009 Depolarization: Phase 1 of the Action Potential

009 Depolarization: Phase 1 of the Action Potential

Leslie Samuel IBTV, The Nervous System 171 Comments

The action potential can be a complicated thing to understand, unless you are dealing with little white plusses on a table :)

In this video, I help you visualize the first phase of the action potential – the Depolarization phase.

Go ahead and watch the video and you should get a clear understanding of the events that cause depolarization of the neuron.

- Leslie Samuel

Transcript of Today’s Video

Hello and welcome to Interactive Biology TV, where we’re making biology fun! In this episode, we’re going to be talking about depolarization, which is the first phase of the action potential. We’re going to go into some more detail than we’ve been doing in the past when it comes to the action potential. It’s going to be a little different. You’re not going to be looking at me talking about something. You’re actually going to be looking at another one of my elaborate set-ups, and I’m going to call this set-up the “Action Potential Simulator.” I hope that gets you excited about it.

Alright, so what we have here is, this side represents outside the cell, outside the axon, and this side represents inside the axon. Here we have a bunch of pluses, and these pluses are representing sodium ions. Now, sodium ions have a positive charge, and that’s why I chose these very attractive pluses to represent sodium ions.

Now, here in the center, we have what we’re going to refer to as voltage-gated sodium channels. What happens is, you have all these sodium ions on the outside of the cell, and these sodium ions want to get into the cell, but they cannot get into the cell. Why can’t they get into the cell? Because the voltage-gated sodium channels are closed. In order for them to get into the cell, this channel needs to open.

Now, why does sodium want to get into the cell? Well, if you go back to Episode 6, this can refresh your memory a little bit, we spoke about Donnan equilibrium. And we spoke about the fact that the membrane potential at rest is somewhere between -50 to -70, -80 millivolts. At that negative charge, sodium ions are not happy. In order for these guys to be happy, the membrane potential needs to be around +58. And on the inside of the cell, we have a negative charge, so this is not something that sodium likes.

So how does sodium want to counteract this? Well, sodium wants to rush into the cell so that it can make the membrane potential more positive, which will be closer to the Donnan equilibrium potential for sodium ions. I hope that makes sense, if not, once again, go back to Episode 6 and refresh your memory on how the Donnan equilibrium works.

So, someone touches you or there’s some stimulation and there needs to be an action potential that’s sent along the axons of the neurons that are stimulated. What happens is, when the membrane potential reaches threshold, that is enough charge to cause voltage-gated sodium channels to open. Once those voltage-gated sodium channels open, sodium ions can then go into the cell.

Now, what is this going to do to the charge inside the cell? What is it going to do to the membrane potential? Well, now you see you have all these positives on the inside, so that’s going to make the membrane potential more positive. This is the process of depolarization. It is the first phase of the action potential, and this is how the charge starts that gets sent along the axon.

So once again, when the membrane potential reaches threshold, voltage-gated sodium channels open and once those open, sodium ions are going to rush into the cell, making the membrane potential more positive, and that is depolarization. I hope that makes sense to you. If you have any questions, as usual, feel free to ask them in the comments below, and I’ll be happy to answer your questions. And who knows, maybe even make a video answering your specific question. That’s it for this video, and I’ll see you in the next one.

Comments 171

  1. Farzana

    WOWOWOW this video is amazing and I am really starting to understand the process of the action potential. THANK YOU SOOOOOOOOOOOOOOO much for making these videos!!!!!!

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  2. CoolItisBand

    I want to thank you about this great explanation. I’m a medice student from
    Argentina, my english level is not as good as i would like but I understood
    the whole video. Thank you.

  3. InteractiveBiology

    @DeMuslim You should go to my channel and check out the episode 047. It’s
    about the action potential in Cardiac muscle cells.

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  4. NeedsAHardOne

    how is the threshold potential reached?
    I’ve read that the axon is MUCH more permeable to K+ than Na+,
    so is the threshold potential initially reached because of some K+ flowing in?
    Would that also answer why there are K+ ions in the axon?

  5. InteractiveBiology

    @kvmackay1321 Glad to know it made it all clear for you. We have more
    Biology videos that you might find useful, as well. Stay tuned!

  6. ravinderguliasls

    thanx for such a nice videos but need video of spinal cord, amygdala, hippocampus, stratum,

    I will be than full to you.

