How do we smell odors? We can smell many, probably thousands of different odors. Whenever we smell something, odor molecules are reaching the olfactory receptors in our nose. Usually it is a mix of many different odor molecules which in combination gives us a certain smell. Coffee odor for example is a mix of dozens of different odor molecules. Odor molecules are chemical substances. Even if we are smelling an odor from a natural source, chemical substances are released from the odor source and which reach our nose. An odor source is everything which has a smell. For example, one of the main components of the smell of cloves is eugenol, a chemical substance. We can buy eugenol in the pharmacy and smell it. It smells exactly like the cloves we can buy in the grocery store (although it may smell a bit stronger).
In order that we can smell odors, odor molecules have to reach the inside of our nose, the nasal cavity. This usually happens when we breathe in. During every breath, the air surrounding us is soaked into our lungs. Within this air we find many different odor molecules. If we are standing in a bakery, many different odor molecules from bread will be all over the room. Every time we breathe in, these bread odor molecules will also be inhaled with the room air. And every time we breathe in, we will smell the nice odor of fresh bread.
Odor molecules do not have to go all the way to the lungs in order to be smelled. Instead they just have to reach the so called olfactory mucosa, which is located in the nasal cavity. Another term for olfactory mucosa is olfactory epithelium. As every opening of our body, the nasal cavity is lined with mucosa. However only in the top portion of the nasal cavity, the nasal mucosa carries certain cells, the olfactory receptor cells. And the odor molecules have to reach these olfactory receptor cells in order for us to smell them.
When we look at our own face in the mirror, we see our nose in the middle of the face. Everyone thinks he knows his nose very well. However, one may be surprised to hear that the portion of the nose which is visible from the outside is only a minor part of it. In fact, our nose is constructed similar to a gothic cathedral, and we can only see the façade. It is only once we enter the gothic cathedral by the gate (the nostril), we see the inside. Our nose-cathedral is very narrow, but goes very far back, and very high up. In the very back, something like 5 to 8 centimeters inside the nasal cavity, we reach the nasopharynx, which is the uppermost part of our throat. From here we can descent towards the lungs. On the way there we could reach our mouth (from backwards), the esophagus, which leads to the stomach and the wind pipe or trachea, which leads to the lungs. But we are interested in looking upwards. When we look up to the ceiling of the nasal cavity, approximately 5 cm away from the nostril, we are looking directly onto the olfactory mucosa. When looking from outside, the olfactory epithelium is located right between our eyes. So, odor molecules have to reach the top of the nasal cavity in order to be smelled.
When we inhale normally, most of the odor molecules stay on the floor of the nasal cavity, and only few reach the top of the nasal cavity. When we sniff, however, we are causing turbulences in the nasal cavity and much more odor molecules will reach the olfactory mucosa at the top of the nasal cavity – and we will perceive a stronger smell.
Dr. Frasnelli specialises in odor perception. He conducts research in the field of neurophysiology of smell and taste as well as therapy in loss of the chemical senses. Frasnelli is a graduate of the Medical Schools of the University of Vienna (Austria; 2001; Dr. med. univ.) and the Technical University of Dresden (Germany; 2009; Priv.-Doz.). Since 2006 he work in Montreal, first as an Academic Trainee at the Montreal Neurological Institute, since 2008 as a Postdoctoral Fellow at the Department of Psychology at the Université de Montréal. He currently hold a fellowship of the CIHR. Dr. Frasnelli research interest is the neurophysiology of smell and taste as well as therapy in loss of the chemical senses.
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