>>Dr. Ketchum: So in this video lecture we are going to focus on membrane-bound mediated
responses. And specifically what we will be focusing on within those membrane mediated
responses are what are called ligand gated channels. So this is just to give you an introduction
to these ligand gated channels, and then we’ll have some specific information for you in
future videos. So these are the types of channels. Now you’ve already been introduced to leak
channels, and remember that leak channels are always open. Remember that leak channels
also exhibit specificity. So we’ve already seen example of sodium leak channels and potassium
leak channels. Now there are also a type of channel called gated channels. And there are
four different types of gated channels. Here we will only focus on chemically or ligand gated
channels, later in the semester we’ll talk about voltage gated, mechanically gated and
thermally gated. So what this means if you’re looking at a ligand gated channel is here
we have all of these ligands: here’s sodium, here’s acetylcholine—a ligand that you’ll
become quite familiar with later on in the semester. And here is your channel in blue.
And so, notice how the shape of the ligand binds. It’s the same shape as the site on
your receptor and that binds to the channel, and the channel will respond to that.
So now let’s go ahead and look at the two different types of ligand-gated channels.
So we have what are called fast ligand-gated channels, and we have slow ligand gated channels.
So we’re going to talk about both. So a fast ligand gated channel, the receptor is
the same protein as the channel, and a slow ligand gated channel, the receptor is a different
protein than the channel. Now fast channels are called that because they cause a response
within milliseconds, whereas a slow ligand gated channel will take minutes. So let’s start
with a fast ligand gated channel. So with a fast ligand gated channel, remember it’s
fast, so the receptor and the ion channel are the exact same protein. So how does this
work? We’re going to start with your messenger, which is called the ligand. Your ligand binds
to the receptor. You could also say the ligand binds to the channel since they’re the same
protein. So then once that happens, the channel opens. And it does not close; it will only
open when a ligand binds to it. That’s really important. So when the channel opens, then
you have ions that can move into or out of the cell. Now some of these channels will
exhibit specificity. So perhaps you will only have chloride moving through this channel,
or you’ll only have sodium or maybe potassium. Or in some cases, you could have both sodium
and potassium moving through the same channel. So in this example, let’s just assume that
you have sodium moving into the cell. So we have what’s called sodium influx. So sodium
carries a positive charge, and since sodium is moving into the cell, the cell becomes
or the intracellular fluid becomes more positive. So what we’ve done then is we’ve changed
the electrical properties of the cell.
Now here’s an example of a fast ligand gated channel called a calcium channel. And so in
this particular situation since it’s called a calcium channel, we know that calcium is
going to be the ion moving through the channel. So here’s our ligand. It’s bound to our
channel, which again is also the receptor. And when the channel opens, calcium enters
the cell through the channel, and then that causes a number of target cell responses.
Since there are positive charges coming into the cell that changes the electrical property,
right? The intracellular fluid is more positive. The calcium that’s moving into the intercellular
fluid can be used for muscle contraction; it can cause secretion. You can also use calcium
as a second messenger, which we’ll talk about later. Calcium then can combine to something
called calmodulin, which can activate an enzyme called protein kinase. And we might as well
go ahead and remind you that protein kinase functions via covalent regulation: It’s
an enzyme and covalently regulates. And so what protein kinase always does is it always phosphorylates
a protein. And then you get some sort of response inside of the cell. So again, this is just
to give you an idea of what a fast ligand-gated channel is and how it can change the properties
within the cell. So you can change electrical properties, you can have contraction occur.
It just depends on what specific tissue that you’re in. And we haven’t gotten to what
tissue we’re in yet. Right now it’s just understanding what a membrane-bound mediated
response is and the fact that you can have these fast ligand gated channels, the channel
will always open, the channel and the receptor are the same protein, and you get some sort
of response in the cell.