Gustation – structure and function | Processing the Environment | MCAT | Khan Academy
- Articles, Blog

Gustation – structure and function | Processing the Environment | MCAT | Khan Academy

Voiceover: We have 5 main tastes. Each taste depends on
a particular receptor that’s localized somewhere on the tongue. So the 5 different things
that we’re able to taste are bitter compounds, salty compounds, sweet compounds, sour compounds, and one more thing known as umami. This is basically the
ability to taste a particular molecule known as glutamate, so glutamate. Now these 5 tastes all depend on a particular receptor. So let’s imagine that
here we have a tongue and this is the front of the tongue, this is the back of the tongue. We basically have taste buds sprinkled throughout the tongue so these taste buds are kind of all over the place and for the most part they are very, very highly localized in the more anterior aspect of the tongue so they’re most localized in this anterior aspect of the tongue moreso than in the back. There are 3 different types of taste buds. There are taste buds that
kind of look like this and these are known as
fungiform taste buds. I’ll write that down. Fungiform taste buds and these are the ones
that are mostly found over here in the anterior
part of the tongue. There are also taste
buds that look like this. where the little buds are over here and these are known as foliate taste buds and these taste buds are
mostly found over here on the side of the tongue. And finally there are taste buds knows as circumvallate taste buds
which look a little like this. And the taste buds themselves are actually over here on the sides and these circumvallate
taste buds are mostly found back here in the back of the tongue. Now each one of these taste buds, if we actually zoom in, so if we kind of zoom into
an individual taste bud what we would find we
would find a little pore. And this pore would have a bunch of different types of cells. It would have like one cell over here. It would have another
cell right next to it. It would have a third
cell over here and so on. And basically each one of these cells is responsible for one of these 5 tastes. So for example, we can take
this pink cell over here to be able to detect salty things. For example we could imagine
that this whitish cell would be able to detect sweet things and so on and so forth. So the general idea is
that each taste bud, so this thing would be a taste bud, each taste bud contains all
of the different taste cells so every small taste bud
is able to detect bitter, salty, sweet, sour and umami compounds and these taste buds are found all around the tongue mostly in the anterior part of the tongue. So another way to say
this is that we are able to taste all 5 of these different tastes everywhere in the tongue so it’s not as if bitter compounds can only
be tasted in this region and salty compounds can only be tasted in this region and sweet compounds only in this region. That’s not the case. This is not the case
so it’s not like that. Instead we basically are able to taste all 5 of these different
compounds throughout the entire tongue and mostly most of the taste buds are concentrated over here in the anterior part of the tongue. So what I want to talk about next is something known as
a labelled lines model. So here we have a taste bud and as I mentioned before we have all the different types of cells and each one of these cells is specialized to a particular one of the 5 tastes and what you can see down
here is that each cell has a little axon projecting from it and what is interesting
is that these axons actually remain separate
all the way to the brain so all these get projected and they eventually reach the brain and they actually synapse on different parts of the brain so
let’s imagine that this is the gustatory cortex so
let’s imagine that this is the part of the brain that receives input from various taste cells while each one of these different axons will synapse on different parts of the brain. So here we’ve got this
axon that’s going to go it’s going to synapse over here and every other axon,
every other taste cell that is this color blue so let’s just say this is a sweet cell. Let’s say this is a sweet taste cell every single sweet taste
cell will send its axon to the brain and it will all end up in this one region of the gustatory cortex so we can say this is the sweet region of the gustatory cortex. I keep using this word gustatory and taste can also be thought of as
our sense of gustation, so gustation. Similarly all
the cells that are green and we can say that green could be bitter so we can say the green
cells are bitter taste cells, they’re really bitter. They will all synapse in
one part of the cortex which we can imagine to be the bitter part of the cortex. It’s very mad at the world. So this is basically how taste cells send their projections to the brain. They actually send them
through dedicated axons and there is no mixing and this is known as the labelled lines model. So let me just write that down here. Labelled lines model. And again what the labelled
lines model is saying is that each one of these cells has its own dedicated labelled line and this distinction
is basically carried on all the way to the cortex itself. Now let’s look at an
individual taste cell itself. So let’s look at an individual taste cell. So here’s the axon. This is the cell nucleus and little hairs that kind of project out into the tongue. So what happens is let’s say
that this is a sweet cell. A sweet taste cell so it’s
sensitive to sweet molecules so let’s imagine that we’ve got a little molecule of glucose. So let’s imagine you’re eating cake. Some glucose will hit the tongue and some of the glucose will find its way over to a sweet cell and
