After we understand how the tongue is organized, with the papulae, the taste bud, and the sensory cells. So, what kind of taste can we sense? In fact, normally when we say the tongue can taste, we say that we can taste taste bitterness, sourness, saltiness, and sweetness. But there are more than four different tastes. So what are they? And they actually have some important implication. For example, bitterness as we said. In fact, bitterness is caused by compounds such as phenylthiourea, polythenols, which we found in a lot of plants, flavanoid, terpenes, organic bases like glycosides. So, these are usually associated with toxic substance. How about sourness? Actually what we are sensing is the amount of hydrogen ions present in the food. So it always comes with the acid such as the lactic acid, acetic acids, citric acid, like that. How about the saltiness? It's largely because of the minerals such as sodium, is because of all this ionization of organic salt that gives the sense of saltiness. How about sweetness? In fact what we are monitoring is the presence of the hydroxy group in a lot of these sugars such as glucose or fructose, and sometimes even ethanol or glycerol. Now, the final one in fact is umami. I said that is a taste of amino acid. What exactly it is, amino acid, we say that actually, and the nucleotides. Both of them can give you the sense of umami, and that is largely because of this glutamate structure. Now, these taste substance, they have one common property. Whatever it is, acid, minerals, these polyphenols, or amino acid, they are all polar, that means they carry charges, and they are water soluble, so that they can easily be dissolved in water and so that when we taste, we want them to be in the water as a solution, so that we can perceive them. And most of them, they are relatively stable in water because they are not volatile. So they are not going to be vaporized and disappear gradually. All right. So this is the collection of tastes that we're going to sense. And a lot of times when you read the textbook you may see that well, along the tongue we have different regions of the tongue that they can taste different kind of Bitterness sourness saltiness or sweetness. And they have a taste map like that. Later on in this particular section, we're trying to go through another exercise to find out whether it's real. Now let me talk a little more about the science of it. We say that in the sensory cell in the taste bud they have a lot of micro-villi there. What they have is that in fact in this particular cell on this micro-villi, they have a lot of all these receptors and these receptors can be generally classified into two. One of it we call it the ion channels basically look at that and say that's a channel sitting on the surface of a cell. And whenever there is any salt or hydrogen ion, they come through it. They would depolarize the cell, and therefore it trigger the firing of this neuron. The other type of it is for the sensation of the sweetness, bitterness and the umami. These, we call them G-protein coupled receptors. Sounds difficult but very simply they are simply receptors, they have very specific configurations so that they would be able to capture the bitterness molecule, the umami molecule, or the sweet molecule, and when they are activated they can trigger the firing. How do they do that? In fact, they all come into the cell. Leading to the depolarization of the cell, depolarization of cell through a common machinery. Now in this process when the cells get activated. What do they do? They fire out signal. And this, the polarization process results in the nerve, they would have their membrane potential being altered so that they are going to fire and trigger a spike with influx of the ion but then they quickly restore it so that this particular signal, maybe transmit it along the axon, so that they can go from one part of the neuron to the other end of the neuron. And then through some chemical connection, they reconnected to other neurons in the brain. Now, in this process, in fact, we need to ask ourselves, are there parts of our tongue which are more sensitive to a particular favor or all parts of the tongue they are equally sensitive to same flavor? Now, this it is quite a bit of dispute because it varies from people to people. So, let me put it into some of the examples. For example we say that the taste bud we can sense the sodium choride only. Or actually we can sense both the sodium choride and the sucrose. We find that in fact within the same taste bud there are different senosry cells some of the sensory cells in fact they can sense sodium choride. but some of the sensory cells in fact they can also sense sucrose so therefore what it means is that within one taste bud, in fact they can sense both sodium chloride and sucrose. Now so but then question is that what's the composition of all the sensory cells in this taste bud? Sometimes they have more sensory cells which sense one. Sometimes they have more sensory cells than the other. So therefore, that would result in a different kind of sensitivity. A second part of it, we say that the bitter receptor, in fact they are expressed in some, a subset of the taste cell in all the papillae on the tongue. This is done in the experiment in the rats. but however what we find is that these receptor, they are more densely populated at the side and the back of the tongue. So therefore usually people would say well when I sense bitterness usually it's coming from the back of your tongue or the side of your tongue. Now, this tells you that the taste map again, it varies. If you look at the sensitivity of our tongue to all the tastes, they are distributed across the entire tongue in different regions. So, what we need to consider is that, when we say that we are sensitive to particular taste, we need to question whether it is because of the density of the papillae, the density of the sensory cell, and also how the sensitivity is related to the number and amount of receptors available in that particular region. Now, very easily we can use an example as an illustration. We say that when we sense sugar we are sensing glucose. And in fact the sensation of sugar is triggered by a receptor we call the T1R3. this particular receptor is a G protein couple receptor. A receptor. it is present in our tongue. Are they evenly distributed? The answer is no. They are not evenly distributed. They're more heavily focused on the tip of your tongue. But the question is that can different regions of our tongues sense the sweetness? The answer is yes. In fact we find that this receptor is not just present in the tongue. In our entire intestinal tract in fact there are also this receptor present. So, you recall that we talked about, sometimes we want to sense fullness after eating some meal. And for experiments what we do with animals, we can inject glucose into their duodenum. And in fact they can still feel the fullness. So, somehow, that must be a relationship of that. So, in the real life, when we look at this particular receptor, look at where they express, in fact we find they are expressed in the taste buds. That means your taste buds have the receptor to sense the sweetness. But it is more than that, we see it is present in the nasal epithelia cell. Well, somehow In your nasal cavity. So why are they there? we have this receptor in the trachea near the lung. We have it in the stomach. We have it in the bile duct. And we have it in the intestine. So basically what it says is that in fact these receptors, they are not just there for us to perceive the taste. They probably would have additional function elsewhere along the intestinal tract so that allow them to monitor the presence of glucose somewhere in the body
