Posted by Thierry Page on Thu, Nov 01, 2012 @ 01:34 PM
We are honoured to present you a special blog edition written by our guest author Dr. Johannes Frasnelli.
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.
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This blog follows: On the way to understand how we perceive odors
For many years neuroscientists have worked in order to determine the basic dimension of odors, or, in other words, to find out in which basic categories we can divide odors into. Sometimes it is helpful to look around a bit, as the world sometimes provides us with wonderful ideas. We can look at perfumers, which are specialists from the point of view how we perceive odors. They use four basic categories for odors. They divide fragrances into the categories “woody”, “fresh”, “oriental”, and “floral”. While these categories may be very useful for perfumers, they are unsatisfactory for a neuroscientist, because there is no corresponding receptor or anatomical structure for these categories. For example, in taste, we can distinguish five (or maybe six) different basic taste qualities. They are sweet, sour, salty, bitter, and umami (and maybe fatty or metallic). These basic tastes however correspond to different receptors – located in the tongue’s mucosa. In the olfactory system, we know that there are hundreds of different receptors. We only know for few of them which odor perception they are responsible for – but there certainly does not appear to be a general distinction into “woody”, “fresh”, “oriental”, and “floral” receptors.
Therefore researchers instead of investigating the nose, started to look more closely at the brain, in order to maybe find anatomical features which may then be linked to odor perception. They indeed found brain regions that respond differently to odor aspects. In fact, they found several aspects of the odors brain regions respond particularly to.
The first of these is the chemical structure of the odorant. Here, researchers let their participants smell several odorants. Some of those odorants smelled like lemon, others smelled like vegetables. The researchers however applied a special trick: in both categories there were odorants which shared chemical properties. Specifically, they used aldehydes and alcohols: some of the aldehydes smelled like lemon, some smelled like vegetables.
The same was true for the alcohols: again, some smelled like lemon, some smelled like vegetables. By applying this trick, the researchers could look if there is any brain structure that responds specifically to differences in chemical structure (aldehyde or alcohol) or in odor quality (lemon and vegetable). They were able to find a brain region that specifically responded to chemical structure, and another one, just next to it, that responded specifically to odor quality.
Another group of researchers was able to determine that there is a brain region that specifically responds to different odor intensities. I have outlined before that odor intensity is also a basic odor category. The same group of researchers also found a brain region that responded to specifically to pleasantness of odors, which therefore may also be one basic odor category. Other categories have been put forward, although there is not yet a brain structure linked to it. For example, edibility of the odor appears to be a basic odor category. This means that an odor which stems from food is fundamentally different from an odor which comes from a non-food odor source.
We have not yet reached a final answer to the question of which is the most basic odor category. There are at least four basic odor categories, namely chemical structure, quality, intensity and odor pleasantness. So, we have some hints towards which direction researchers should look into. From the point of view of a neuroscientist there is not much support for the subdivision of odors into “woody”, “fresh”, “oriental”, and “floral” – other than that perfumers use it.
However, the very same perfumers are able to create wonderful scents, so maybe they are right, after all?
Dr. Johannes Frasnelli
Links:
http://ucdenver.edu/academics/colleges/medicalschool/centers/tastesmell/Pages/tastesmell.aspx
http://www.vcu.edu/ent/ent_clinic_smell_taste.htm
http://www.tasteandsmell.com/
http://personal.ecu.edu/wuenschk/anosmia.htm
en français:
http://www.hcuge.ch/~infotec/rhino/index.htm
Posted by Thierry Page on Thu, Aug 16, 2012 @ 08:37 PM
We are honoured to present you a special blog edition written by our guest author Dr. Johannes Frasnelli.
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.
___________________________________________________________________________
When studying the sense of smell it is always a good idea to look at the other senses as well. Many features in the individual senses have the same characteristics, because evolution and biology is economic. So if we know something about seeing or hearing, then there is a certain chance that the sense of smell works in a similar fashion. And we do know a lot more about seeing or hearing than about smelling.
So, if one wants to know about how we perceive odors, it is a good idea to first look how we perceive images or sounds. Here I am not talking about what happens in the eyes, ears or in the nose, because that is pretty different for each individual sense. What we are looking for now is how the brain treats the information from the different senses. In order to understand how we analyse smell information, let’s first look how we analyse visual or auditory information.
