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Non-Pacemaker Action Potential – Heart Rate and Electricity

by Thad Wilson, PhD
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    00:01 So, now, we can look at a ventricular myocyte action potential.

    00:07 So, this is not a pacemaker.

    00:08 This is the ventricular myocytes, the ones that do all the work.

    00:13 So, not the pacers, the ones that do the work.

    00:16 All right.

    00:17 So, here, we start off with what – I know it's the heart.

    00:22 We started with four.

    00:23 Why do we start with four? I don’t know.

    00:25 We start off with four.

    00:26 That’s how they’re always going to term it.

    00:28 So, you have to know it.

    00:29 They start with 4.

    00:30 Four is resting membrane potential.

    00:34 But notice, for a ventricular myocyte, the line is flat.

    00:40 There's no spontaneity in phase 4.

    00:45 Phase 4 is flat.

    00:47 It doesn't slope up.

    00:49 So, there's no way it's going to reach potential on its own.

    00:52 It needs to be stimulated to cause an action potential.

    00:56 What is it stimulated by? Well, pacemaker cells.

    00:59 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.

    01:09 Let's look at phase – the next phase.

    01:12 If you have a signal via the gap junction that travels through and stimulates what? Fast sodium channels.

    01:22 You get phase 0.

    01:24 So, phase 0 is when the sodium rushes into the cell.

    01:30 Note that phase 0 is steep.

    01:33 It’s fast.

    01:35 A lot of sodium travels through.

    01:38 Now, that is different than phase 0 in the pacemaker cell.

    01:44 That was driven by calcium.

    01:46 So, there's a difference between these two cells based upon which channels open.

    01:52 So, phase 0 involves fast sodium channels, a little bit of calcium, and also there is this transient outward potassium current.

    02:03 And that’s that little blip you see at the top.

    02:06 The prolonged portion is governed by calcium.

    02:11 So, this is a calcium current that travels for a longer period of time.

    02:16 That is what phase 2 is.

    02:19 Phase 3 involves that repolarization, that delayed potassium response, and that brings membrane potential back down to phase 4.

    02:31 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 3, then back to 4 again.

    02:42 So, you didn't think we were going to skip numbers, did you? We didn't.

    02:46 We just had to get them by bringing up the ventricular myocyte action potential.


    About the Lecture

    The lecture Non-Pacemaker Action Potential – Heart Rate and Electricity by Thad Wilson, PhD is from the course Cardiac Physiology.


    Included Quiz Questions

    1. Calcium
    2. Sodium
    3. Potassium
    4. Chloride
    1. Phase 0
    2. Phase 1
    3. Phase 3
    4. Phase 2
    5. Phase 4

    Author of lecture Non-Pacemaker Action Potential – Heart Rate and Electricity

     Thad Wilson, PhD

    Thad Wilson, PhD


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