Cardiac Action Potential

00:01 So let's go through the electrical activity in the heart.

00:04 So, when a myocardial cell depolarizes; that is it has a resting potential electric current across its membrane, and then suddenly that goes to zero.

00:15 When that happens in a process called the action potential it results in a trigger of the myocardial contractile machinery.

00:24 So you have the electrical depolarization, contraction, electrical depolarization, contraction.

00:30 So, there is a pacemaker which spontaneously depolarizes high in the right atrium.

00:37 That's the little round green dot high in the right atrium in the diagram on the left here.

00:44 And that area has very specialized myocardial cells. They depolarized spontaneously.

00:52 Sodium spontaneously leaks into the cell through some channels that are called the funny channels cuz no other sodium channel leaks like this.

01:02 And when they get to a certain point, it triggers the action potential.

01:06 And then what happens is the wave of electrical depolarization passes from the sinus node through the atria down into the septum of the - between the two ventricles.

01:19 There's a pause at the AV node and that's the second little round green circle. Why a pause? Because if it went immediately into the ventricle, both the atria and the ventricle will be contracting at the same time.

01:32 Not a good idea because then nothing would go off the aorta, there would be no effective cardiac output.

01:39 So there's a pause while the atria finishes its contraction, filling the ventricles.

01:46 And then the way the depolarization passes down, past the AV node into the His-Purkinje system and throughout the ventricle, passing down first the septum to the apex and then backwards to the base of the heart.

02:01 And when that happens, it triggers contraction and the ventricles squeezes.

02:06 And here you see - here's a whole series of action potentials for the different myocardial cells as the wave of depolarization passes down.

02:15 This is the - you'll see the action potential for the sinus node which is as you can see slowly rising.

02:25 There's a spontaneous depolarization.

02:27 But once you get into the atria and to the ventricle, the depolarization is not spontaneous, it's immediate as the wave of depolarization arrives.

02:36 And so - then here we see again going down through the atrium and into the - past the mitral valve, past the tricuspid valve into the ventricle, you're in the His-Purkinje system now.

02:49 And then eventually, you finish and you get to the T wave as everything resets itself, getting ready for the next beat.

02:58 It's interesting that the wave of depolarization doesn't go through the ventricle all at the same place.

03:07 It comes down the septum, arrives at the apex and then turns back and does the base.

03:13 So what that means is that if you're an observer - you're an electrode watching this, when the electrical current and the wave of depolarization is coming towards you, the EKG records an upstroke.

03:27 When the electrical depolarization passes the apex and starts to go back towards the base, in other words going away from an electrode, you see a negative deflection.

03:37 And that's why for example, you saw on the QRS, there's some positive and there's some negative depending on where the electrode is looking.

03:45 If an electrode's looking from here, it's going to see different than if the electrode's looking from here because the way the depolarization from here will be coming towards this one at a certain point when it's going away from the one over here and vice versa.

03:59 And we'll see some examples of that.

04:01 And here in the -- we continue -- we see the wave of depolarization passing from the apex back up to the base until the entire - both right and left ventricle have depolarized and have contracted.