Cardiac Pacemakers – Heart Rate and Electricity

00:01 Heart rate and electrical propagation.

00:05 Here, we'll need to think a lot about where the electrical propagation starts.

00:11 The cardiac pacemakers are the most important aspect to get everything started.

00:16 From here, we'll usually look at the SA node, which is the sinoatrial node, as our primary pacemaker.

00:25 This will start off the whole electrical activity of the heart, which will make sure each heartbeat beats in unison, so you get a lot lub-dub sound and also, so you can have an intrinsic rate.

00:42 The AV node is the secondary pacemaker.

00:46 If the SA node is not operating correctly, it can take over the job.

00:50 However, it has a lower intrinsic rate than the SA node, so you always have a slower heart rate when the AV node is engaged.

01:00 We have other areas of the heart, such as, as you go from the base to the apex, that will also have Purkinje fibers that will help you propagate that action potential out to all the ventricular myocytes.

01:14 But, now, let's talk about how in the world you get these sparks to start.

01:19 The sparks start based upon action potentials.

01:23 So, there are two different types of action potentials we need to deal with.

01:26 One are pacemaker cells, which are the SA node, the AV node, and maybe the Purkinje fibers.

01:34 Then, we have non-pacemaker action potentials and those usually involve the ventricular myocytes.

01:41 So, what do we need to know specifically about pacemaker cells? What makes them so important? Well, they have no resting potential.

01:51 They also have spontaneous depolarization and repolarization.

01:56 They are also slower in their rise of depolarization than a non-pacemaker cell.

02:04 So, those are the three important principles.

02:07 What do we really mean by no resting potential? That sounds like an odd thing to say.

02:13 But what it means is there is no flatline during resting.

02:17 It's always progressively either depolarization or repolarization.

02:23 There's not a isoelectric line.

02:27 The spontaneousness or spontaneity of the response involves that this drift always happens until you reach some sort of threshold.

02:37 Once you reach threshold, an action potential can occur and you can propagate that action potential down the whole system.

02:44 Let's contrast this now to non-pacemaker cells just so we have something to compare our pacemaker cells to.

02:52 They have a true resting membrane potential.

02:55 What do we mean by that? You have flat lines at the edge of each of the action potentials.

03:01 Flat lines.

03:04 Those flat lines allow for there to be longer amount or a stable baseline.

03:11 They will not spontaneously depolarize or re-polarize because of those flat lines.

03:18 They also have another principle that's different and that is their action potential widths are longer, have a wider width.

03:27 That is a prolonged repolarization.

03:30 The other thing that's interesting is that depolarization rate is really fast.

03:37 So, those are the key differences between pacemaker and non-pacemaker cells.

03:43 Now, let's talk through how this works throughout the whole heart to make sure you get that heartbeat – lub dub – to occur.