How does the sense of smell works?

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Whether it is the perfume of a rose or the aroma of a cake, the perception of odours is very much linked to emotions. The explanation is… neuroscientific, and is related to our evolution!

A smell is never a simple odour: it provides immediate, irrepressible sensations, as sublime as it is appalling, and rushes up the memories. It cannot be seen, touched or heard, and yet it has the power to capture our whole being. Like it’s talking directly to our unconscious. And for good reason: of all our senses, the sense of smell is the most intuitive, the most animalistic.

In the brain of the first vertebrates, the perception of odours occupied a central role, and when the cortex developed 190 million years ago in mammals, it was primarily to process olfactory informations: they made it possible to locate food, mark a territory, show that we were available for reproduction…

The sense of smell has diminished in importance as we have evolved

When sight gradually supplanted our ancestors’ sense of smell in terms of importance, they emancipated themselves from this mode of communication, preferring artificial scents to body odours, elaborated to please the nose and rise above their animal nature.

Except that the sense of smell has remained deeply rooted in us. It belongs to our history, even to the point of waking up memories: the simple scent of a rose can remind us of the emotion of a first love… A smell is never strictly a smell, but a world. How is that possible?

The power of our sense of smell has diminished as human beings have evolved
The power of our sense of smell has diminished as human beings have evolved

From nasal mucosa to cortex

It all begins with the odorous molecules that float in the air: they dissolve in the nasal mucus, where they tickle the olfactory cells. These send nerve signals to the brain in response, which will pass through nerve structures linked to emotions. First step: the olfactory bulb, at the front of the brain. Here is found a primitive cortex, where 4 types of neurons, organized in glomerules (pelotons), analyze the odours, making a first classification in categories, for example according to the length of the carbon chain in the molecule.

Unlike what happens for the other senses, this information does not then go directly to the conscious areas of the brain (situated in the upper cortex), but passes through deep zones, also primitive (hippocampus, tonsil, hypothalamus…), which add an affective component to it. It is only after having felt the emotions associated with our memories of a smell that its identification becomes conscious!

Five percent of our genes dedicated to smell

But genes also have a connection with odours: 950 of them (almost 5% of our genome!) govern the production of receptor proteins, allowing olfactory cells to recognize odorous molecules. Different from one person to another, these genes explain the very personal sensitivities to odours. It is estimated that the human nose can identify 1000 billion smells!

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