  7. InteractiveBiology

    I always find it interesting when people TELL me what they NEED me to do as opposed to asking me if I would be able to do it. You would think they are paying me ;)

  8. psipsychologytutor

    Oh very cool~ and here am I at the table with students using cardboard axons and spreading rock salt all over the shop ~:-)

  9. AmoebaLasVegas

    I love your hands-on approach to this very tricky topic.
    The only thing I would suggest is to avoid statements such as “Sodium wants…”, “Sodium likes…”. It is pedantic, but I have found that when I have made such statements in the past (and it is really hard not to) students start to think that, for example, atoms have human traits. Anyway, keep up the great work.

  10. Gerald Carey

    I love your hands-on approach to this very tricky topic.
    The only thing I would suggest is to avoid statements such as “Sodium wants…”, “Sodium likes…”. It is pedantic, but I have found that when I have made such statements in the past (and it is really hard not to) students start to think that, for example, atoms have human traits. Anyway, keep up the great work.

  11. Gerald Carey

    I love your hands-on approach to this very tricky topic.
    The only thing I would suggest is to avoid statements such as “Sodium wants…”, “Sodium likes…”. It is pedantic, but I have found that when I have made such statements in the past (and it is really hard not to) students start to think that, for example, atoms have human traits. Anyway, keep up the great work.

  12. olivia

    Hi Leslie,Thank you soooooo much for doing these videos for us!!These videos help alot!!!
    Btw i found the power point of the neurons system and just wondering where is the video bout this PW?

  13. hmds88

    I sincerely appreciate your existence and the time you took out of it to create these wonderfully helpful explanations. Trust me, hungry minds cherish this stuff. Thank you

  14. hmds88

    I sincerely appreciate your existence and the time you took out of it to create these wonderfully helpful explanations. Trust me, hungry minds cherish this stuff. Thank you

  15. Laurie

    What would I do without your quizzes and your videos? I’ve learned more from your site than in class. So,I want to thank you for all your time that you put into sharing your knowledge with the world!

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  16. Azalea Johnosn

    I wish I found these videos last year. However they are a huge help to me now I’m in my second year of study. Thanks a lot.

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  17. Em

    Hello Leslie! I am a postgrad student in the UK doing experimental psychology, trying to revise action potentials for an exam in a few weeks and wanted to let you know that your videos are wonderful. Succinct and neat – the perfect revision and study tool! Thank you so very much!

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  18. ramesh

    Thank you Samuel. It was refreshing going through your videos. keeping doing this for the fun of learning biology.

  19. Alexa


    Excellent videos! I’m using them to brush up on my cardio stuff before my rotation with a cardiologist. This is a big help. You have a really nice voice and explain things very concisely. Keep up the great work!

    Best of luck,


  20. PmCZAkuan

    What would happen to the resting membrane potential if the extracellular potassium concentration was increased and how would this affect the neuronal action potential?

  21. DrMythology

    Hi, i understand why the entry of Na will cause depolarization for example from -90mV to -10mV the polarity ‘negativity of the cell’ is being lost.. this is totally understandable but here is where i get mixed up >>> when the cell is in a depolarizing state when it reaches to ”’+” mV for example +10mV isn’t that being also polarizing to the other side ?? why is it still called depolarizing?? can anyone explain to me .. Thank you

  22. bautista1

    At rest, the action potential is negative due to more Sodium ions flowing out of the axon correct? And it is less Potassium that flows inside, correct?

  23. SplashFlip

    Just wondering if there is a chapter 8? i couldn’t find it in youtube though i searched. I don’t wanna miss any of these awesome videos!

  24. luvlyvero14

    I love the first line of all his videos.. “Hello and Welcome to interactive biology tv. Where we’re making biology fun!” lol.. sounds like he really enjoys his subject matter. Pleasant voice for learning..

  25. Tasha Tolbert

    Wow! You are an EXCELLENT teacher!!! I have been watching your videos all day going over some of the subjects that I did not understand in class. Your videos have made me more interested in the material, and I have actually been understanding the information as well!! I haven’t gotten bored at all!! You are a truly gifted man. God bless you:-) You are quite handsome as well:-) Thank you.

  26. OriginalRipndip

    West Indians apply a simple approach and logic to learning. feel free to merge the 2 vids on this subject we don’t have the attention span of a neuron we are here to learn for our intense classes. thx doc

    do u have playlists grouped by systems?

  27. NaniiStarlight

    I’m from The Netherlands and I’m studying Psychology and the English Biology books we have to use are soooo hard but this is amazing!! it makes it really easy to understand, thank you!

