21 Jun Smells, olfaction, and well-being
Smells are sensations that we perceive through our nose, and more specifically through our sense of smell or olfaction. To smell and identify an odour, we use our olfactory system, which is directly connected to the brain! Smells have exceptional properties. Do you know what they are?
What is an “odour”?
It’s a chemical response to one or more odorous molecules. When an odorous compound reaches our nose, it binds to olfactory receptors located in the mucous membrane of our nose, triggering a series of reactions that send a signal to the brain. The brain then processes these signals to give an odor perception.
An odour is therefore one (or more) chemical molecule(s) with very specific characteristics. An odorous molecule is transported through the air to reach our noses! This means that the odorous molecules are small enough to travel, they are said to be volatiles and sufficiently hydrophilic to penetrate the mucus of a nasal cavity.
There are thousands of different odorous molecules, each with a unique chemical structure that gives it its particular smell. For example, limonene is a fragrant molecule found in citrus fruits, giving a fresh, lemony smell. Allicin is another odour molecule found in garlic and onions, giving a pungent, sulphurous smell. And who can forget the distinctive smell of freshly ground coffee which is due to a molecule called 2-furfurylthiol?
How do we identify a smell?
Our noses can distinguish between thousands of different smells. This is made possible by the presence of olfactory receptors in the nasal mucosa. These receptors are specialised cells capable of detecting odorous molecules floating in the air.
But how does our brain manage to perceive a smell? To understand this, a little neuroanatomy is needed. Electrical signals from olfactory receptors are sent to a region of the brain called the olfactory bulb. This region processes the signals and transmits them to other parts of the brain, in particular the amygdala and the hippocampus (emotions and memories).
How do you specifically identify an odour?
Most odours are complex, i.e. they are made up of several odorous molecules. An odour is picked up by different types of olfactory receptors.
Humans have around 400 different types of olfactory receptor, which are activated in different ways depending on the odour molecules they breathe in. Depending on the combination of receptors activated, we can identify the smell. To put this in perspective, it’s like tuning a piano with 400 keys.
Everyone is different when faced with a smell, which explains the wide variability depending on experience and culture.
How do we memorise a smell?
Have you ever been transported through time and space by a familiar smell? Perhaps you remember the smell of your grandmother’s house or the smell of cut grass on your summer holidays. But how is it possible that we can memorise these smells and recognise them years later?
The answer lies in the way our brain processes olfactory information. When we first smell a scent, our brain processes the odour molecule and creates an “olfactory imprint”. This imprint is stored in our long-term memory, where it can be recalled when we encounter the same smell again.
But how does this happen? Our brain processes olfactory information in an area called the olfactory bulb, which is located in the lower part of the brain. The olfactory bulb is connected to other brain areas, such as the amygdala and the hippocampus, which are involved in processing emotions and memories.
When we smell a familiar odour, the olfactory imprint stored in our long-term memory is activated, and this information is sent to the amygdala and hippocampus. These areas of the brain allow us to link smells to memories and emotions, which explains why smells can instantly transport us to the past.
But olfactory memories are not only linked to positive experiences. In fact, unpleasant smells can also be remembered particularly effectively, because our brains see them as a potential threat. This explains why a simple smell can trigger a stress or fear response.
In short, our ability to memorise smells is a wonderful feature of our brain, allowing us to retain memories in a deeply personal way.
Why is smelling essential to well-being?
Since the COVID 19 pandemic, loss of smell (anosmia) or smell-related disorders (parosmia = mixed smells) are sometimes still present 6 months after the disease.
The cause is inflammation of the olfactory bulb, which breaks the odour identification chain. But what impact does the loss of smell have on people’s daily lives?
The sense of smell is essential for both humans and animals. From an evolutionary point of view, it is one of the oldest senses. Olfaction enables us to identify:
- food,
- partners, fellow creatures
- predators,
Smelling odours also provides hedonic pleasure: the smell of flowers or perfume, the food on our plate, but also warnings of danger. Examples include spoiled food or chemical hazards such as smoke. For both humans and animals, it is one of the important ways in which our environment communicates with us.
But smells are not just interesting from an environmental point of view – they are also associated with many anecdotes and memories. For example, do you remember the smell of your grandmother’s house, or the first time you smelled the sea? These smells can trigger deep memories and emotions, as they are closely linked to our brain and our past experiences.
So, what happens when we no longer smell anything?
- A feeling of unease, which in more serious cases can lead to withdrawal and depression.
- eating disorders, as the sense of smell plays a major role in tasting food.
- a safety problem: when you can’t smell anything, you can’t detect a danger signal such as the smell of smoke.
- poor health, linked to an inability to detect bad smells.
Although human beings are more highly developed in terms of vision and touch, the sense of smell is essential to our well-being.
For more information on this subject:
https://lejournal.cnrs.fr/articles/de-la-molecule-a-lodeur
https://odourobservatory.org/fr/about-odours/
