013 A Review of the Action Potential

013 A Review of the Action Potential

Leslie Samuel IBTV, The Nervous System 291 Comments

So we’ve gone over Depolarization, Repolarization and Hyperpolarization in some detail. It’s time to do an overall review.

Watch the video above to put everything into perspective and solidify your understanding.

And as usual, you may leave your questions/comments below.

- Leslie Samuel

Transcript of Today’s Episode

Hello and welcome to Interactive Biology TV, where we’re making biology fun! My name is Leslie Samuel. This is Episode 13, where we’re going to be talking about the action potential. I’m basically going to be giving you a review of the concepts that we’ve been talking about up to this point when it comes to the action potential.

Over here, we have our neuron. The part of the neuron that we’re going to be focusing on is the axon, so that’s this region here, starts here and it goes to about there. Now what I’m going to do is I’m going to take a section of the neuron, let’s say I’m going to take this part here and I’m going to draw it down here. So here we have the axon. This is inside the axon, and this is outside the axon.

If you remember from a previous episode, outside the axon, we have a lot of sodium ions. So I’m going to draw sodium ions here, and they are all outside the cell. Now let’s look over here. Here we have a stimulus that’s happening. You can see there’s a first stimulus that does not reach threshold, so nothing happens. Another stimulus comes, it does not reach threshold, so we still do not get an action potential. If we have a stimulus that’s strong enough, and I’m going to draw another stimulus in here. So let’s say we have a stimulus that’s that big. That’s going to reach the threshold and cause an action potential.

When that happens, voltage-gated sodium channels are going to open, and that’s going to cause sodium to rush into the cell. Of course, that’s going to start at the axon hillock. Sodium is going to rush in, making the membrane potential even more positive, causing more channels to open, more sodium to rush in along the axon.

What is that going to do to the membrane potential? You can see right here, this is where we get depolarization. The membrane potential goes up, and it’s trying to reach the Donnan equilibrium for sodium ions. That equilibrium potential is somewhere around 58 millivolts. Sodium is rushing in because of the driving force causing sodium to go in. Sodium wants the membrane potential to go up to its Donnan equilibrium, and that is around 58 millivolts. At this point, voltage-gated potassium channels have enough voltage in order for them to open.

So I’m going to erase all of this now, and we’re going to take at the axon again, I’m going to draw it here in blue. Inside the cell, we have a bunch of potassium ions. Now we have such a positive charge on the inside that potassium ions want to leave, because positive repels positive. When voltage-gated potassium channels open, potassium can now leave the cell. So we have a lot of positive leaving the cell, and what that is going to do is cause repolarization, where the membrane potential is going down. Just like with sodium, potassium wants to reach its equilibrium potential, which is somewhere around -93 millivolts. That is why the membrane potential is going down as it’s leaving, because potassium wants the membrane potential to be at -93 millivolts. That is where it is most comfortable.

Considering that the resting membrane potential is around -70, the membrane potential goes significantly lower than that -70, and this phase we call hyperpolarization. All along this process, we have sodium-potassium pumps that are pumping 3 sodium ions out of the cell, and 2 potassium ions back in. That’s going to cause the membrane potential to eventually reach back to its resting state.

This is the entire action potential, looking at depolarization, repolarization, hyperpolarization, and it getting back to the resting state. I hope that makes sense. If you have any questions, feel free to leave them in the comments below. I’d be happy to answer your question, and even maybe make a video answering your specific question. That’s it for this video, and I’ll see you in the next one.

 

Comments 291

  1. Mohammed

    Thank you so much for the videos sir, they are very helpful .. i have a question if you could answer it please .. after hyperpolarization ( when the na+/k+ pump take 3na+ out and 2K+ in ) are all the na+ now outside the axon ? and all the K+ ions inside the axon ?! or do some Na+ remain in before another action potentail occurs.

    again thank you very much Sir

  2. Shawn

    I have been trying to find a video about the resting membrane potential. I stumbled onto your videos, and I really like the way you explain things, but I’m not finding a video simply on resting membrane potential. Does such video exist?

