How Does our Olfactory system work? How do we Smell? It turns out that quantum mechanics plays a big role. What you may not realize is that inside your nose rests a very sensitive quantum device that uses complex physics to give you the ability to distinguish, according to the latest estimates, 1 trillion different smells.
In order for you to smell anything, molecules from that thing have to make it to your nose. Everything you smell is volatile is some way, meaning it is giving off molecules that float into the air and land inside your nose.
But how do these molecules trigger the sense of smell? At the top of your nasal passages behind your nose, there is a patch of special neurons about the size of a postage stamp called the olfactory epithelium. They have hair-like projections called cilia that increase their surface area. Any molecule from anything with a smell binds to special smell receptors on these cilia and triggers the neuron causing a signal to your brain. This signal ends up in a primitive part of the brain called the limbic system which is associated with emotions and memory – so that is why smells can trigger strong memories and emotional reactions.
These neurons are unique they come directly from your brain, and are out in the open where they can come into contact with the air. So this is the only place on your body where your central nervous system is directly exposed to the environment.
Now the questions is how do the neurons get triggered? The standard explanation going back to the 1950’s had been that the receptors at the ends of these olfactory neurons can only accept particular shapes of molecules. There are 400 such smell receptors. And the exact type of smell is determined by how the molecules from the smelly compounds fit in the set of receptors of the receiving olfactory neurons. By triggering a particular combination of the 400 smell receptors, the brain interprets a particular kind of smell. It’s like a lock and key. The key is the shape of molecules of the smelly compounds. And the lock is the olfactory receptors. One combination can trigger the smell of a rose, another combination can trigger the smell of rotten eggs and so on.
This was the standard explanation for the mechanism of smell, until researchers discovered that you can have molecules of different shapes that have the same smell. So for example cyanide smells the same as benzaldehyde, a bitter almond smell. But they are vastly different shapes. So scientists concluded that there must be something more than just the shapes that determines smell. Well it turns out that although their shapes are different, cyanide and benzaldehyde have the same vibration. And new research suggests that it is not just the shapes of the molecules, but also the vibrations of these molecules that determines how we perceive the smell.
All molecules vibrate with a certain frequency, and tempo, based on their structure, bonds and weight. Its analogous to sounds coming from various instruments due the their various shapes. Luca Turin, a biophysicist at the Alexander Fleming research center in Greece conducted experiments using the the smell of sulfur compounds. What he found was that these compounds with the same vibrational frequency of sulfur do indeed smell like sulfur, even though their molecules are shaped completely differently.
In quantum mechanics, so-called particles like electrons are really waves of probabilities until the moment they are measured. The probability wave is such that when the particle encounters a barrier, the probability wave does not stop at the barrier, but continues through for a short distance. So there is non-zero probability that the electron will go straight through the barrier and show up on the other side of a barrier, or tunnel through this barrier. This is called quantum tunneling.
It is theorized that the vibrations of certain molecules might allow electrons from the receptors to tunnel through to other receptors, and trigger signals in the neurons, and to the brain. Different molecules with different vibrations can cause different rates of tunneling. When the molecule of the odor has a certain frequency that matches the energy of the receptor, it opens a gateway for an electron to tunnel more preferentially than when the molecule is not present.
In a way, you might say our noses not only smell shapes, but hear shapes too using the complex physics of quantum mechanics to do it. How could our noses have evolved such a sophisticated sensory mechanism? Turin has an interesting answer, he says “Four billion years of R&D with unlimited funding is a long time” – indeed evolution is a process we have probably greatly underestimated.
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