Membrane Potential – Heart Rate and Electricity

by Thad Wilson, PhD

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    00:02 To start off talking about action potentials, we need to set up some basic parameters.

    00:08 The basic parameters just so happens to be what's inside a cell versus what’s outside the cell.

    00:15 So, we have three ions that we’re going to concern ourselves with.

    00:18 Why only three? Because they're the most important.

    00:22 We have potassium.

    00:23 We have sodium.

    00:24 And we have calcium.

    00:26 The big thing to remember is that potassium is high within the myocyte, and these are cardiac myocytes, and it’s low outside.

    00:35 Sodium and calcium are the opposite.

    00:37 They are high in the interstitial fluid, but low within the cardiac myocyte.

    00:43 Why or how can we utilize these ionic differences? It is the ion differences that will change the conductance across the cell membrane, which generates that action potential.

    00:57 So, simply by having two ions in different concentrations.

    01:01 That concentration gradient can be used to spark electricity or voltage changes to happen.

    01:09 One of the other things, besides conductance, which we almost always label as G – so, whenever you see a G, know we’re talking about conductance across the membrane.

    01:19 What helps set up these concentration differences are pumps.

    01:24 These electrogenic pumps, such as the sodium potassium ATPase, which pumps out three, pumps in two sodium and potassium molecules.

    01:38 With this allows us to do, though, is create a difference in the potential because we've exchanged different amounts of positive charges – three versus two.

    01:49 So, you always set up a little bit of a difference between the two across the membrane because of this 3 to 2 relationship.

    02:01 The other thing to think about when you're talking about a membrane potential change is it matters how fast that conductance happens.

    02:11 Does the conductance change occur very quickly, like we saw in the non-pacemaker action potential, or does it occur slower as what would happen if you're in a pacemaker action potential? Those are the key things to keep in mind.

    02:30 So, now, let's talk through the key ions – potassium, sodium and calcium.

    02:37 Why are they changing at different times and how does this work with the various channels? So, let's start with our resting potential.

    02:46 During rest, the potassium channels are open, which allows potassium to move from inside the cell to outside the cell.

    02:55 Now, now, now, why does it want to do that? Because of the concentration difference.

    03:01 It’s high on the inside and low on the outside.

    03:04 So, more of the potassiums within the cell want to move out into the interstitial space simply because of that concentration difference.

    03:14 Now, both sodium and calcium, they want to get in.

    03:17 Why? Because they’re high outside, they’re low inside, they want to travel into the cell, but unfortunately, the gates are closed.

    03:26 So, the only thing that's operated in this condition is that potassium is leaving the cell, sodium and calcium are prevented from entering the cell.

    03:38 This yields a hyperpolarized state.

    03:41 What do we mean by hyperpolarized? It means that the resting membrane potential is going to be lower than normal.

    03:50 Lower than normal.

    03:52 Why? About 90 mV.

    03:56 This happens to be about what the potassium equilibrium potential is for having the amount of sodium within the cell versus out of the cell.

    04:07 It can simply be calculated.

    04:09 So, resting membrane potential of 90 mV is very common for ventricular myocyte.

    04:15 Now, so that's what happens at rest.

    04:18 So, even though, you're at rest, your ions are still moving.

    04:21 They're not really stationary.

    04:23 But how about when you get an active process, such as an action potential, to occur? In the action potential environment, we close the potassium channels.

    04:35 We open up the sodium and the calcium channels.

    04:39 So, sodium and potassium – sodium and calcium want to rush in the cell.

    04:44 Why do they want to rush in? Remember, they’re at high concentration outside the cell, low concentration inside the cell.

    04:51 So, they simply want to travel down that gradient.

    04:54 Potassium is prevented from leaving.

    04:58 So what this does is depolarize the cell.

    05:02 So, why is it depolarized? Depolarization means that membrane potential increases.

    05:09 Why would the membrane potential increase? Because a positive molecule, like sodium, traveled in, so it became more positive.

    05:17 Calcium traveled in, became more positive.

    05:20 You’ve prevented the sodium from – sorry, the potassium from leaving, which means more positives build up.

    05:26 So, you have three good reasons why the membrane potential will increase.

    05:31 And this is a depolarization.

    05:33 So, you might go up to maybe a positive 10 mV, so that it contrasts resting potentials and action potentials.

    05:42 Why do they occur? Concentration gradient differences between the ions and then which channels are open and which channels are closed.

    About the Lecture

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

    Included Quiz Questions

    1. Potassium channels
    2. Sodium channels
    3. Calcium channels
    4. Chloride channels
    1. Na low, K high, Ca low
    2. Na low, K high, Ca high
    3. Na high, K low, Ca high
    4. Na low, K low, Ca low
    5. Na low, K low, Ca high
    1. -90 mV
    2. -75 mV
    3. 90 mV
    4. 75 mV
    5. -60 mV

    Author of lecture Membrane Potential – Heart Rate and Electricity

     Thad Wilson, PhD

    Thad Wilson, PhD

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