Olfactory perception: the mechanisms of the sense of smell

olfactory perception is a well-established mechanism that allows our nose and our brain to work together

Since COVID19, we realized how precious sense of smell was. It is therefore important to understand the well-oiled mechanism that operates between our nose and our brain. From inspiration to scented memories, how does olfactory perception take place? How can we identify and differentiate between a multitude of smells, yet find it difficult to describe them in simple words? Let’s discover the journey of odors, from detection to perception.

But in fact, what is a smell?

As human beings, we are able to identify and distinguish several thousand odors. So before we understand how olfaction works, let’s take a look at what a smell is. In fact, almost all smells are a mixture of hundreds of different odorous molecules. The warm and comforting smell of coffee that tickles your nose in the morning is actually made up of about 800 different olfactory substances! These volatile molecules are released into the air and finally reach our nostrils. Our brain is not able to identify precisely all the molecules that enter the nose. It only needs a few compounds to detect a specific smell. For example, the odorant molecule ethylvaniline smells like vanilla and isoamyl acetate smells like banana. Each of us will perceive a different number of odors depending on our abilities and the training of our nose (find some exercises to become a champion of olfaction).

The three-beat waltz of olfactory perception

From detection to perception and “registration” in our brain, odor follows a precise path through several stages in different areas. This process is guided by a brain system, present in all mammals: the olfactory system.

The meeting of smells

Before getting to the heart of the matter, and in the back of our nose, the very first step is in the air. Thus, when we breathe in, several hundred odorous molecules that were floating in the air enter our nose. At the origin of all olfactory perception, there is a molecule volatile enough to be carried by the air and hydrophilic enough to be absorbed by the mucus of our nasal cavity. The molecules therefore travel to the bottom of our nostrils, in our nasal cavity, to bathe for a few moments in the mucus, the liquid that flows generously from our nose when we have a cold!

Capture odorous molecules

The detection of odors takes place in the depths of our nose and not in our nostrils. Here, we find several million olfactory neurons. These neurons are equipped with cilia which constitute our olfactory receptors. Each neuron has a single type of receptor, which is reproduced several thousand times. By the time the odorous molecules are retained in the mucus, the olfactory receptors are activated to capture and detect the smell. Humans have 400 different types of receptors, while rodents have more than 1000 and elephants have about 2000! But this does not mean that we can only smell 400 different scents. A receptor can identify several molecules, and a molecule can activate several receptors: this allows a multitude of possible combinations.

Coding and decoding: reading the smell

In order for a molecule to become an odor, it is necessary to continue the path a little further in the brain. When the molecules are captured, the information about the smell is transmitted to an organ present in all vertebrates: the olfactory bulb. Located between the two eyes and about the size of a large Tic Tac, this area is also composed of neurons capable of deciphering the information received from the olfactory neurons. The bulb will then code this data, building a sort of identity card for each smell.

A fine analysis

Library of memories and emotions

This coded information is then transmitted to the different areas of the brain linked to olfactory perception. It is first the piriform cortex (which looks like a pear, hence its name) that will read the information. This area is responsible for recognizing all the olfactory identity cards. It is this cortex that defines the perception of one object or another. When you smell coffee, the piriform cortex will find the identity card of this odor already recorded. It is here to inform you that you are smelling coffee right now.

The smell is also sent to the hippocampus, the place where our memories are built and stored. It is the seat of our olfactory memory, a real library of more or less conscious events. It sends us back to our childhood as well as to more recent days.

Finally, the information is sent to the amygdala, an almond-shaped area of our brain, which is linked to emotions. It is here that the “I like” or “I don’t like” will be formed. The brain will thus tint the smell with a positive or negative aspect. Whether the smell of coffee comforts you or, on the contrary, disgusts you, it is in the amygdala that it happens.

Olfactory perception, a vital mechanism

All the areas of our brain related to olfactory perception communicate with each other. But it is finally the orbitofrontal cortex, located just above our eyes, that will make the decision. This is the last step in the integration of the smell, which will then lead to an action. To approach a smell or to move away from it, to prefer this perfume rather than that one, to drink or not to drink this coffee. This area is one of the most active for perfumers, who use it assiduously. And it is thanks to this cortex that the sense of smell appears as a primary mechanism, even more developed in our animal friends. It is this cortex that takes into account all the information to keep us away from the danger that an odor may represent.

It is therefore a formidable tool that we all have at our disposal, which we can educate and train in order to benefit even more from the infinite diversity of smells that surround us!

Did you know the mechanisms of olfactory perception?

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