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      Author
      Leslie

      Hi Shawn,

      First of all, I’m glad you are finding value in my videos. That makes it all worth the effort I put in.

      To answer your question, I don’t have one specific video that explains resting membrane potential. However, here are some videos that talk about things pertaining to that topic. Hopefully that will help:
      1. Ion Channels: the Proteins in the Membrane of Neurons
      2. The Isoelectric Point of Proteins
      3. Donnan Equilibrium and Driving Force

      If you understand those concepts, you will have a pretty decent idea of what contributes to the Resting Membrane Potential. All the best!

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  3. varsha

    thank you very much for the wonderful video.i had a doubt in the concept.why is it that the inner part of the membrane is negatively charged while the outer part of the membrane is positively charged when the neuron is in resting potential??

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      Leslie

      For a number of reasons:
      1. The Sodium-Potassium pump pumps 3 Sodium ions out and 2 potassium ions in, both of which have a positive charge. Because both are positively charged, more positive is leaving than entering.
      2. There are negatively charged proteins inside the cell.

      Hope that helps Varsha. Take care!

  4. Denise Smith

    Thank goodness for your videos they have really helped me to study for my exam. Your interactive biology is so much easier to understand than my college classes. Leslie you really do make biology fun. Thankyou so much. you are an inspiration.

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  5. courtcusimano022

    I do have one question. My notes say that the unstimulated membrane is permeable to potassium but not sodium. Says something about resting permeabilities reflecting the properties of passive leakage channels within the membrane. I was wondering if you could clarify for me?

  6. sathyahema

    Hi Leslie,

    My professor talked about Sodium Chemical channels opening during depolarization. When do they come into the picture, before the Sodium Voltage gated channels or after? Thank you so much for these videos, they are so helpful!

  7. InteractiveBiology

    @Amethystiii Wow! You’re very welcome. Leslie is great at explaining these complicated things making them seem so simpler. You can go to our site for more of these Biology videos. I’m sure you’ll learn a lot more! :)

  8. richardus

    mmm… ok… there is one thing I do not understand… the stimulus becomes an electric current, right? and that is a group of electrons travelling… so Na/K help the current to keep flowing???

    I do not really understand what the function of Na and K is…

  9. InteractiveBiology

    @richardus Hi! Thank you for watching the video. Unfortunately, Leslie won’t be able to answer specific questions as he is busy at the moment with a lot of work. But, he’ll also be working on additional Biology videos, so please stay tuned for more!

  10. mihae

    This is probably the most helpful YouTube channel I have found–universities and colleges should model the way that Leslie teaches and incorporate it into their own teaching! amazing!

  11. punkvijay224

    I should really say this, you are just too amazing!! I have my exam due coming tuesday, and your help was just at the right time!! THANKS A LOT!!

    Something that I don’t understand is why do college professor don’t find videos like this so they could improve their performance in the class!!

  12. punkvijay224

    I should really say this, you are just too amazing!! I have my exam due coming tuesday, and your help was just at the right time!! THANKS A LOT!!

    Something that I don’t understand is why do college professor don’t find videos like this so they could improve their performance in the class!!

  13. preacherman9

    I have an exam next Monday and I was recommended to do this. Reading this in a text book was blowing my mind and freaking me out. But watching your 6 videos which took 20 mins and i got it. that is a testament to how good you explain this. Thanks a lot buddy. My biopsychology exam does not seem so bad.

  14. brownboys16

    @InteractiveBiology I LOVED UR VIDEO it was really helpfull with my midterm question 3) Describe how an action potential begins and what happens within the neuron during its transmission. thank you.

  15. brownboys16

    @InteractiveBiology I LOVED UR VIDEO it was really helpfull with my midterm question 3) Describe how an action potential begins and what happens within the neuron during its transmission. thank you.

  16. brownboys16

    I LOVED UR VIDEO it was really helpfull with my midterm question 3) Describe how an action potential begins and what happens within the neuron during its transmission. thank you.

  17. brownboys16

    I LOVED UR VIDEO it was really helpfull with my midterm question 3) Describe how an action potential begins and what happens within the neuron during its transmission. thank you.

