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The Sense of Smell / All About the Sense of Smell
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submitted by Dr. Gary Farr - Contact the author here.
Last Updated December, 8, 2011
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The Sense of Smell
The sense of smell is known as olfaction. Olfaction, along with taste, is a form of chemoreception. The chemicals themselves which activate the olfactory system, generally at very low concentrations, are called odorants. The chemical reactions that cause the sense of taste are not stored in our memory system as well as other senses are; such as the information received by our visual and auditory (echoic) systems.
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 releaesed 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.
 The sense of olfaction is complex. Odor perception is influenced by many factors unique to each individual as well as external environmental factors. The basis of odor perception is the contact between chemical molecules, mainly in the gaseous state, which can be detected by the olfactory epithelium.
The perception of an odor by humans results from a stimulus. It includes key information as the odor intensity and odor quality. Our ability to collect this information makes the olfaction a very complex sense. All the biochemical parameters are not yet fully understood by specialists.
 The perception of odorant molecules in the human nose is achieved thru an array of sensory cells called the olfactory epithelium. These cells act as biological sensors that react to the presence of odorant molecules (for example aldehydes, ammonia, butanol, dimethyl sulfide, toluene, + 10000 more!) by generating electrical signals. These signals are sent to the brain thru the olfactory cortex and travel along the olfactory nerve.
The cell body is in the mucosa, the lower parts (the cilia) with the receptors are actually on the surface of the mucosa. On the upper side, the olfactory receptor cells carry an extension, the so called axon. This axon travels from the nasal mucosa through the bone of the skull to the brain. SWhen we say that the olfactory receptor cells send the information to the brain, they do it via these structures. The axons of all olfactory receptor neurons together form the olfactory nerve, the first cranial nerve. The axons reach a brain structure called the olfactory bulb. The olfactory bulb is just above the nose, but already part of the brain. Within the olfactory bulb, axons end in some ball-like structure, the so called glomeruli. These are however very small balls, they measure approximately a tenth of a millimeter.
 Here something interesting happens. The axons of all the olfactory receptor cells carrying one specific receptor (for example, the “A-receptor”) all terminate at the same glomerulus. Furthermore, at this glomerulus no axon from other olfactory receptor cells end. We can therefore call it the “A-glomerulus”. Whenever an odor molecule reaches the olfactory mucosa and activates some olfactory receptor cells, the according glomerulus gets activated. Since we said we have 200 different olfactory receptors, and 200 different olfactory receptor neurons, we should also have 200 glomeruli in our olfactory bulb.
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