why is this cell sweet? Well it’s sweet because
it actually has receptors in the membrane so it has little receptors in the membrane that
are sensitive to glucose and other sweet molecules so when the glucose binds to the cell it triggers a cascade of events that eventually allows
this cell to depolarize and send an action potential all the way down its
axon and to the brain. Now I mentioned that we
have 5 different tastes. We have sweet, umami,
bitter, sour and salty. These top 3 taste cells over here have similar receptors and these receptors are known as G-Protein. So G-Protein coupled receptors. So basically what a
G-Protein coupled receptor is let me just go ahead and draw it out. It’s an inner membrane protein so this is the protein
and then let’s imagine this is the cell membrane. So this protein is a receptor and the receptor binds to a ligand so let’s imagine that we
have glucose in this case so let’s imagine that a molecule of glucose comes in, hits the receptor. The receptor will undergo
a conformational change that basically causes a G-Protein which is coupled to it so this is a G-Protein over here so when the ligand binds to the receptor it causes the G-Protein to dissociate so this bond is broken and
the G-Protein goes off. The G-Protein can do a
couple of different things inside the cell so this is
inside the cell over here. This is outside the cell. So basically the G-Protein can go off and do a few different things. Now one of the things
that the G-Protein can do when it gets dissociated
from this receptor is it can actually open some ion channels so the G-Protein can go
and open ion channels. So the opening of ion channels the conformational
change can actually cause the cell to depolarize and
fire an action potential. So fires an action potential. So on the other hand these 2 taste cells sour and salty rely on ion channels. So instead of relying on a G-Protein coupled receptor they
actually rely on a receptor. So let’s imagine this receptor over here. So in the case of salty let’s imagine that a molecule of NaCl. So a little salt molecule comes in it will bind to this ion receptor and then cause the receptor to open up. So the receptor will open up and it will allow positive ions outside the cell to flow in. When positive ions flow inside the cell it causes the cell again to depolarize and fire an action potential and that action potential
goes to the brain. So let’s look at what
would happen if we put a salty receptor inside a sweet cell. So here we have a sweet
cell and the reason that this cell is sweet
is because it will have receptors in its membrane
that bind to glucose. So let’s imagine that
we took a salty receptor so as I said previously
the salty receptors are ion channels and
let’s imagine that we put a little salty receptor here. Now if you remember the
labelled lines model basically says that a particular cell, so in this case a sweet cell has an axon that will eventually reach the brain and when it reaches the brain the brain since the axon
is coming from a sweet cell will detect the compound that activated the sweet cell as sweet. So what would happen if we put a salty receptor inside a sweet cell. Well if we have NaCl so some salt comes in and it will activate this receptor and when this receptor is activated it opens up and positive ions outside flow into the cell. This sweet cell depolarizes and causes an action potential to
fire and by the time the action potential reaches the brain the brain isn’t able to
differentiate between a sweet molecule or a salty molecule and a sweet molecule
so both salts and sugar will activate the sugar cell, but since the brain has
already decided that hey, this is a sugar cell and every time this cell normally activates it’s usually something sweet it’s gonna think that the salty compound is actually sweet. It’s gonna detect it as being sweet. So if you were to put a salty receptor in a sugar cell you could
actually trick your brain into thinking that something that is salty such as NaCl, sodium
chloride, was actually sweet.

About James Carlton

Read All Posts By James Carlton

22 thoughts on “Gustation – structure and function | Processing the Environment | MCAT | Khan Academy

  1. foliate, circumvallate and fungiform are tongue papillae where taste buds are located, right? they are not the real taste buds. That is one of the misconceptions.

  2. Ok so the narrator said that intramembranous g protein, after being dissociated, can do a couple of things. One of them is opening ion channels and he did not talk about the other one right?

  3. what? will salt molecule even activate the sweet cells? it's not quite possible naturally, right? only sweet molecules activate the sweet cells.

  4. "and the bitter cells synapse at the bitter part of the cortex because, you know, its very mad at the world", lmbo, didn't anyone else catch this?!!!

  5. But if the salt molecule only activates via ion channels how can it go on a sweet receptor and activate a g protein?

  6. The video is clearly not made by an expert rather a person who just googled five minutes before making the video.Taste buds are different from taste papillae and taste receptor cells do not have axons rather they synapse with neurons having axons going to brain. This video according to me is merely jargons for a layman.

  7. To all mcat students (and actually, all students), I highly recommend to watch the crash course video on taste and smell to clear up a lot of misconceptions/misinformation in this video as many have pointed out in the comments

Leave a Reply

Your email address will not be published. Required fields are marked *