One of the main features, if not the most important feature of an image or a sound is its intensity. So, a very weak visual stimulus will be perceived as pretty dark, a very weak auditory stimulus will be perceived as very silent. On the other hand of the scale, a very intense visual stimulus is perceived as very bright, and a very intense sound is perceived as very loud. And this analogy is true for smell as well. A very weak olfactory stimulus is perceived as a weak smell, and other hand, a very strong stimulus is perceived as a very strong smell.
Source: Wikipedia
What is the basis of a strong or weak stimulus in the different sensory systems? If a light source emits a lot of photons which then reach the retina, we perceive a strong light. If a sound source emits very strong pressure waves which reach the ear drum, we perceive a loud sound. The equivalent of the photons for seeing and the pressure waves for hearing is, in the case of olfaction, the number of odor molecules, or in other words, the concentration. Therefore, if an odor source emits a lot of molecules, we perceive a strong odor. If an odor source emits only few odor molecules, we perceive a weak odor. This is however only true for a given substance. There is the possibility that for one substance we need many molecules to have a very faint odor perception, and for another, a few molecules may be enough to evoke a strong smell.
So, we have seen that the concept of intensity exists and is valid for the sense of smell as it is for seeing and hearing.
But what about the next step, what about more complicated characteristics of the stimulus. What are the next key characteristics in the other sensory systems? For example, in vision, another key feature, next to intensity, is color. Let’s look at color: different colors are evoked by different wavelengths (frequencies) of the electromagnetic wave which reaches the eyes. Short frequencies evoke purple and blue colors; long frequencies evoke red and yellow colors. Again, we have something analogous in the auditory system: different wavelengths (frequencies) of the pressure waves evoke sounds of a different pitch. Short wavelengths (frequencies) evoke a sound with a high pitch; longer frequencies evoke sounds with a low pitch.
Unfortunately, the sense of smell is structured in a completely different way than seeing and hearing. Smell perception is not based on perception of frequencies (although some, but very few, researchers do think so). So, do the analogies between seeing, hearing and smelling stop here?
Fortunately not. But we have to look at the brain from a different angle. We know that those basic features, which are also called dimensions, in seeing and hearing are encoded in brain structures. What does that mean? There are some cells in the area of the brain processing input
from the eyes which react to only one color, let’s say red, but not to another color, let’s say blue. Similarly, in the parts of the brain processing auditory information there are some cells which react to high pitch sounds exclusively, and some others which react to low pitch sounds exclusively. We can therefore be positive that color and pitch are dimensions of vision and audition, respectively, which are encoded in the brain.
Source: wikipedia
So, let’s look at the sense of smell. We know there is no equivalent of wavelength in olfaction. If one would however be able to find cells, zones or regions in the brain which react to one feature of smells, but not to the other, he could conclude that this feature is a dimension
of smell, just as intensity is. The hunt for these features of smells has started some years ago and is not yet over. Several concepts have been put forward, and some of them fulfill the criteria above. We know now that, next to the intensity of odors – which we know is caused by the concentration of the odorant and which is therefore one dimension of odors – those dimensions may include the chemical structure of the odorant, the pleasantness of the odor, and the edibility of the odor source.
I will talk about these in one of my next contributions.
To be continued... Odor categories: what determines how we can smell?
Dr. Johannes Frasnelli
Links:
http://ucdenver.edu/academics/colleges/medicalschool/centers/tastesmell/Pages/tastesmell.aspx
http://www.vcu.edu/ent/ent_clinic_smell_taste.htm
http://www.tasteandsmell.com/
http://personal.ecu.edu/wuenschk/anosmia.htm
en français:
http://www.hcuge.ch/~infotec/rhino/index.htm
Posted by Thierry Page on Tue, Oct 04, 2011 @ 03:52 PM
We are honoured to present you a special blog edition written by our guest author Dr. Johannes Frasnelli.
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.
___________________________________________________________________________
When someone is unable to see we call him blind; if someone is not able to hear sounds, we describe him as deaf. There is an analogous situation for the sense of smell: the complete absence of the sense of smell is called anosmia, whereas a reduced function of the sense of smell is called hyposmia.