  18. brownboys16

    I LOVED UR VIDEO it was really helpfull with my midterm question 3) Describe how an action potential begins and what happens within the neuron during its transmission. thank you.

  19. olivia

    HI Leslie, i dont really get how the sodium-potassium pumping in 3 sodium ions out of the cell, and 2 potassium ions back in that causing the membrane potential back to the resting state…Can you plz explain more about this?
    thanks :)

    cheers
    o.Jane

  20. ske8975

    This was one of the most informative and easy to understand videos I’ve seen on this complex topic. Thank you so much. The combination of your ability to articulate exactly what’s going on inside of a cell during this process and your visual aids was PERFECT! Thank you so much.

  21. michaelyouth

    @InteractiveBiology Great explanation but what I don’t understand is if 2 K+ ions get in the cel and 3 Na+ ions out of the cell, this means that you have a charge of 1 + ( the 1 Na+ ion that goes) that leaves the cell, shouldn’t this mean that there is no hyperpolarization but that the potential even goes lower as a positive charge leaves the cell (netto charge)

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  22. MJ

    your clips are so helpful, but I have a hard time associating it with the heart. We are currently studying about the action potential of the heart, what happens on the ECG, and how it affects the ions in the heart (Na, K, Ca). Do you have a video that puts it all together. That would save me!!!! Thanks, MJ.

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  23. alphaWAYNE

    The best channel so far explaining about biology… and making it more fun! a million thankx to the great teacher Lislie Samuel. Thank you, thank you, thank you.

  24. Jessie

    You are seriously an excellent teacher… i understand this completely :) Please keep this up and save all the students from terrible teachers. You explain things slowly so it makes sense. Again thanks a lot for this!!

  25. bupbebuon00

    please help, i have a lab question and im stuck:(
    ‘With continuous stimulation, how long did the maximal force last ?”thanks

  26. Frea Petra

    nice tutorial :) i have a question about Na+ – K+ pump, is it still working in state of no action potential? thanks

  27. atomikjames

    I have a couple of questions so it would be great if someone could please help me :)
    At the peak of the action potential, is it the concentration of sodium that causes the repulsion of potassium out of the neuron once the pottasium gated channels open? Are the pottasium ions always in the neuron, or did they enter with the sodium during the action potential?
    Also, when the neuron is at rest, what is inside it, which makes it have such a negative resting membrane potential, of about -65mV?

  28. atomikjames

    I have a couple of questions so it would be great if someone could please help me :)
    At the peak of the action potential, is it the concentration of sodium that causes the repulsion of potassium out of the neuron once the pottasium gated channels open? Are the pottasium ions always in the neuron, or did they enter with the sodium during the action potential?
    Also, when the neuron is at rest, what is inside it, which makes it have such a negative resting membrane potential, of about -65mV?

  29. ThePakman4

    I am a professor in Psychology and I glean as much as I can from these sources to make my classes that much more interesting. thanks for your vids they’re a real help

  30. ThePakman4

    I am a professor in Psychology and I glean as much as I can from these sources to make my classes that much more interesting. thanks for your vids they’re a real help

  31. ThePakman4

    I am a professor in Psychology and I glean as much as I can from these sources to make my classes that much more interesting. thanks for your vids they’re a real help

  32. 333teebee

    WOW! Thank you so much for clarifying the action potential! I was so lost before watching, and now it all makes sense!

  33. sandyswimsxoxo

    this is fantastic, thank you so much!!! I just couldn’t get my head around it until you started drawing it out on that diagram! thank you thank you thank you!!!

  34. kajal4eva

    OMG THANK U SO MUCH I LOVE UR VIDEOS…after watchin ur video I think am ready for my test..ur wayyyyyy better than my teacher.

  35. TheLongRain

    I’m pretty sure that at the peak sodium channels are closing as the potassium channels are opening to restore the resting membrane potential of -70mV.

  36. emikoisabel

    You are amazing! Here I was crying about being terrified about my upcoming biology exam and I find your glorious videos. Thank you so much!

