we can look at a ventricular
myocyte action potential.
So, this is not a pacemaker.
This is the ventricular myocytes,
the ones that do all the work.
So, not the pacers,
the ones that do the work.
So, here, we start off with what –
I know it's the heart.
We started with four.
Why do we start with four?
I don’t know.
We start off with four.
That’s how they’re always
going to term it.
So, you have to know it.
They start with 4.
Four is resting membrane potential.
for a ventricular myocyte,
the line is flat.
There's no spontaneity in phase 4.
Phase 4 is flat.
It doesn't slope up.
So, there's no way it's going to
reach potential on its own.
It needs to be stimulated
to cause an action potential.
What is it stimulated by?
Well, pacemaker cells.
Pacemaker cells send that signal,
propagate it down throughout the heart
and that will cause the ventricular myocytes
to want to contract
or depolarize first.
Let's look at phase –
the next phase.
If you have a signal
via the gap junction
that travels through and stimulates what?
Fast sodium channels.
You get phase 0.
So, phase 0 is when the sodium rushes into the cell.
Note that phase 0 is steep.
A lot of sodium travels through.
Now, that is different than
phase 0 in the pacemaker cell.
That was driven by calcium.
So, there's a difference between these two cells
based upon which channels open.
So, phase 0 involves fast sodium channels,
a little bit of calcium,
and also there is this transient
outward potassium current.
And that’s that little blip you see at the top.
The prolonged portion is governed by calcium.
So, this is a calcium current
that travels for a longer period of time.
That is what phase 2 is.
Phase 3 involves that repolarization,
that delayed potassium response,
and that brings membrane
potential back down to phase 4.
So, it is in a ventricular myocyte
that we actually get to use
all the numbers, right?
We use 4,
then we use 0,
then 1 and 2,
then back to 4 again.
So, you didn't think we were
going to skip numbers, did you?
We just had to get them by bringing up
the ventricular myocyte action potential.