Both anosmia and hyposmia are found relatively often. Researchers estimate that 5% of the population of western countries do not perceive any smells. This means that one person out of 20 is not able to smell; these persons are therefore considered anosmic. The numbers for hyposmia are even more impressive; up to 15% of the population have a reduced perception of smells. Therefore, one person in five has either a reduced sense of smell or does not smell anything at all. These numbers are considerably higher than those of blind or deaf persons – but of course anosmia and hyposmia have less devastating effects on the life of the affected persons.
But also persons with anosmia and hyposmia suffer from their condition. They have more depression symptoms and exhibit a reduced quality of life compared to people with a normal sense of smell. Amongst the typical problems people with anosmia or hyposmia describe to have is that they cannot appreciate food, they do not perceive the smell of a loved one, such as their partner or child, and others.
Qualitative olfactory dysfunctions are special forms of smell problems. Here people can perceive smells, but they perceive smells different from what they are supposed to smell like. For example, they can perceive flowers to have a burnt smell. This condition is called parosmia. In addition, some people perceive smells although there is no smell source, sometimes all the time. For example, some people constantly perceive a foul, rotten smell. This condition is called phantosmia. Both parosmia and phantosmia can be very disturbing and affect quality of life considerably.
Of course persons with a reduced sense of smell are running higher risks to be exposed to dangers, since the sense of smell has warning functions. For example, persons with a smell loss would not perceive if food is spoiled. Therefore it is very important for them to pay attention when storing food and to obey the expiration dates of food. If in doubt they should either ask someone else to smell the food or, in order not to run any risk, throw it away. People with anosmia should also consider that they do not perceive the smell of smoke or household gas. They have therefore to be especially careful when dealing with situations in which a fire may be burning or gas may be leaking.
There are different reasons why people can lose their sense of smell. The most important causes of a loss of smell function are different kinds of nasal problems such as nasal polyposis or chronic rhino-sinusitis. Although we do not know the exact mechanisms it is likely that the ongoing inflammation of the nasal mucosa in nasal polyposis and rhino-sinusitis affects the smell receptors in the upper part of the nasal cavity.
Next, there are two other, equally common, causes of a smell loss. The sense of smell may be lost as a consequence of a viral infection such as the flu; the specialists call this postviral anosmia/ hyposmia. This does not refer to a common cold; we all have experienced a common cold during which the nose is blocked, and with a blocked it is impossible to smell. However, in the case of a postviral anosmia/ hyposmia the loss of smell function persists even after the viral infection has healed. Interestingly, postviral anosmia/ hyposmia affect mainly women above the age of 50; we do not know why.
Smell function may also be lost as the consequence of an accident with traumatic brain injury (posttraumatic anosmia/ hyposmia). Although severe accidents are more likely to cause smell loss, even a concussion with no other long term effect may be enough to cause anosmia or hyposmia.
Other, less likely causes of smell loss include brain tumours or exposure to toxic substances. In many cases no apparent cause for the smell loss can be determined (idiopathic anosmia/ hyposmia). Up to 1% of all persons who can’t smell are born without a sense of smell and have never smelled in their life (congenital anosmia). Also, most of the persons suffering from neurodegenerative diseases such as Parkinson’s disease or Alzheimer’s disease have an impaired sense of smell.
There is hope for those who have lost their ability to smell. We know that the sense of smell can return, even after as long as several years. People with postviral anosmia/ hyposmia have been described to have a chance of 30% that their sense of smell will improve over one year. This percentage is lower, in the range of 10% for those who have lost their sense of smell after an accident.
IMPORTANT: The medical clarification of a smell loss has to be performed by a medical specialist such an ENT (ear-nose-throat) specialist or a neurologist. Diagnostic tools include a thorough medical history, nasal endoscopy and magnetic resonance imaging (MRI).
Links:
http://ucdenver.edu/academics/colleges/medicalschool/centers/tastesmell/Pages/tastesmell.aspx
http://www.vcu.edu/ent/ent_clinic_smell_taste.htm
http://www.tasteandsmell.com/
http://personal.ecu.edu/wuenschk/anosmia.htm
en français:
http://www.hcuge.ch/~infotec/rhino/index.htm
Posted by Thierry Page on Sun, Jun 26, 2011 @ 07:33 PM
We are honoured to present you a special blog edition written by our guest author Dr. Johannes Frasnelli.
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.