  37. tingchanghui

    Hello, I have a question: you mentioned that after hyperpolarisation at -93mV, the resting membrane potential is restored by the Na+/K+ pump, where 3 Na+ is pumped out for every 2K+ pumped into the cell. Wouldn’t the pump cause the membrane potential to be more negative (i.e. lower than -93mV) rather than restore the resting membrane potential (-70mV), since there is a net loss of positive charge? Thank you, you’ve done a great job :)

  38. tingchanghui

    Hello, I have a question: you mentioned that after hyperpolarisation at -93mV, the resting membrane potential is restored by the Na+/K+ pump, where 3 Na+ is pumped out for every 2K+ pumped into the cell. Wouldn’t the pump cause the membrane potential to be more negative (i.e. lower than -93mV) rather than restore the resting membrane potential (-70mV), since there is a net loss of positive charge? Thank you, you’ve done a great job :)

  39. Emeka Diribe

    @tingchanhui based on the numbering, the potential seems to be calibrated as V(in) – V(out). This way, as Na+ ions flow into the cell, + V is achieved. As K+ ions flow out, V (out) gets bigger as V(in) gets smaller. So potential becomes more -. Pinch of salt figurative description! Remember that some K+ channels are leaky and the base potential is maintained partially by that leakiness and a balance of electrical and chemical potentials plus the Na+/ K+ pump.

  40. MrBbsingh

    Hey, great vid. I was wondering whether you could explain the chloride ions and the inhibitory effects that they have on achieving action potential.
    Thanks

  41. drpink86

    Great Video!!! Thank you!

    Can you explain to me the accommodation phenomenon on action potentials. I read it over and over and don’t seem to get it :-( Thank you!

  42. Naren Selva

    Hey I studied my book for about 4 hours to understand this. But this video made me understand the whole process within 5 minutes. This is very clear and will help me in my upcoming exam. I really appreciate your efforts.

  43. purpleyellowelf

    I would like to ask, is that after depolarization, an action potential is generated and then follows repolarization? I’m confused when did action potential is generated.

  44. AlMuthanna AlHazmi

    the action potential happend when Na+ reach the equilibrium potential state ^^
    i think not sure
    & the wholly process is actually called” action potential ”
    chaaaw

  45. Edward Aveiro

    as you said how many Na ions and K ions enter the axon per action potential and also you said that 3 Na ions leave the cell and 2 K ions get in… Is that the Neuron your were talking about…..??

  46. Mel Corner

    Everything makes sense except for the equilibrium potentials..how did you come up with these numbers, 58mV and -93mV..are those estimates? because by looking at the graphs, it seemed as if they should be less..

  47. augumentum

    I understand the video. What happened to the CA ions. Are CA K and NA actually in elemental forms during Action Potential in efflux and influx ?

  48. sshackelford89

    When the Na+ and K+ ions are going into/leaving cell during depolarization and repolarization, where are they leaving through? Through the leaky ion channels? thanks

  49. queendiamond1969

    This video was helpful but 1 thing I didn’t understand is when repolarization was ending and going into what you have labeled on the diagram as a refractory period (below -70mv). I was a little confused because at 4:32 you point to that refractory area and say that it’s hyperpolarization. Are both of these terms synonymous or are there two different functions going on? Thanks.

  50. CJNconor

    Repolarization is the process in which the membrane potential is attempting to go back to being polarized, whereas hyperpolarization is the process in which the inside of the axon becomes even more negative than the resting membrane potential. When this happens (the inside becomes overly negative), it is called the refractory period.

  51. woohoozzi

    Hyperpolarization is just when the membrane voltage falls below the resting potential (that dip labeled refractory period). It is caused by all the K+ (potassium) ions leaving, making the membrane extra-negative until the sodium-potassium pumps can restore the resting potential.

  52. woohoozzi

    If you consider the resting potential to be “polarized” then getting more positive is depolarization, returning is repolarization, and going more negative than the membrane voltage at resting potential is hyperpolarization. This coincides with the refractory period because at this point all the sodium is inside the cell and the potassium is outside — so the neuron is not ready to fire another action potential until pumps reinstate the resting potential.

