Voiceover: Have you ever
wondered how our nervous system like our neurons and our
brain, can react so quickly? Well, the reason is because of ligand-gated ion channels. Ligand-gated ion channels are one type of major membrane receptors. The three categories are: ligand-gated ion channels, G protein-coupled receptors, and lastly enzyme-linked receptors. Today we're going to focus on the ligand-gated ion channels. These are also called ion
channel linked receptors. Ligand-gated ion channels are transmembrane ion channels
that open or close in response to the binding of a chemical messenger like a ligand. Like we mentioned, a very common place to find ligand-gated ion channels are in electrically
excitable cells like neurons. The reason why is because
these ion channels react really quickly to
the binding of ligands, and so they're very commonly found in cells that need to react
very quickly to stimulus. When we talk about
transmembrane ion channels, what we really mean is
they are transmembrane or integral proteins
that also have a channel, a hole through them, in which things can move in and out. To start out with, let's
say we have an ion channel that looks something like this. Let's go ahead and color that in. This particular channel
at the moment is closed. Right now, we have our
intracellular environment, this is where our cytosol
and all of that stuff is. This is our extracellular environment. As you can see, this ion channel has a weird kink in it, and this is a place where a messenger, like a ligand or a neurotransmitter can bind. Let's say we have a ligand
that looks like this. This is our ligand. This ligand can bind in there. Notice that the shape of our ligand is complementary to the
shape of our channel, so it fits right in there. Only specific ligands can
bind to specific channels. This is what we call our "lock and key", or a more updated one
is called "induced fit". Once this ligand binds,
what it'll actually do is it'll cause this closed channel to actually open up. We'll actually see this channel open up, kind of like that. Now this is our open channel. One really interesting thing about ligand-gated ion channels, is if you look at where this ligand binds, the binding site of this ligand is not anywhere near the actual channel. The reason why is because this is what we call allosteric binding. The ligand binds to what
we call an allosteric site. This is a place that's
away from the ion channel. But what happens is,
once the ligand binds, it can control the opening and closing of the ion channel by altering the protein conformation
of the entire protein. Once it binds, a channel
opens in a different place, and the ion permeability of the entire plasma
membrane can quickly change. Remember this is not
just one thing happening. When these ligands are binding, there are many of these channels scattered throughout these cell membranes, so all of these are opening
and closing all at once. Once this channel opens it'll let ions, like potassium, sodium,
chlorine or calcium being the most common, move
through the open channel. Once these ions are moving in and out, this will cause a change in the electrical properties of a cell. In other words, you'll convert this extracellular ligand signal, into an intracellular electrical signal. Once these ions move in, or they can also move out, you'll have an intracellular
electrical response, electrical signal happen inside the cell. There are two things I'd
like to note real quick. The first is that the allosteric
binding site of a ligand, this area that is
complementary to the ligand, can be intracellular, it
can be inside the cell, but that's considered pretty rare. Why might that be? Well, we'd have to think about the main purpose of membrane receptors. We have to realize that they generally are meant to respond to
extracellular signals, to things that are on
the outside of the cell. Generally speaking,
ligand-gated ion channels will have the binding site
on the extracellular side. The second thing to note is
that it is possible for there to be multiple allosteric
binding sites for ligands. It's possible that there
are multiple of these kinks, of these complementary shapes, that let the ligands
bind for each protein. Finally, I'd like to clear
up two quick misconceptions. Ligand-gated ion channels
are easily confused with two types of ion channels. They are not the same as
voltage-gated ion channels. Voltage-gated ion channels rely on the difference in membrane potential. As we recall, ligand-gated ion channels actually respond to the
binding of a ligand. The voltage-gated ones only depend on the difference in membrane potential. The second one they're
easily confused with is what we call
stretch-activated ion channels. As the name implies,
stretch-activated ion channels depend on what we call the deformation of the cell membrane, or the cell membrane stretching and being pushed and being stressed. In summary, ligand-gated ion channels are one type of membrane receptors. They are transmembrane ion channels that open or close in response to the binding of a chemical
signal like a ligand. You'll notice that here
our channel is closed, and once this ligand binds
to an allosteric site, which is a site that's
not on the channel itself, it'll let ions such as
potassium and sodium and so on move through the membrane. This will actually cause an intracellular electrical signal. We have one outside ligand, and this will allow us to have an intracellular electrical signal. This will actually tell
the cell to do something. Ligand-gated ion channels
are not to be confused with voltage-gated channels, which only rely on a difference
in membrane potential, and they are not to be confused with stretch-activated ion channels, which are affected by a deformation of the cell membrane.