The signal then gets sent down the olfactory tract to end up in the primary olfactory cortex (POC). The POC connects to various centers of the brain. The connections allow for the scent to be quickly interpreted. But how is one smell being distinguished from another?
Just as there are millions of olfactory receptors, there are millions of neurons connected to olfactory neurons. The distinct combination of these neurons that are activated by specific binding events are what permit the POC and the rest of the brain to differentiate various odors. The way the brain is able to distinguish between even extremely similar compounds and scents is fascinating. The system is perhaps analogous to a computer cache. In the same way that a frequently visited file will be recognized and accessed more quickly by the computer, a combination of signals sent to the brain by olfactory neurons that has been encountered before will be more easily recognizable to you.
Not every compound has an odor that is detectable in humans, and not every odorant is detectable in all humans. The odors we can detect can elicit responses within us that go beyond just acknowledging a smell exists. The POC’s connection to other parts of the brain are what make us feel nostalgic when we smell scents associated with memories: your family’s cooking, the elementary school gymnasium floor, your best friend’s shampoo, freshly mown grass. Odorants are not your run of the mill compounds thanks to the complex olfactory system within us.