Hello students. In this second week of the class we're going to cover two areas. We're going to talk about taste, and smell. Now in this particular class I'll try to focus on a few areas, trying to understand how the basic structure of our taste bud is like, so that we would be able to perceive different kind of taste. And of course I think the most important element is that we want to understand what are the factors that affects our perception of taste. And also not just about taste. We have all this taste but then when we eat food we don't eat the food for one single taste. A lot of times there would be interaction of different tastes. So that we need to understand how the taste can be modified. How they would interact with each other, supressing each other or enhancing each other. So, that's the focus. So, lets look at how things can taste good. Now, I want to remind you again it is important that when we eat there is a purpose. The purpose is that we want to have nutrients. We want to obtain energy, and a lot of time as we pointed out last week we want to be satisfied. We want to feel rewarding. All this is the major drive for us to take up food. But at the same time there's another element of it. We want to stay away from all the poisonous stuff, as well as food which is rotting. Now, so, with that, in fact, in the evolution we developed to have different sense perceptions for all this taste. Now let's focus again on each of them. For example, when we talk about sweetness. Actually what is sweetness? The very easy things that you come to is that glucose. fruits, vegetables, when you eat them, they are sweet. Why? Because they carry a lot of carbohydrates or sugars so that they would provide energy. Now, how about salty? We mentioned earlier that salty things in fact is related to availability of minerals, and have minerals available. What it means is, it's going to help to balance the electrolytes in our body, because we need minerals to function properly. Now, how about sour? Now, some of us they, we, we, like sour food but, in fact sourness by itself is always associated with acid. And when do you have lot of acid? Usually it's because when the food is rotting, and when they are rotting they become sour. So, we want to differentiate whether it's rotting or not rotting, so that gives us a sense of whether it's safe to eat the food or not. How about bitterness? Well, in fact, a lot of the natural poisons such as some of the plants you may recognize that when you eat a lot of herbs. They have a very bitter taste. Why? Because usually they are toxic and we don't want to eat them and want to stay away from them. So therefore, that bitterness tells us that, well, this is not necessarily something good that you want to try. And how about another one, which is Umami. We seldom talk about it, but then it's important then, because umami actually is a taste of amino acid. Where do we get amino acid? Protein. That's where you have the meat. And so, therefore, all these combine together. They give you a very strong sense of what is good, and what is bad. And of course I need to emphasize that this is not a one-on-one matching. Sometimes sour food can be good. Sometimes sweet things, they can be bad too. So, altogether it just gives you a general idea how this different taste is related to the quality of the food. Now, let's look at. How we taste. This is the surface of the tongue, in fact the tongue is the instrument that we use to taste the food and to see actually whether it is good or bad. On it you may notice that, well there are some tiny structures on surface of tongue, so you can go to the mirror and take a look at it and see what your tongue is like. There are tiny structures there. These are called papillae. And what happens is that papillae in fact, they are present all over the surface of the tongue. What are they made of? Papillae in fact, is like it comes on the tissue together. On them in fact, they have a lot of taste buds. And this taste buds, they are located on the papillae. And they are made up of different types of cell, which I'm going to introduce to you a little later. And altogether in fact they have a large number of cells, 50 to 100 or so, and at the tip of each of these taste buds they have an opening, and that opening allowed all the food with the taste. The taste material would come into the opening, and will be in contact with all the sensory cells inside. And food activation of the cells, we perceive the taste. Let's look at how exactly it's alike. This is a diagram of the papillae. Okay? What you see is that this is a papillae with a lot of the taste buds located inside the cleft. All right. And if you blow up those taste bud, that's what you see. On each of the taste bud, you have an opening, with this opening, inside there are cells. These are sensory-receptor cells. And these sensory-receptor cells, they have a lot of cilia-like structure stand out. And where they would have a lot of the receptors, sensory receptors, so that they can perceive the quality of the food. And at the end they would have the nerve fiber going all the way to your brain, and of course there are other support cell that associated with it. Now let's look at what are those cells. First of all we say that there are support cell you don't want to put a whole group of neurons sitting there without anything supporting them. You remember last time we talked about the nervous system. We have supporting cells to ensure that the neurons, they are kind of like a protector somehow. So we have this supporting cell, and we have the sensory receptor cell. These are the cells that exactly they are standing out there, they are trying to perceive the signal whether there is any glucose, there is any acid or so. And these are sensory receptor cells, and very important. In these days, we talk about stem cells. In fact, in our taste buds we do have stem cells. These are the cells that are sitting right at the bottom of the taste bud, and what they do is that whenever there's some sensory cell they are old, they are because of wear and tear they are eliminated you need to replenish the taste bud with sensory cells. So this stem cell which we call a basal cells, from the bottom of the taste bud would continue to produce more new cells. And replenish the entire taste bud with the different sensory cells. So on average, every ten days or so they would replace all these sensory cells. Now these are the cells present in the taste bud. And important part of it, on each of the sensory cell, I want to remind you. We need to use some sort of like a receptors, and these receptors they would be sitting on a surface of the cell, which will be in contact with the environment where the taste molecule would come in from the taste bud at a pore position. Okay? And each of these sensory cell they would have one or more different type of these receptors. So what happened is that if these receptors they can be matched with specific taste molecules. What I mean is that each of the sensory cell, in fact they can elicit response based on the stimulation by a different kind of molecules, or the taste molecule. Now, then we may ask ourselves, well are there people who are more sensitive, are there people who are less sensitive to what's taste? So, we often use this as example. We say that are there supertasters? The answer is yes. because we can look at this particular case, we say that well, on the tongue, on the papillae, in fact some of us, we are extremely sensitive to some of the compound. For example, in this case it's called n-propyl-thiouracil. some of us, in fact, we have more of this receptor, and some of us, we have fewer. So the range would go from, if you look at the normal population, is let's say on average, normal taster, have about 50% of the population have the normal taster. But then about 1/4 of them in fact they have a lot more of these receptors so that. They can sense more of this particular taste, and they can perceive more sensitively. So what happens is that in fact they have a lot more papillae and on their tongue so that they can perceive this particular taste very strongly. And of course we also have individuals which they have fewer of this particular papillae, and so end up that they are not as sensitive. So what happens is that this particular n-propyl-thio-uracil. In fact, where is it coming from? It is coming from a lot of the green vegetables or the fatty foods. So what happens is that some people, about one quarter of the population, in fact they are very sensitive to it, they find it very bitter, and they don't want to taste it. But some of them when we eat it we don't feel anything at all, so therefore it reflects a variety of responses, so called super tasters or non-tasters in the population. Now, when you look at the tongue, we say that the tongue, in fact, there are different regions of it. Well, you probably can't read all these words, but the most important part is that we want to tell you that the tongue have different regions. And different regions it seems that they can sense different kinds of taste. Now, the most important part that I want to link it all together, recall what we talk about last time, we say that all this taste perception is coming from the sensory cell. They come in as a neuron and then from the neuron signal will be sent towards the brain and the brain is how things are being processed. But remind you, in the brain in fact there is different component. For example, there would be area which is allowing us to have the conscious perception of what exactly taste is, whether it is sweet, whether it is bitter, whether it is sour. We also have another component in the brain including the hippocampus, that's the area that somehow can also be triggered by this sensory input. It's just to remind you that actually what happen in the past your experience, experience counts. And, of course, the final thing is that when we eat food, one very important purpose that we want to have is feel rewarding, feel satisfied. So only when all three of them, after the sensory input is integrated. We'll be able to perceive whether the food is good or bad. Okay, so that gives you an overview of what we want to cover in this particular class.