______________________________________________________
The advantage of having two eyes is obvious. Everyone covering an eye with a hand will realize how difficult it is to navigate with only one eye open. Two eyes allow us to see in 3D. With 3D (or stereoscopic vision) we can also perceive the depth of the image in front of us. The huge success of movies such as Avatar is due to the vividness of 3D images.
The same is true for ears. We have two ears in order to hear in stereo. We do not have to move our head to be able to localize the source of sound. If we sit in front of an orchestra, we can close our eyes and point very accurately where the trumpets are, and where the timbale is.
For both, ears and eyes, our brain takes advantage of the fact that it receives slightly different information from each ear (or eye). This slight difference allows the brain to calculate where the sound (or the image) came from. The best thing about this is that we do not even realize how the brain calculates; it comes all natural to localize a sound or an image.
We have another sensory organ, which is built in a pair: we have two nostrils, just as two eyes and two ears. But it is not only that. Inside our nose there is a wall in the middle – the septum - which separates the left side from the right side. So, in fact, we have two noses within our nose, which are completely separated from each other. But why do we have these two noses which are completely separated from each other, why do we have two nostrils?
After the examples from ears and eyes, one can speculate that, the two nostrils enable us to localize where a smell comes from. But before talking about us humans, let us first see, how the specialists do it. Let’s see how dogs track the scent of a prey.
You can look at it on the left hand of the image below. Here a dog is following the scent of a pheasant. You can see the pheasant’s track by the yellow line. A dog, which follows the track, does not follow this line closely. The dog’s path is highlighted in red. The dog rather crosses over the pheasant’s track, until he realizes to have lost it, then he returns until he finds it again. We are able to do the same: on the right side of the image you can see the results of an experiment in humans, performed at theUniversityofBerkeley. Here a chocolate track was put on a lawn, and subjects had to track it based on the smell only. You can see that the red line of the subject tracking the chocolate scent in yellow. In this experiment the researchers showed that we can track scents. Just as dogs do, we zigzag towards the source of the smell.
Source: University of California, Berkeley
However, apparently, the sense of smell works differently than seeing or hearing. If we move towards a sound (or to something we see), we are not using a zigzag pattern. Instead we are very well able to move towards a sound (or to something we see) in a straight line. In fact in order to approach a source in a zigzag pattern, we do not need input from two nostrils; we could do that with one nostril alone, too. As long as we smell the odor we move on. As soon as we do not smell it anymore we move back. By doing so, we will approach the odor source.
So, this experiment showed that we humans are able to track scents; but to do so we do not need two nostrils. Therefore, the question remains: Why do we have two nostrils?
The answer may lie somewhere completely different. There is a so called nasal cycle: the mucosa of one of the nostrils is always a little bit more swollen than the other. What happens if the nasal mucosa is swollen? The blood vessels in the mucosa get larger, and more blood gets into the mucosa. The blood stays in the vessels, but the mucosa as a whole gets thicker, and thus there is less space for air in the nose. After two or three hours one the side changes and the mucosa of the other nostril is more swollen. This goes on and on, day and night.
Usually we do not realize if the mucosa is swollen, since the swelling is only slight. When we have a cold however, both nostrils are swollen anyways. Then the nasal cycle is more evident. We may lie in bed and try to sleep. We have difficulties breathing, because we have a cold, and the nose is blocked. Then we realize that - finally - the nostril which was giving us a hard time is opening up. In the same moment, the other nostril gets blocked. Voilà, this is the result of the nasal cycle together with the effect of a viral infection.
As we discussed, the nasal cycle is also present when we have no cold, we just have difficulties in perceiving it. But what is it good for? There is no definite answer to it, but the idea has been put forward that the nasal cycle is important for our immune system. The nose serves as an air filter so that all kinds of harmful substances (toxins, viruses, bacteria, etc) within the air we inhale do not get to more sensitive organs such as the throat or the lungs. So, the mucosa of the nose is exposed to all these dangerous things, and since they stay in the nose, the immune system has to handle them there.
The hypothesis of the nasal cycle goes therefore as follows: One side of the nose is maximally open, so that the body can breathe air. Toxic substances, viruses, bacteria, etc. are filtered and stay in the nasal mucosa. At the same time, blood vessels in the other nostril are open, so that immune cells in blood have maximal access. In a certain way, this nostril gets cleaned up while the other nostril is maximally at work. After a couple of hours, the side switches. Now the nostril that was at work before gets a clean up, while the other is back in business.