  28. TheAdmiston

    hey! sorry a have a dilema.,i see that you respownd at this question..how is possible that one small stimulation at the one part of neuron can be transmited to oposite part ..neurons can be very long..over 30 cm.. i don’t underestand how ions chanels is opened..i understand the first chanel is open by the stimulation but after that? i look all over the youtube but i find smth like this..”the signal is transmited afther that to all over the cel..”how more exactly?who opens the others ion chanels?

  29. zmerka1

    I want to sincerely thank you for this fountain of knowledge. Your videos have gotten me thru A&P classes and biochemistry classes. I am always amazed at the generosity of people like you who put so much time and effort bringing something like this to the world.

  30. Tan Run Yu

    2:46 how to make the membrane potential reaches threshold ? is it by increasing the amount of Na ions outside the cell ?

  31. caxsmith

    Is there food you can eat that will affect the threshold? I’m thinking of people with gluten reactive disorders, not celiac disease as such. And is this related to the threshold of pain a person has?

  32. Coco Duncan

    A friend advised me to come check out your videos and I am understanding so much from you than my own anatomy&physiology professor. Your videos are a great study tool. Thank you for your expertise.

  33. Dragonflowi

    Hey excellent video !!! I have a question. I dont understand why inside the membrane the charge is negative if it contains Potassium K+ that also has a positive charge ? Could you explain to me why is this ? Thx in advance

  34. Underground308

    When something “clicks”, Its the best feeling in the world!!! I understand!!! After feeling hopelessly lost for so many night, I get it!!! l! You sir are awesome!! Thanks!!

  35. vladbio

    Hi Leslie, great video – as usual.
    I was wondering what stimulus can create the initial difference sufficient to open Na gated channels?
    Thanks in advance.

  36. Hayah Bellatrix

    thanx a lot 4 ya video brother. May Allah reward u.. will u please guide me to ur episode 6… i’m unable to find it!

  37. Ray

    Great vids, but can you Please remove the sign up box that pops up from the beginning of every window? Because even though I’ve signed up, it still pops up before every videos and it’s hard to keep writing that info in when you’re watching 40-50 videos and you have to write it in before each one. But other than that, amazing videos!!!

  38. Ravneetu

    Hey! Could you please make a video on Cardiac action potentials, SA node,and ventricular action potentials? Along with Cardiac muscle contraction? Maybe even the Cardiac cycle if you have time! You would be a lifesaver!!!

  39. Foxy Roxy

    I love your videos. As I read each chapter of my Anatomy & Physiology book I watch one of your videos for a clear understanding of what I’m reading. Your videos are easier to follow. Keep up the good work!!! You have been a great help.

  40. Karen Peaden

    thank you! I am 56 yrs old and taking ECG and need all the help I can get! the heart is very complicated and you are great at explaining things! you are great at teaching! thank you again!

  41. Akash Ahmed

    Hello everyone. Fabulous video clip. My close friend had been fat. He converted himself from 283lbs of pure fat to 215lbs of purely natural muscle. Everyone was amazed. I just subscribed personally coz I’m planning to get stronger muscle mass. He used the Muscle Building Bible (Google it)…

    1. teresa

      You have been soooooo helpful. I am thankful for your videos. Really, because of them I have passed difficult tests in the last year!
      Thank you so very much <3

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  42. leo

    Great videos !!!!
    just a question how can it reach the threshold if the voltage gated channel is closed? is that with the passive transport ?
    thans a lot

  43. Ingrid

    Hi Leslie!

    Thank you SO SO very much for these wonderfully composed and easy to understand lessons; the combination of visual video with simple, well-written transcripts increased my rate of learning and absorption by ten-fold (at least). I am an English major attempting to get into a Nursing program and thus am spending my summer filled to the gills with biology and physiology courses. I’d been pouring over my textbook, our class lab book, and website after website trying to understand the connections between concepts that you made so rapturously clear in your beautifully organized lessons all across your website. Keep up the good work, you’re helping a great number of people, and I wish you the best of luck in whatever path you choose to take.

    Thanks again,
    (Bethesda, Maryland USA)

  44. barbara r.

    This has been super helpful, I swear I learned this before but every time I start up on the information again I realize I forgot again. Pharmacology is kicking my arse. I have googled, I have looked at old notes, I have watched other videos but this has been most helpful, great thanks to you

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