To start a long day, what would be better than having a good cup of coffee? How do I smell coffee? Let's look at it. Well, in fact if you want to smell the coffee, it smells good. You need to use your nose. And for our nose we do a few things. We use our nose to breathe. We use our nose to smell and sniff and in fact breathing and smelling. You can't really separate them, because why? When we are breathing what happens is that in this cross section of our skull we will find that air goes into our nasal cavity. And then pass through the cavity where we'll be able to detect what type of aroma is in this particular breath. How about sometimes when we breathe it out normally, something inside body it comes up the lung and also goes through the same nasal cavity and it comes out. But you remember that sometimes if you want to smell something, you try to sniff, what do you do? [SOUND] So what happens is that the air would come way up there and directly be in contact with the sensory organ you have in your nasal cavity. And with that it allows the movement of all this volatile aroma molecule to go into this airway. And it is through this olfactory epithelium in the nasal cavity. You sense what the molecule is. Now what exactly is the olfactory epithelial cell that I'm talking about? So here's all these components. Think about that in the nasal cavity, in this particular reason, in fact we have a structure which is called olfactory bone. That's the place a lot of our neurons are located. On that we have a lot of the interneurons that would be able to integrate all the signal input. But the most important part of it, this olfactory part is protected by a layer of bone underneath of it. And below that in fact all the food go to a surface which we call the nasal epithelium cell. On it in fact we have all this sensory receptor cells that allow us to taste or sense what the aroma molecule is there. So, it is very interesting that if you look at this olfactory epithelium, you'll find that there are a lot of different kind of cells there. And I'm going to show you a little bit later. And the most important part of it, you'll find on this layer, in fact, we call it as mucosal layer. That means, actually, is very mucous in nature, why is it mucous? Because mucous help us to trap water on it. And the important thing is that aroma molecule need to first go into this water before they will be sensed by these particular receptors on these cells. And this particular chamber what we find is that all the sensory cells also have a lot of microvilli and they have a lot of ciliated surfaces. What are they doing? They try to increase the surface area so that they would be able to trap a lot of those odorant molecules so that we can sense. Now, having that in mind, think about it. When we smell or when we sniff, what are we doing? We have a very fast and very turbulent movement of all this air coming into our nose. How much can we take in? We're taking about 250 mil of air per second. A large volume. Think about that. Only a small volume of them, in fact they will be able to go into this olfactory tract. And what it does is that when we breath deeply we will be able to capture more of all this air. And when there is important molecules that we want to sense, then we will be able to perceive there are some aroma, some flavor, right? Interestingly, you probably have experienced that when you have a cold and you have a running nose. You find that actually you don't smell very well, why? Because the amount of mucus produced by your nose, in fact it's way more than enough. So what happens is that they cover all the nasal epithelial cell. And in fact you can have molecules dissolved in the water. But then they are so far away from the sensory cell you can't sense it or you can't smell it, all right? If you look at this particular layer of cells it's very interesting. I want to remind you, on it in fact we have something which is called basal cells. They are right at the bottom of it. And they are whole bunch of cells which are supporting cells. But along them you will find that there will be sensory cell that sends their process to the very bottom of it. That's the place, it's the surface where the odor molecules would be able to dissolve into. So there are sensory cells for the sensation, support cells to make sure that they are in a healthy conditions and we have basal cells for replenishment. Because normally some of these cells, they are going to be dying or they will be removed from time to time over time, okay? And when you learn about that, do you remember what happens in the taste buds? Do we have the same kind of cells? We do have basal cells, we do have support cells, we all have the sensory cell. In fact in terms of the overall organisation, very similar we have a lot of microvilli, a lot of sensory cilia and those are the places we want to put those sensory receptors on them. So that we would be able to perceive a sense. Now, that's one element about the structure. Think about the surface area, the surface area for human olfactory epithelium is about 10 centimeters squared. Very small, ten centimeters is around that, ten centimeters squared. But think about that, for the dogs, in fact they have a very large olfactory epithelial layer. What they have is about 70 times larger than what we have, 170 centimeter square. And the most important part of this is dogs they have a large number more of all this receptors per centimeter square. So you add them all together, you have 17-fold more area. And you have 100 time more receptor per area. So therefore, they are having thousands of those more receptors on the olfactory epithelial cell. And so what it mean is that they would be able to detect some kind of smell, even though they exist in a very minute amount. And so therefore, the number of the sensory receptors and the type of sensory receptors all combine together. Making dogs much more sensitive to smell. And in fact, what happened is that, we know that in our nose in all these sensory cells, each of sensory cell they produce one type of receptor. And for us we have roughly about 40, 50 millions of these sensory receptor neurons all combined together. So what exactly it is is that well, we would now be able to smell. So if I give you a particular sense, in fact it will be able to stimulate a large number of neurons distributed on this olfactory epithelium. And so what you see is that using this particular colored coded stimulation graph, we show that a particular odorant would be able to stimulate some area and the other area, but not those blue area. So, in a way, they are stimulating different type of olfactory neurons. Okay, let's look at how having a sensory response in our olfactory bulb, and the olfactory epithelial cell's being activated. How's it related to our ability to perceive the world? Using all this odorant receptor, 1,000 of them in human being, we are sharing, in fact, the same number of odorant receptor as all the other mammals, such as dogs and mouse. And what happens is you find that in human being, 70% of them, they are pseudogenes. Meaning that actually these other genes, they are not functional at all. So, meaning that we are only having about one third the number of the odorant receptors which is performing its functions compared with a mouse. So you think about that. In that case no wonder our ability to perceive the world by the smell of a chemical sensation of the environment around us is much weaker than the mouse or the dog. Now, having that in mind, then we need to ask ourselves, only having 347 functional sensory receptors. Is it sufficient for us to define the world around us. The answer's very simple, you can get the best you can with what you have. You have only 347, but don't worry. If you think about it, how many different tastes can we sense? We would be able to tell whether it's sweet, whether it's sour, whether it's salty or whether it's bitter. Or whether it carries a sense of umami. And yet when we taste all the dishes, they're all kind of combination that tell us that what is a delicious dish. The same thing now, we have 347. So, in fact we can have a huge combination of all these odorants being sensed by us and in the combination, it gives us a perception of what is really around us. Now, having this odor receptor being activated and go through the olfactory bulb and translating all the signals and send it back into the brain. We will stimulate the frontal cortex, which is the place where we get to have a conscious analysis of what kind of flavor, aroma that we are detecting. It also stimulates our hypothalamus and amygdala, the alman body, that tell us whether it's pleasing or not pleasing. And also it stimulates our hippocampus to tell us to recall some of the old memory that is associated with this particular sensation. With this particular combination. All together, it really gives us a sense of what is around us and what kind of memory and experience we should be able to integrate together to give us a good sense of pleasant perception.