This gives us the answer why we have two nostrils. It is advantageous to have two nostrils, because it gives the body the possibility to simultaneously clean up the air filter of the nose, while one can still breathe with the other nostril.
To know more:
http://berkeley.edu/news/media/releases/2006/12/18_scents.shtml
http://www.ncbi.nlm.nih.gov/pubmed/21310764
http://www.ncbi.nlm.nih.gov/pubmed/20451578
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About the author: Dr. Johannes Frasnelli Ph.D.
Dr. J. 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.
Contact information: johannes.frasnelii@umontreal.ca
Personal links:
Posted by Thierry Page on Tue, Mar 01, 2011 @ 05:47 PM
We are honoured to present you a special blog edition written by our guest author Dr. Johannes Frasnelli.
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.
______________________________________________________
We have known for a long time that the occipital cortex is the brain region which is active if we are watching something; it is located in the very back of our brain. If we listen to music, on the other hand, the temporal region of the brain is active; this region is located just beneath the ear (interestingly, the left temporal region responds to sounds from the right ear and the right temporal region to sounds from the left ear). However, for a long time it was unclear with which part of the brain we smell, or, to put it into scientific terms, in which part of the brain olfactory information is processed. Over the last 30 years new imaging methods have become available which allow us to have a look at the brain at work, without harming the brain, such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET).
Before fMRI and PET were available, researchers had to dissect the brains of dead bodies and follow the nerve bundles to see with which region the olfactory bulb was connected. But since PET and fMRI entered the stage we have gained much more insight into how our sense of smell works. The three most important brain regions are:
(1.) the orbitofrontal (from Latin: between eye-socket and forehead) cortex is located just above and behind our eyes;
Source: Paul Wicks
(2.) the insula (from Latin: island) is located deep beneath our ears;
Source: Wikimedia
(3.) the piriform (from Latin: pear shape-like) cortex is located just between the two other brain areas.
In addition, the olfactory brain includes smaller, but still very important brain regions (e.g., the anterior olfactory nucleus, the olfactory tubercle, the amygdala, and the entorhinal cortex). There are several interesting features related to the anatomy of the olfactory parts of the brain:
First, the olfactory regions do not only serve for smelling, but are also used, if we experience emotions and when we are memorizing events. This is the reason why odors can evoke very strong associations and memories of situations and place from a long time ago. Probably everyone knows an anecdote where he smelled a food or a perfume, and was brought back to early childhood and remembers exactly the circumstances of when he smelled that odor.
There is a second special characteristic of the sense of smell. For all the other senses (seeing, hearing, tasting, touching), the information from the sensory organs (the eyes, the ears, the skin, the tongue) travels through a brains structure called the thalamus (from Greek: room). The thalamus is something like a gate to our consciousness. If we focus on one sense (say, on vision while reading a very interesting book), we can blind out information from other senses (and not hear that someone was talking to us). This is done by the thalamus, who decides what we are aware of and what not. Since smell information is independent of the thalamus, we cannot blind out olfaction: we either smell an odor, or we do not smell it.
This particularity of not traveling through the thalamus is also responsible for another characteristic of the sense of smell. In all other sensory systems, a moderate (and for some even a light) sensation is enough to interrupt sleep: if we are asleep, a light, a sound or someone touching us is enough to wake us up. Unlike the other senses, a smell is not enough to wake us up; therefore we all have to install smoke detectors at home. If there is a fire, we would only wake up, if the smoke is so strong that it becomes stinging (and therefore is like a touch sensation). This may then be too late.
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About the author: Dr. Johannes Frasnelli Ph.D.
Dr. J. 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 FRSQ. Dr. Frasnelli research interest is the neurophysiology of smell and taste as well as therapy in loss of the chemical senses.
Contact information: johannes.frasnelii@umontreal.ca
Personal links:
Interesting links:
Posted by Thierry Page on Sat, Dec 18, 2010 @ 11:28 AM
We are honoured to present you this second part of a special blog edition written by our guest author Dr. Johannes Frasnelli.
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.
