But what is a smell exactly?
It is a set of volatile molecules released by a substance in the air and detected by our sense of smell, through the receptors located in our nasal cavities. There are two activation pathways:
The 'Gazette'
Olfaction is one of the five human senses: it allows us to detect and perceive odorous molecules, through our nose but also through our mouth. Each of us has around 400 types of olfactory receptors. Whereas some are specific to an odorous molecule, others may be associated with several of them.
It is a set of volatile molecules released by a substance in the air and detected by our sense of smell, through the receptors located in our nasal cavities. There are two activation pathways:
– The orthonasal pathway – or direct pathway: where the compounds pass through the nostrils when inhaling.
– The retronasal pathway – or indirect pathway: where the compounds pass through the nostrils when exhaling.
Generally, we talk about odour when we smell through the orthonasal pathway, and aromas when we smell through the retronasal pathway.
The olfactory system’s orthonasal and retronasal pathways
In order to fully understand how the perception of a smell is decoded by our brain, we must first look at the structure of our olfactory system. In short, it is composed of:
The olfactory mucosa: this is the organ that receives the odorous molecules.
The olfactory bulb: the first relay of olfactory information to the brain.
In summary, although the functioning of the olfactory system is very complex, we can still identify 4 main stages in the perception of odours:
Olfactory receptors are the keystone to olfaction. Everything begins with a volatile molecule that travels to our nostrils to reach the olfactory receptors. Once bound to one (or several) receptors, the molecule activates a kind of switch that starts the creation of a mental image for the smell in the brain. In the next article, we’ll look at the perception of an odour and break it down into three levels.
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Meierhenrich, U. J., Golebiowski, J., Fernandez, X., & Cabrol-Bass, D. (2005). De la molécule à l’odeur. L’actualité chimique, 289, 29
Murphy, C., & Cain, W. S. (1980). Taste and olfaction: independence vs interaction. Physiology & Behavior, 24(3), 601-605
R&D | December 2024 | Dr. Magali Picard