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.


  1. REENA SAJI September 13, 2013 at 12:52 pm #

    thanx …. u rock


  2. cagirl085 September 15, 2013 at 10:59 pm #

    Does all three phases of action potential involve equilibrium?


  3. safar September 17, 2013 at 10:40 pm #

    potasium also has positve charge why when goes out side the membrane make it negative or repularization?


  4. Georgina Jahnel September 18, 2013 at 1:45 pm #

    Thank you! You are helping me so much! Without you I wouldn’t even understand a thing of biology. Keep on doing these videos!
    Greetings from Germany:)


  5. Jose Martinez September 18, 2013 at 9:30 pm #

    Thank you so much, as an IB student this definitely helps with the huge neurobiology chapters!


  6. Ema Akw September 25, 2013 at 4:13 am #

    I have a question, and by the way your videos are really awesome. My question is: why is it that the more negative a membrane potential gets, the less likely it is for that cell to get depolarised and the more positive a cell gets, the less likely it is to get depolarised. I thought it shud be the reverse. ?.


  7. Allyson Rodriguez November 2, 2013 at 1:47 pm #

    You are the best!


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