010 Repolarization: Phase 2 of the Action Potential

IBTV, The Nervous System
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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.

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TAGS

action potential, membrane potential, nerve impulse, repolarization, voltage-gated K+ channels, voltage-gated potassium channel

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  3. @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 🙂

  4. It is very helpful. I have a question that how the membrane potential approaches to equibilium potential for potassium during repolarization

  5. 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!

  12. 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!

  13. 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!

  16. 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

  17. @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.

  18. 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.

  24. 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.

  25. @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.

  26. 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.

  27. 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.

  35. 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

  36. 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

  42. 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?

  43. @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

  44. 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?

  45. 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

  48. @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

  49. 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

  64. @whatzupdud You’re welcome. Please stay tuned for more Biology videos, and fun! 🙂

  73. wow…thank you! I was having difficulty understanding this but thank you for making it clearer.

  80. what was very confusing is now not so much…thx greatly!!! I do hope you are teaching Bio somewhere to someone b/c you make this fun!!!

  81. Ahhh… I’ve been trying to understand this for weeks… I just couldn’t get it, but this helped so much. It’s nice to have something explained in a easy to understand way. Haha

  82. I am confused where calcium comes into the mix of these channel? I am so confused as to why the body needs these channels etc…??

  83. please can u mak a video 4 mechanism of excitation conduction in unmyelinated and unmyelinated fiber

  88. Thank you so much! Im studying anatomy and physiology at university and your videos make it a lot easier to understand! Wish my lecturers explained it like this!

  90. hello sir! do you mind if I use your examples for my presentation in class? I’ll be sure to credit you for the awesome information 😀

  91. Hi, definitely you can use them. Only make sure to link back to the site at Interactive Biology. Good luck and enjoy!!

  92. can you answer my question I need to know the answer to this question please : explain what is happening at the threshold, rising phase, peak, falling phase of the action potential. thank u

  97. Sir,this is really informative…but why does the leaving of the potassium ions make the inside negatve again rather than making the outside more negative ? I’m having bit of a difficulty in understanding that, could you please explain ?

  99. i appreciate you video i am nursing school and i was having a hard time with p wave and qrst, your video help me to understand it better

  100. So calcium doesn’t play any role in Action Potential’s, I thought they did?

  101. Great video bioscience is bit easier, and this was a question we needed to answer for class, thank you.

  104. Thank you! but I still got a question.. so does it means depolarisation = removing negative charge and repolarisation = removing positive charge?

  106. I will try to answer your question…Polarization occurs when their is a difference of charges between two regions. In this case positive outside the axon and negative inside the membrane (before depolarization). Therefore, when their are no longer two opposite charges being attracted to each other like when the sodium rushes in the axon, there is no longer opposite charges attracting which is why scientists call this depolarization (because the neuron loses its polarity!). hope this helped!!

  110. 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:)

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

  112. 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. ?.

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