012 The Absolute and Relative Refractory Periods

012 The Absolute and Relative Refractory Periods

Leslie Samuel IBTV, The Nervous System 131 Comments

Refractory Period? What is that? If you are asking that question, then you want to watch this video.

It explains why you can’t stimulate another action potential at certain times regardless of how strong the stimulus is and why it takes a stronger stimulus to cause another action potential in specific situations.

Check it out, and if you’re left with a question or comment, leave it below.

- Leslie Samuel

Transcript of Today’s Episode

Welcome to another episode of Interactive Biology TV. My name is Leslie Samuel. In this episode, Episode 12, we’re going to be talking about the absolute and relative refractory periods. But before we talk about these refractory periods, let’s look a little bit at voltage-gated sodium channels. Now, we’ve been looking at the action potential, and we’ve said that when a stimulus comes and it makes the membrane potential go above the threshold, we get an action potential. The reason why we get this action potential is because voltage-gated sodium channels open.

Now, voltage-gated sodium channels are very unique, in that they have 3 states that you can find them in. They can either be closed, or they can be open, or they can be inactive. How this works is very simple. They have an activation gate, and they also have an inactivation gate. When the stimulus causes them to open, the activation gate opens, and after 0.5 to 1 millisecond, the inactivation gate automatically closes. What is special about these voltage-gated sodium channels is that once it’s open or inactive, it cannot be re-stimulated to open, because it’s either already opened, or it’s inactivated. With that in mind, let’s go and take a look at what causes the absolute and relative refractory periods.

Here, I am looking at a neuron, and you can see the neuron over here to the right. You know by now the parts of the neuron. Here we have a soma, and then here we have the axon. The main part that we’re going to look at today is what is happening in the axon, like we’ve been looking at in the last few episodes.

If I’m going to look at an action potential and I’m looking at what is happening to the membrane potential, here you can see that we have a stimulus, but it doesn’t cause an impulse. We have another stimulus, but still it does not cause an action potential. But if the stimulus reaches the threshold, we have depolarization. For a review of depolarization, see the episode on depolarization.

What this means is voltage-gated sodium channels (I’m going to write Na+ for sodium ions) open, and sodium rushes into the cell, causing the membrane potential to become more positive. Now, while that is happening, this means that the voltage-gate sodium channels are either open, or they are inactive. So they open first, and after a short period of time, they become inactive. While this is happening, no matter what you do, you cannot cause another action potential, because this one is already on the way, and the voltage-gated sodium channels are either open or inactive. So it does not matter what you do, we will not get another action potential. This is called the absolute refractory period. So we have the ARP, for the absolute refractory period, because the voltage-gated sodium channels are either open or inactive.

As I said in the previous slide, in order for another action potential to happen, those voltage-gated sodium channels need to be reset to close. When we reach the repolarization phase and potassium rushes out, the membrane potential starts going down. As it starts going down, the voltage-gated sodium channels start resetting to their closed state. Once they start resetting to their closed state, you can stimulate it to do another action potential.

However, if there are only a few sodium channels reset, it’s going to take a significantly stronger stimulus to cause the membrane potential to reach the point where we can stimulate the action potential to happen again. I’m going to say that again. When the voltage-gated potassium channels open and potassium ions rush out of the cell, the membrane potential is going to start going down because it’s repolarizing. Once that starts happening, voltage-gated sodium channels start being reset to their closed state. You can stimulate it to have another action potential, but it’s going to take a stronger stimulus since you have fewer channels being reset. That is the relative refractory period (RRP). And that continues, more and more channels are being reset to the closed state, and when they’re all reset to the closed state, that is the end of the relative refractory period.

So you stimulate the axon, you get an action potential, voltage-gated sodium channels are either open or inactive, and you cannot stimulate it again. That is the absolute refractory period. Once they start resetting, you have the relative refractory period, where you can stimulate another action potential, but you will need a stronger stimulus. So that’s the absolute and the relative refractory periods.

I hope it makes sense. That’s all for this video, and I’ll see you in the next one.

Comments 131

  1. Akbar M.

    Could you please elaborate a bit more on the inactive state of the Na+ channel? How does it differ from the closed state?

    P.S.
    I am a pharmacy student and I think that your videos are great! You explain things so well! Thank you!

    1. Post
      Author
        1. Post
          Author
          Lrsamuel

          Hi Marie, unfortunately, I can’t take any special requests anymore due to time limitations. I’m making the videos as I need them for the classes I’m currently teaching.

          All the best!

  2. daniel

    how could a cell be modified to increase or decrease the maximal flux of a solute across the plasma membrane by meditated transport mechanism?

    thank you

    1. Post
      Author
      Leslie

      Hi Daniel,

      That’s an excellent question. Think about it like this – The factor that regulates the ability of a solute (ions) to cross the membrane would be the presence of those ion-specific channels. It’s kind of like having doors in a building. In order for people to get in, they need to use the door. In order for ions to cross the membrane and get into the house of the cell, it needs to have channels.

      So the question is – If you want to maximize how many people can enter a building, what would you do? Well, you’d probably put more doors. If you want to maximize the flux of a solute across the plasma membrane by mediated transport mechanism, you would want to have more channels available. So, if a cell were to be modified to have more channels, that would increase it’s conductance and allow for a greater flux of the solute.

      Hope that answers your question :)

  3. InteractiveBiology

    @BreatheinStandstill LOL, it’s kinda hard to understand gibberish ;) Glad
    you found value in the videos!

  4. fandanstan

    @ediniz101 Basically, a graded potential that causes membrane potential to
    go above threshold initiates an action potential, which causes the ion
    gates to open. The K voltage-gated channels open slower than the Na ones
    do, which is why they open at different times.

  5. crackowacko87

    Hi again Leslie, is it possible you do a lecture video on calcium channels involved in heart contractions? It would really help as I have a thesis to do and I need some tips and assistance in understanding the concepts before I start writing them. Thanks!

  6. sarva khitana Bunbulama

    Oh I found it. please disregard my request about making a vid about
    refractory period. now I understand, the sodium chanel doesn’t close like
    bang. it closes gradually right? and this causes that the second stimulus
    needs to be stronger than the first one to produce an action potential?!
    thanks for this clip

  7. Djalitana

    Oh I found it. please disregard my request about making a vid about refractory period. now I understand, the sodium chanel doesn’t close like bang. it closes gradually right? and this causes that the second stimulus needs to be stronger than the first one to produce an action potential?! thanks for this clip

  8. skinnimelon

    The OCR A2 textbook stated that “For a short time after each action potential it is impossible to stimulate the cell membrane to reach another action potential”. