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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.
About The Author Leslie Samuel
Leslie Samuel is the creator of Interactive Biology. He created this site to help Make Biology Fun and has the goal of making this the biggest and best biology resource on the net.
24. March 2011 at 10:29 pm
@kathysayan1975 That’s awesome to hear. Glad it helps. Make sure to check out the others then, because I go through a lot of detail. Also, tell your classmates about it so they can learn too
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28. April 2011 at 12:30 am
It is very helpful. I have a question that how the membrane potential approaches to equibilium potential for potassium during repolarization
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Lrsamuel Reply:
April 28th, 2011 at 8:19 am
Because when the V-gated Potassium channels open, Because of the strong positive charge inside the cell, potassium (which is also positively charged) will be pushed out of the cell. It wants to be at it’s equilibrium potential, so it rushes out of the cell to accomplish that equilibrium potential, which is a negative value. Hope that helps!
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VG Reply:
June 16th, 2011 at 4:02 am
Hi, very helpfull videos!!! I have one question tough:
In the previous video you only talk about NA+-ions rushing into the axon.
When we start this video we have NA+-ions in the axon but also K+-ions.
Where do they come from? Did they also rush into the celle with the NA+?
They couldn’t already be in the cell because they are also + charged.
Maybe i looked over it in another video but i thought it wouldn’t mind to aske the question to be sure.
Keep up the good work, it is helping me very much with my Biology-studies!
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Lrsamuel Reply:
June 16th, 2011 at 8:21 am
Glad to know that you are finding the videos valuable.
Actually, yes, K+ ions were in the cell before the channels open. Here’s why. The Na/K Pump pump 3 Na+ ions out of the cell and 2 K+ ions into the cell. Since more + is leaving than is coming in, that will be one of the causes for the inside having a – charge. However, as a result, there will be more K+ inside the cell.
K+ wants to be outside, so as soon as the channels open, they will rush out.
Hope that helps. All the best!
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VG Reply:
June 16th, 2011 at 2:49 pm
That helps a lot!
Thank you very much!
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Lrsamuel Reply:
June 16th, 2011 at 6:21 pm
You are very much welcome!
1. June 2011 at 10:10 pm
awesome video nice and clear… love ur work helped me for my exm
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2. June 2011 at 8:38 am
@genek121 That’s great to hear. Glad to know it helped you!
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4. June 2011 at 1:27 am
Thank you so much! these videos were so helpful =)
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4. June 2011 at 9:17 pm
@lilReeseyRed You are very much welcome. Glad it helps!
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6. July 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
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6. July 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.
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6. July 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.
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22. August 2011 at 9:55 pm
i was just wondering what would happen to the membrane action potential if the delayed rectifier didnt exist
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22. August 2011 at 9:57 pm
Sorry, but I’ve never heard of a delayed rectifier.
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22. August 2011 at 9:57 pm
@hannnahhhh1 Sorry, but I’ve never heard of a delayed rectifier.
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27. August 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.
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27. August 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.
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27. August 2011 at 10:16 pm
@ramyashraf333 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.
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29. August 2011 at 11:08 am
thank u
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Lrsamuel Reply:
August 29th, 2011 at 5:38 pm
You’re welcome
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4. September 2011 at 3:21 pm
Oh god..you just saved me..i wish i had teachers like you..
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5. September 2011 at 6:39 am
Glad to know that it helps
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5. September 2011 at 6:39 am
@juhidgenius Glad to know that it helps
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9. September 2011 at 8:46 am
at depolarisation inside of the cell is less negative than resting potential and at repolarisation inside of the cell is getting close to resting potential. i think I got it. thank you so much
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12. September 2011 at 6:12 pm
Thanx, you’re grat! I finally understand! Keep making those video’s- love theme!
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12. September 2011 at 6:13 pm
Thanks for the feedback. Glad you are enjoying them!
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12. September 2011 at 6:13 pm
@1508maikel Thanks for the feedback. Glad you are enjoying them!
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30. September 2011 at 4:28 am
When K+ ions leave, I understand that that phenomenon causes hyperpolarization. But what happens after hyperpolarization? In Resting Membrane Potential, there should be more K+ inside the cell while Na+ needs to be dominant in the extracellular fluid. As shown at the end of this video, all of the Na+ is inside the axon while K+ is outside. Does this mean that Resting Potential can have Sodium inside of the cell while K+ lies in the Extracellular fluid?
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30. September 2011 at 7:18 am
@09BANGBANG All questions are answered in the Interactive Biology community forums from now on. Go to the website in the description and then visit the community. This is to make it as efficient as possible as we have multiple people over there to help answer questions.
All the best
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11. October 2011 at 1:01 pm
what about the sodium potassium pump???
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12. October 2011 at 6:10 am
@thedarkpoets All questions are answered in the Interactive Biology community forums from now on. Go to the website in the description and then visit the community. This is to make it as efficient as possible as we have multiple people over there to help answer questions.
All the best
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1. November 2011 at 3:20 pm
Thank you, so much clearer
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3. November 2011 at 12:09 pm
I thought that it was
1.Polarization
2.Depolarization
3.Propagation
4.repolarization
?
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7. November 2011 at 12:09 pm
You’re very much welcome! Stay tuned for more!
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7. November 2011 at 12:09 pm
@twinee10 You’re very much welcome! Stay tuned for more!
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4. December 2011 at 7:09 pm
thank u sir
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4. December 2011 at 9:01 pm
You’re welcome
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4. December 2011 at 9:01 pm
@raiamirr You’re welcome
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20. December 2011 at 12:02 pm
thank you very much!!!
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21. December 2011 at 1:46 pm
@whatzupdud You’re welcome. Please stay tuned for more Biology videos, and fun!
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6. January 2012 at 2:05 pm
This video is popular on Egypt
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12. January 2012 at 12:07 pm
Your video is a favorite on Vanuatu
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25. January 2012 at 10:58 pm
thank you (:
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12. February 2012 at 8:47 pm
wow…thank you! I was having difficulty understanding this but thank you for making it clearer.
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12. February 2012 at 8:47 pm
wow…thank you! I was having difficulty understanding this but thank you for making it clearer.
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11. March 2012 at 9:17 pm
Thank you! You saved my life! You are so clear and amazing!
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25. April 2012 at 12:07 am
You are great. Thanks for posting!
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