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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.

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Leslie Samuel is the creator of Interactive Biology. His mission is to use this site to Make Biology fun for people all over the world.

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Leave An AWESOME Comment

113 Responses to “010 Repolarization: Phase 2 of the Action Potential”

  1. ailemahm February 4, 2011 at 1:09 am #

    very helpful!

  2. InteractiveBiology February 19, 2011 at 6:48 pm #

    @tiffanylynn9704 You are very much welcome!

  3. InteractiveBiology March 24, 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 :)

  4. Yu April 28, 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

    • Lrsamuel April 28, 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!

      • VG June 16, 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!

        • Lrsamuel June 16, 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!

          • VG June 16, 2011 at 2:49 pm #

            That helps a lot!
            Thank you very much!

          • Lrsamuel June 16, 2011 at 6:21 pm #

            You are very much welcome!

  5. Gene Kehoe June 1, 2011 at 10:10 pm #

    awesome video nice and clear… love ur work helped me for my exm

  6. genek121 June 1, 2011 at 10:10 pm #

    awesome video nice and clear… love ur work helped me for my exm

  7. InteractiveBiology June 2, 2011 at 8:38 am #

    @genek121 That’s great to hear. Glad to know it helped you!

  8. InteractiveBiology June 2, 2011 at 8:38 am #

    @genek121 That’s great to hear. Glad to know it helped you!

  9. lilReeseyRed June 4, 2011 at 1:27 am #

    Thank you so much! these videos were so helpful =)

  10. InteractiveBiology June 4, 2011 at 9:17 pm #

    @lilReeseyRed You are very much welcome. Glad it helps!

  11. 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

  12. 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.

  13. 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.

  14. 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

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

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

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

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

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

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

  18. 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.

  19. 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.

  20. InteractiveBiology August 27, 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.

  21. 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.

  22. InteractiveBiology August 28, 2011 at 2:16 am #

    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.

  23. archana August 29, 2011 at 11:08 am #

    thank u

  24. juhidgenius September 4, 2011 at 3:21 pm #

    Oh god..you just saved me..i wish i had teachers like you..

  25. juhidgenius September 4, 2011 at 7:21 pm #

    Oh god..you just saved me..i wish i had teachers like you..

  26. InteractiveBiology September 5, 2011 at 6:39 am #

    @juhidgenius Glad to know that it helps :)

  27. InteractiveBiology September 5, 2011 at 6:39 am #

    Glad to know that it helps :)

  28. InteractiveBiology September 5, 2011 at 10:39 am #

    Glad to know that it helps :)

  29. Djalitana September 9, 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

  30. Djalitana September 9, 2011 at 12:46 pm #

    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

  31. 1508maikel September 12, 2011 at 6:12 pm #

    Thanx, you’re grat! I finally understand! Keep making those video’s- love theme!

  32. InteractiveBiology September 12, 2011 at 6:13 pm #

    @1508maikel Thanks for the feedback. Glad you are enjoying them!

  33. InteractiveBiology September 12, 2011 at 6:13 pm #

    Thanks for the feedback. Glad you are enjoying them!

  34. 1508maikel September 12, 2011 at 10:12 pm #

    Thanx, you’re grat! I finally understand! Keep making those video’s- love theme!

  35. InteractiveBiology September 12, 2011 at 10:13 pm #

    Thanks for the feedback. Glad you are enjoying them!

  36. 09BANGBANG September 30, 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?

  37. InteractiveBiology September 30, 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

  38. 09BANGBANG September 30, 2011 at 8:28 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?

  39. InteractiveBiology September 30, 2011 at 11:18 am #

    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

  40. thedarkpoets October 11, 2011 at 1:01 pm #

    what about the sodium potassium pump???

  41. thedarkpoets October 11, 2011 at 5:01 pm #

    what about the sodium potassium pump???

  42. InteractiveBiology October 12, 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

  43. InteractiveBiology October 12, 2011 at 10:10 am #

    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. twinee10 November 1, 2011 at 3:20 pm #

    Thank you, so much clearer :)

  45. twinee10 November 1, 2011 at 7:20 pm #

    Thank you, so much clearer :)

  46. ultrapsyk0 November 3, 2011 at 12:09 pm #

    I thought that it was
    1.Polarization
    2.Depolarization
    3.Propagation
    4.repolarization
    ?

  47. ultrapsyk0 November 3, 2011 at 4:09 pm #

    I thought that it was
    1.Polarization
    2.Depolarization
    3.Propagation
    4.repolarization
    ?

  48. InteractiveBiology November 7, 2011 at 12:09 pm #

    @twinee10 You’re very much welcome! Stay tuned for more!

  49. InteractiveBiology November 7, 2011 at 12:09 pm #

    You’re very much welcome! Stay tuned for more!

  50. InteractiveBiology November 7, 2011 at 5:09 pm #

    You’re very much welcome! Stay tuned for more!