  53. Nikki Clark

    Thankyou you’ve made this really simple and it’s so much more concise you can’t get lost in all those processes, thank you!!!! Short sharp explanations are better than long winded ones.

  54. YouHearAboutPluto

    ohh man you are a beast my friend that was right on point everything was explained i understand it now thank you so very much

  55. IknowUThinkImCool

    No, there isn’t. The plateau phase in the cardio-action potential is caused by the “balancing out” of the efflux of K+ ions with the influx of Ca2+ ions…
    :D

  56. sunny4lady

    Great video, I understood the process but was missing some information that my book didnt explain so thank you – it clarified it!

  57. João Luis Carvalho de Souza

    Hi, you made some mistakes in your lecture such as 1) Donnan Equilibrium for Na+, 2) K+ leaves the cell cause positive repels positive and 3) the NaKPump causes the membrane voltage go back to the resting potential after hyperpolarization phase.
    And also how do you want to explain action potentials without say what is threshold?

  58. João Luis Carvalho de Souza

    Now I can realize how people don’t understand some kind of phenomena like action potential. People are enjoyng this video and saying that “it was wonderfull’ !!!

  59. eniola fujah-sanni

    thank you so much I found this very useful ,because am currently doing my A levels in biology and I never realy understood the action potential but now I do . can u please explain the nerve junctions ,thanks and God bless

  60. Tan Run Yu

    2+ in, 3+ out means that the inside is getting more negative than the outside of the cell. but the graph (hyperpolarization) shows an increase in the voltage potential ?

  61. Tan Run Yu

    2+ in, 3+ out means that the inside is getting more negative than the outside of the cell. but the graph (hyperpolarization) shows an increase in the membrane potential ?

  62. peggywonghk

    so why is the membrane potential more positive when three sodium ions are pumped out and two potassium ions are pumped into the cell?? Wouldn’t that be more negative as one positive ions is gone????? i am so confused!

  63. AM loki

    wait a minute, if the Na+ voltage-gated channel only open when the stimulus exceeds the threshold, then how inside of the cell became slightly relatively positive BEFORE it reached the threshold?
    I mean, the Na+ ions can still come in to the cell before it reached the threshold? how?

  64. mage nama

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  65. josiejad

    Thank you so much! My classmates did a presentation in class, and I couldn’t fully grasp it, but this video really helped me to understand it!

  66. xTvxN41Fx

    I have read that Na/K pumps do to return sodium ions and potasium ions to thier locations and return potential to resting potential after hyperpolarization, now pump give extracelluar fluid positive charges therfore it make intracellular fluid more negative to no end, this menas is resting potential willn’t return, so what happen?! thanks.

  67. Mead Badir

    i dont get it,after hyperpo’, you still pump out 3 K+ for 2 Na+ in, meaning you are doing an efflux of cations, which only drives the potential to a more negative voltage….so why does the membrane go back to its resting point?

  68. Lightbluefire

    the way I understand it, before hyperpolarization, the voltage is – outside of the axon and + inside, and K+ keeps flowing out, but at a point before it reaches equilibirium, the 3 Na+ for every 2 K+ pump kicks in, meaning that there will be more of a negative gradient inside the cell than outside, where more Na+’s are.

  69. Lightbluefire

    the way I understand it, before hyperpolarization, the voltage is – outside of the axon and + inside, and K+ keeps flowing out, but at a point before it reaches equilibirium, the 3 Na+ for every 2 K+ pump kicks in, meaning that there will be more of a negative gradient inside the cell than outside, where more Na+’s are.

  70. Sarah Kostanich

    Thank you so much! you make it easy and clear to understand and actually finding it quite interesting now :)

  71. Anosh Kanag

    needs to be corrected : this is only relevant to the nodes of ranvier between the sections of the neuron circled not the section circled which is myelinated and does not deal with sodium and potassium channels. apart from that this video is very good

  72. BlueV Jones

    I really like these videos but I have a query. From refractory period to resting potential: If the Na+K+ATPase pumps out 3 Na+ ions for every 2 K+ in the neurone, then the inside of the membrane becomes more negative, and does not restore resting potential. Can you clarify this please? ( I thought it was to do with K+ diffusion from the outside to inside that restored the resting potential)

