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.
King L. ChowProfessor
Lam Lung YeungAssociate Professor
