To start off talking about action potentials,
we need to set up some basic parameters.
The basic parameters
just so happens to be
what's inside a cell
versus what’s outside the cell.
So, we have three ions that
we’re going to concern ourselves with.
Why only three?
Because they're the most important.
We have potassium.
We have sodium.
And we have calcium.
The big thing to remember is that
potassium is high within the myocyte,
and these are cardiac myocytes,
and it’s low outside.
Sodium and calcium are the opposite.
They are high in the interstitial fluid,
but low within the cardiac myocyte.
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.
So, simply by having two ions in different concentrations.
That concentration gradient can be used
to spark electricity or voltage changes to happen.
One of the other things,
which we almost always label as G – so, whenever you see a G,
know we’re talking about
conductance across the membrane.
What helps set up these
concentration differences are pumps.
These electrogenic pumps,
such as the sodium potassium ATPase,
which pumps out three,
pumps in two
sodium and potassium molecules.
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.
So, you always set up a little bit of
a difference between the two
across the membrane
because of this 3 to 2 relationship.
The other thing to think about
when you're talking about
a membrane potential change
is it matters how fast that conductance happens.
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.
So, now, let's talk through the key ions –
potassium, sodium and calcium.
Why are they changing at different times
and how does this work with the various channels?
So, let's start with our resting potential.
During rest, the potassium channels are open,
which allows potassium to move
from inside the cell to outside the cell.
Now, now, now, why does it want to do that?
Because of the concentration difference.
It’s high on the inside
and low on the outside.
So, more of the potassiums
within the cell
want to move out into the interstitial space
simply because of that concentration difference.
Now, both sodium and calcium,
they want to get in.
Because they’re high outside,
they’re low inside,
they want to travel into the cell,
the gates are closed.
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.
This yields a hyperpolarized state.
What do we mean by hyperpolarized?
It means that the resting membrane
potential is going to be lower than normal.
Lower than normal.
About 90 mV.
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.
It can simply be calculated.
So, resting membrane potential of 90 mV
is very common for ventricular myocyte.
Now, so that's what happens at rest.
So, even though,
you're at rest,
your ions are still moving.
They're not really stationary.
But how about when you get an active process,
such as an action potential,
In the action potential environment,
we close the potassium channels.
We open up the sodium and the calcium channels.
So, sodium and potassium –
sodium and calcium want to rush in the cell.
Why do they want to rush in?
Remember, they’re at high concentration outside the cell,
low concentration inside the cell.
So, they simply want to travel down that gradient.
Potassium is prevented from leaving.
So what this does is depolarize the cell.
So, why is it depolarized?
Depolarization means that membrane potential increases.
Why would the membrane potential increase?
Because a positive molecule,
so it became more positive.
Calcium traveled in,
became more positive.
You’ve prevented the sodium from –
sorry, the potassium from leaving,
which means more positives build up.
So, you have three good reasons
why the membrane potential will increase.
And this is a depolarization.
So, you might go up to
maybe a positive 10 mV,
so that it contrasts resting potentials and action potentials.
Why do they occur?
Concentration gradient differences between the ions
and then which channels are open
and which channels are closed.