______________________________________________________
When odor molecules reach the olfactory mucosa at the top of the nasal cavity, they get in contact with certain cells, the olfactory receptor neurons. The main part of these cells (the cell body) is located within the mucosa; but some branches, called cilia, reach the surface of the mucosa and are therefore exposed to the air in the nasal cavity. On these cilia we find the olfactory receptors. We humans have approximately 200 different olfactory receptors. Some animals, such as rats and dogs have many more olfactory receptors. The interesting thing is that, although we have so many different olfactory receptors, each and every olfactory receptor neuron carries only one receptor. Since the olfactory receptor neuron is therefore characterised by the receptor it carries, we can say that we have approximately 200 different olfactory receptor neurons in our olfactory mucosa. We could give our odor receptor names, for example “A-receptor”, “B-receptor”, “C-receptor”, etc. Then the olfactory receptor cells which carry the “A-receptor” would be a “A-specific” cell, or an “A-cell”. So, we have “A-cells”, “B-cells”, “C-cells”, etc. Within the olfactory mucosa, the different olfactory receptor cells are distributed completely at random.
As soon as an odor molecule reaches an olfactory receptor, the olfactory receptor cell is activated and sends a signal to the brain. But not every odor molecule activates all olfactory receptor neurons, because then all odors would smell the same. In order for the olfactory receptor neuron to be activated the odor molecules has to fit to the olfactory receptor as a key fits into a lock. However, this key-lock relationship is not very specific, but it is rather like middle age keys and middle age locks. In those old days one key could open several different locks; similarly an odor molecule fits into several different olfactory receptors and can therefore activate several different olfactory receptor neurons. So, for example, a ROSE odor molecule could activate the “R-cells”, the “O-cells”, the “S-cells”, and the “E-cells”.
In addition, one lock could be opened by several different keys. Similarly, several different odor molecules fit into the same olfactory receptor; therefore the olfactory receptor neuron which carries this particular receptor could be activated by several different odor molecules. For example, the “S-cell” could be activated by a ROSE-molecule, but also by a JASMINE-molecule, etc.
Now we can imagine what happens when we smell: Odor molecules reach the nasal cavity and there the olfactory mucosa. The odor molecules will reach olfactory receptors and activate the according olfactory receptor cells. Then the different olfactory receptor cells will send their signal to the brain. In order for the brain to recognize a certain odor, the complete information has to arrive. If the brain receives just the signal from the “S-cells”, it may be able to tell that this was a flowery odor, but it will not be able to tell whether the odor was ROSE or JASMINE. In order for the brain to be able to distinguish between many odors, the brain needs the information from all olfactory receptor cells.
Remember that 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 millimetre.
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.
Source: Patrick J. Lynch, medical illustrator
Figure legend:
- Olfactory bulb
- Mitral cells
- Bone
- Nasal Epithelium
- Glomerulus
- Olfactory receptor cells
In the figure, the receptor cells carrying different names are drawn in different colors.
Now we know everything to understand how the olfactory system works: When we smell a ROSE-odor, the ROSE-molecules reach the nasal cavity. They will fit into different receptors (the “R-receptor”, the “O-receptor”, the “S-receptor”, and the “E-receptor”) and therefore activate 4 different sets of olfactory receptor neurons (the “R-cells”, the “O-cells”, the “S-cells”, and the “E-cells”). Then the information will travel to the olfactory bulb, and four different glomeruli will light up: the “R-glomerulus”, the “O-glomerulus”, the “S-glomerulus” and the “E-glomerulus”. Then the brain has nothing more to look on the olfactory bulb and recognize the pattern of activated glomeruli. In our case, the brain would see that R, O, S, and E light up in the olfactory bulb and then conclude that we smell a rose.
______________________________________
About the author: Dr. Johannes Frasnelli Ph.D.
Dr. J. 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 FRSQ. Dr. Frasnelli research interest is the neurophysiology of smell and taste as well as therapy in loss of the chemical senses.
Contact information: johannes.frasnelii@umontreal.ca
Personal links:
Interesting links:
Posted by Thierry Page on Wed, Dec 15, 2010 @ 05:50 PM
We are honoured to present you this special blog edition written by our guest author Dr. Johannes Frasnelli.
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.
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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.
Source: Patrick J. Lynch, medical illustrator
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.
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About the author: Dr. Johannes Frasnelli Ph.D.
Dr. J. 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 FRSQ. Dr. Frasnelli research interest is the neurophysiology of smell and taste as well as therapy in loss of the chemical senses.
Contact information: johannes.frasnelii@umontreal.ca
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