  51. raiamirr December 4, 2011 at 7:09 pm #

    thank u sir

  52. InteractiveBiology December 4, 2011 at 9:01 pm #

    @raiamirr You’re welcome :)

  53. InteractiveBiology December 4, 2011 at 9:01 pm #

    You’re welcome :)

  54. raiamirr December 5, 2011 at 12:09 am #

    thank u sir

  55. InteractiveBiology December 5, 2011 at 2:01 am #

    You’re welcome :)

  56. whatzupdud December 20, 2011 at 12:02 pm #

    thank you very much!!!

  57. InteractiveBiology December 21, 2011 at 1:46 pm #

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

  58. InteractiveBiology December 21, 2011 at 6:46 pm #

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

  59. tybruce37 January 6, 2012 at 2:05 pm #

    This video is popular on Egypt

  60. tybruce37 January 6, 2012 at 7:05 pm #

    This video is popular on Egypt

  61. elijahunderw25 January 12, 2012 at 12:07 pm #

    Your video is a favorite on Vanuatu

  62. elijahunderw25 January 12, 2012 at 5:07 pm #

    Your video is a favorite on Vanuatu

  63. nibblesxsips January 25, 2012 at 10:58 pm #

    thank you (:

  64. nibblesxsips January 26, 2012 at 3:58 am #

    thank you (:

  65. BajanGirlBeautiful February 12, 2012 at 8:47 pm #

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

  66. BajanGirlBeautiful February 12, 2012 at 8:47 pm #

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

  67. BajanGirlBeautiful February 13, 2012 at 1:47 am #

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

  68. YaelTheFearless March 11, 2012 at 9:17 pm #

    Thank you! You saved my life! You are so clear and amazing!

  69. YaelTheFearless March 12, 2012 at 1:17 am #

    Thank you! You saved my life! You are so clear and amazing!

  70. pinkpillow16 April 25, 2012 at 12:07 am #

    You are great. Thanks for posting!

  71. pinkpillow16 April 25, 2012 at 4:07 am #

    You are great. Thanks for posting!

  72. imcrazzy4ever June 3, 2012 at 7:07 pm #

    u didn’t say making biology fun! :(

  73. imcrazzy4ever June 3, 2012 at 7:07 pm #

    u didn’t say making biology fun! :(

  74. allthruu August 2, 2012 at 4:26 am #

    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!!!

  75. 5431marie August 20, 2012 at 1:49 am #

    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

  76. christine September 11, 2012 at 11:43 pm #

    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…??

  77. felicita843 October 7, 2012 at 9:04 pm #

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

  78. felicita843 October 7, 2012 at 9:04 pm #

    explain laws of excitation

  79. 3393amanda October 10, 2012 at 11:02 pm #

    This was so clear and really helpful, thank you!

  80. jsulema07 October 21, 2012 at 4:15 am #

    THANK YOU SO MUCH BRO!

  81. Fiona Umy October 22, 2012 at 12:27 am #

    THis is so helpful sir.
    Thanx

  82. edwaak November 11, 2012 at 7:19 am #

    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!

  83. rawan salem November 19, 2012 at 11:35 am #

    thank you sir :)

  84. RaphaelChino November 20, 2012 at 5:25 am #

    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 :D

  85. InteractiveBiology November 20, 2012 at 7:03 am #

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

  86. bryusuf December 9, 2012 at 4:38 am #

    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

  87. Deborah Pollack December 15, 2012 at 10:39 pm #

    Your videos are amazing!! I love your examples!! You are really helping me out. Thank you!

  88. jmk8792 January 23, 2013 at 11:01 pm #

    This is so helpful – Thanks!

  89. Emad Alanazi February 8, 2013 at 12:37 pm #

    you are GOOOOOD .
    thank you

  90. tsizemore1982 February 14, 2013 at 1:05 am #

    Great video. I was confused prior to watching your video. Now I understand. =) Thanks!

  91. B PHARM February 27, 2013 at 2:37 am #

    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 ?

  92. swiss emerald February 27, 2013 at 7:57 am #

    thanx alot Sir….stay blessed….

  93. Zoonice March 5, 2013 at 5:36 pm #

    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

  94. jesse021891 March 8, 2013 at 8:47 am #

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

  95. TTerminatorr March 16, 2013 at 12:18 am #

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

  96. jhee anthony April 12, 2013 at 6:16 pm #

    Thank you for the video! Totally appreciate it!

  97. catherine ireneo April 24, 2013 at 6:53 am #

    you are amazing!!!!

  98. ching lau May 10, 2013 at 2:26 pm #

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

  99. lazer1235 June 8, 2013 at 11:38 am #

    you just earnt yourself a subscriber

  100. Daniel Fowler August 2, 2013 at 3:44 pm #

    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!!

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

    thanx …. u rock

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

    Does all three phases of action potential involve equilibrium?

  103. 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?

  104. 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:)

  105. 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!

  106. 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. ?.

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

    You are the best!