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.
Time ones again, the link those currents with the associated channel in the myocyte action potential.
Here we're gonna go all the way from phase 4 through 0, 1, 2 and 3.
The most important channel to link up with its associated current, is the voltage-gated sodium channel.
This is our primary item that it's engaged during threshold or phase 0, and then it's deactivated during phase 1.
Its scientitic name is NAV 1.5.
Now, besides this sodium channel, we have a couple of potassium channels to deal with.
The first one are these fast-acting voltage-gated potassium channels.
And these are really only inacted during phase 1, these are very --
in doing that transient potassium current.
The L-type calcium channel, also very important during the cardiac myocyte,
here, the L-type calcium channel or the CAV 1.2,
are activated during phase 1, phase 2, and are deactivated during phase 3.
They're most associated with the plateau portion of phase 2.
And our final current to channel correlation that we need to make sure we bring out here,
is our delayed potassium channels.
These ones are activated primarily during phase 3 to bring membrane potential down closer to baseline,
and then finally also, during the phase 4, they are active.