010 Repolarization: Phase 2 of the Action Potential

Ok, so by now you should have an understanding of Depolarization: Phase 1 of the Action Potential. If not, then what are you doing here? Don’t watch this video as yet. Check out the previous video first :)

Now your ready to learn about Phase 2, which is Repolarization. If you need a refresher on what an Action potential is, check out the episode entitled What is and Action Potential.

If you have any questions, leave them below. Enjoy!

- Leslie Samuel

Transcript of Today’s Episode

Hello and welcome to Interactive Biology TV. My name is Leslie Samuel. In this episode, we’re going to be talking about repolarization, which is the second phase of the action potential. Now, if you haven’t watched Episode 9 as yet, stop this video right now and go back to Episode 9. Watch that first, and then watch this second, because this is the second phase of the action potential.

Now, in the first phase, we said sodium rushed in, making the membrane potential more positive because the voltage-gated sodium channels open. Now, you’re going to see a little addition to the set-up, the “Action Potential Simulator” that we had, and you’ll see we have these blue marbles. These blue marbles are to represent potassium ions, or K+. These potassium ions also have a positive charge.

Now, we have all of these positive ions inside the cell, and we have so many potassium ions inside the cell that potassium wants to rush out. But once again, normally, potassium ions cannot just rush out. The voltage-gated potassium channels, which you can see here by this yellow divider, need to open first.

So, sodium rushed in, making the membrane potential very positive, relatively speaking. And because the membrane potential is that positive, that’s enough now to open these voltage-gated potassium channels. And what’s going to happen when the voltage-gated potassium channels open? Well, you guessed it. Potassium is going to do what it wants to do: it’s going to rush out. The equilibrium potential for potassium is negative, so it wants the charge across the membrane to be negative. So, by all of these potassium ions leaving, that makes the membrane potential more negative, and that process is called repolarization.

So first we had depolarization, now we have enough charge for the voltage-gated potassium channels to open. Once those open, potassium ions are going to rush out, making the membrane potential more negative. That’s the second phase of the action potential, repolarization.

If you have any questions about that, as usual, leave me a comment. I’ll be happy to answer your question, and maybe even make a video answering your specific question. That’s all for this video, and I’ll see you in the next one.

113 Comments

  1. LimaBravoSJA July 6, 2011 at 3:53 pm #

    I’m just not clear on why the inside of the axon is so negative after K+ leaves. There are still a lot of Na+ in there, doesn’t the influx of Na+ help keep the inside relatively positive? At least more positive than it was before? Thanks

    Reply

  2. InteractiveBiology July 6, 2011 at 4:34 pm #

    Because of the negatively charged proteins that are normally in the cell, and the fact that the Sodium Potassium pump is constantly working, pumping Sodium out.

    Reply

  3. InteractiveBiology July 6, 2011 at 4:34 pm #

    @LimaBravoSJA Because of the negatively charged proteins that are normally in the cell, and the fact that the Sodium Potassium pump is constantly working, pumping Sodium out.

    Reply

  4. hannnahhhh1 August 22, 2011 at 9:55 pm #

    i was just wondering what would happen to the membrane action potential if the delayed rectifier didnt exist

    Reply

  5. InteractiveBiology August 22, 2011 at 9:57 pm #

    Sorry, but I’ve never heard of a delayed rectifier.

    Reply

  6. InteractiveBiology August 22, 2011 at 9:57 pm #

    @hannnahhhh1 Sorry, but I’ve never heard of a delayed rectifier.

    Reply

  7. InteractiveBiology August 23, 2011 at 1:57 am #

    Sorry, but I’ve never heard of a delayed rectifier.

    Reply

  8. ramyashraf333 August 27, 2011 at 8:14 am #

    Hi This Video is great! But I just wanted to ask why potassium ions are in the axon now in this video before repolarization ?
    Thx in advance.

    Reply

  9. ramyashraf333 August 27, 2011 at 12:14 pm #

    Hi This Video is great! But I just wanted to ask why potassium ions are in the axon now in this video before repolarization ?
    Thx in advance.

    Reply

  10. InteractiveBiology August 27, 2011 at 10:16 pm #

    Because of the activity of the Sodium Potassium pump. It pumps 3 sodium ions out and 2 potassium ions in. Check out episode 004 and you’ll see. It’s called “Ion Channels: The proteins in the membranes of Neurons.

    Reply

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