  73. Lara Romero

    Thank you so much… I was having such a hard time understanding my professors lecture and the textbook. You explained it so well. KUDOS! :-)

  74. MrTinkasaurus

    Thank you! I did not understand this in class at all today but I totally get it now. You have a really friendly voice, it makes me feel calm haha :)

  75. Laurie Bergman

    THANK YOU FOR MAKING THESE VIDEOS. YOU HAVE SAVED ME FROM FAILING MY PSYCHOPHARMACOLOGY CLASS! I HAVE NOT BEEN IN COLLEGE FOR OVER 35 YEARS AND I WAS LITERALLY LOST WHEN I HAD TO LEARN ABOUT THE CNS. YOUR PRESENTATIONS WERE SO EASY TO UNDERSTAND. YOU MAKE IT INTERESTING TO LEARN ABOUT A SUBJECT WHICH WAS INITIALLY OVERWHELMING TO ME TO GRASP. I AM NOT A SCIENCE PERSON BUT YOUR STYLE OF INSTRUCTION HAS MADE ME FEEL MORE CONFIDENT THAT I CAN MASTER THE CONCEPTS. THANK YOU AGAIN SO MUCH.

  76. sam aryan

    Hi mr.samuel
    these 3 questions have baffled me for a while,
    Q1:when exactly dose the sodium-potassium pump’s job start?
    1 right after the hyperpolarization 2 as soon as the cell reaches the resting state
    if the assumption #2 is correct then in that case the hyperpolarization is sending K+ out while the pump is pumping K+ in?? is’nt it contradictory or maybe the assumption #1 is correct.
    (plese check out my next comment because youtube doesnt let me to post it all in one comment)

  77. sam aryan

    please read the comment below first…
    Q2:IS refactory period the same as the activity of sodium-potassium pump? if the assumption 1 is correct then it can not be said that all that part of the graph which is under the line of resting state is all”refractory state”
    Q3:why the Q and S in ORS complex are under hre isoelectric line?and which one causes the U wave, the depolarization of the purkinje fibers or repolarization of them
    I would mean a lot if you take your time answering my question

  78. daras1871

    Hello! I have a question:
    Wouldn’t the potassium stop leaving as the cell starts becoming more negative (cell potential becomes lower than resting potential)?

  79. Alex Seb

    The answers is yes! As the cell potential reaches -70mV the potassium channels are closing but “slowly” by the time they close cell potential passes past the -70mV. In other words potassium channels have lag time.

  80. Sheyna Hoitsma

    Hello, you are wonderful for doing these videos!
    Can you help me understand the second messengers cAMP and camKII in LTP? also if you could break down LTD that would be very lovely!

  81. Pierre Marcelin

    very informative,it is so clear. Now I have a better understanding of the action potential. 1 question I’d like to ask, why you called at one time of your explanation after K+ open during refractory period, hypopolarization?

  82. Shane Cronin

    How does this “hyperpolarization” period go back to its resting potential? Do Na+ or K+ enter/re-enter the cell or…?

  83. Lizzette Rojas

    The Na+/K+ pumps will release 3 Na+ ions and bring 2 K+ ions back in (thats only one cycle but it happens a lot). This will eventually make the membrane potential reach its resting state again. Hope that helped.

  84. hasib nafis

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  85. hasib nafis

    Hey guys. Fabulous clip. My dad used to be a fat. He changed his body from 284 lbs of pure fat to 211lbs of natural lean muscle. I couldn’t believe it! I just joined myself as I wanna strengthen. He made use of the Muscle Building Bible (Search on Google)…

  86. Eargasm75

    For repolarization, why would potassium leaving the cell make it more
    negative? I thought K was a negative and sodium was positive.

  87. Eargasm75

    For repolarization, why would potassium leaving the cell make it more
    negative? I thought K was a negative and sodium was positive.

  88. crystal

    My book makes this so hard to understand, and I watched this video of yours and was still a slight bit confused until I went back and reviewed the 2-3 videos you made before this one and wow what a difference that made, thank you tons for what you do!

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