00:02
To start off talking about action potentials,
we need to set up some basic parameters.
00:08
The basic parameters
just so happens to be
what's inside a cell
versus what’s outside the cell.
00:15
So, we have three ions that
we’re going to concern ourselves with.
00:18
Why only three?
Because they're the most important.
00:22
We have potassium.
00:23
We have sodium.
00:24
And we have calcium.
00:26
The big thing to remember is that
potassium is high within the myocyte,
and these are cardiac myocytes,
and it’s low outside.
00:35
Sodium and calcium are the opposite.
00:37
They are high in the interstitial fluid,
but low within the cardiac myocyte.
00:43
Why or how can we utilize these ionic differences?
It is the ion differences that will change
the conductance across the cell membrane,
which generates that action potential.
00:57
So, simply by having two ions in different concentrations.
01:01
That concentration gradient can be used
to spark electricity or voltage changes to happen.
01:09
One of the other things,
besides conductance,
which we almost always label as G – so, whenever you see a G,
know we’re talking about
conductance across the membrane.
01:19
What helps set up these
concentration differences are pumps.
01:24
These electrogenic pumps,
such as the sodium potassium ATPase,
which pumps out three,
pumps in two
sodium and potassium molecules.
01:38
With this allows us to do, though,
is create a difference in the potential
because we've exchanged different amounts of positive charges –
three versus two.
01:49
So, you always set up a little bit of
a difference between the two
across the membrane
because of this 3 to 2 relationship.
02:01
The other thing to think about
when you're talking about
a membrane potential change
is it matters how fast that conductance happens.
02:11
Does the conductance change occur very quickly,
like we saw in the non-pacemaker action potential,
or does it occur slower
as what would happen
if you're in a pacemaker action potential?
Those are the key things to keep in mind.
02:30
So, now, let's talk through the key ions –
potassium, sodium and calcium.
02:37
Why are they changing at different times
and how does this work with the various channels?
So, let's start with our resting potential.
02:46
During rest, the potassium channels are open,
which allows potassium to move
from inside the cell to outside the cell.
02:55
Now, now, now, why does it want to do that?
Because of the concentration difference.
03:01
It’s high on the inside
and low on the outside.
03:04
So, more of the potassiums
within the cell
want to move out into the interstitial space
simply because of that concentration difference.
03:14
Now, both sodium and calcium,
they want to get in.
03:17
Why?
Because they’re high outside,
they’re low inside,
they want to travel into the cell,
but unfortunately,
the gates are closed.
03:26
So, the only thing that's operated in this condition
is that potassium is leaving the cell,
sodium and calcium are prevented from entering the cell.
03:38
This yields a hyperpolarized state.
03:41
What do we mean by hyperpolarized?
It means that the resting membrane
potential is going to be lower than normal.
03:50
Lower than normal.
03:52
Why?
About 90 mV.
03:56
This happens to be about
what the potassium equilibrium potential is
for having the amount of sodium within the cell
versus out of the cell.
04:07
It can simply be calculated.
04:09
So, resting membrane potential of -90 mV
is very common for ventricular myocyte.
04:15
Now, so that's what happens at rest.
04:18
So, even though,
you're at rest,
your ions are still moving.
04:21
They're not really stationary.
04:23
But how about when you get an active process,
such as an action potential,
to occur?
In the action potential environment,
we close the potassium channels.
04:35
We open up the sodium and the calcium channels.
04:39
So, sodium and potassium –
sodium and calcium want to rush in the cell.
04:44
Why do they want to rush in?
Remember, they’re at high concentration outside the cell,
low concentration inside the cell.
04:51
So, they simply want to travel down that gradient.
04:54
Potassium is prevented from leaving.
04:58
So what this does is depolarize the cell.
05:02
So, why is it depolarized?
Depolarization means that membrane potential increases.
05:09
Why would the membrane potential increase?
Because a positive molecule,
like sodium,
traveled in,
so it became more positive.
05:17
Calcium traveled in,
became more positive.
05:20
You’ve prevented the sodium from –
sorry, the potassium from leaving,
which means more positives build up.
05:26
So, you have three good reasons
why the membrane potential will increase.
05:31
And this is a depolarization.
05:33
So, you might go up to
maybe a positive 10 mV,
so that it contrasts resting potentials and action potentials.
05:42
Why do they occur?
Concentration gradient differences between the ions
and then which channels are open
and which channels are closed.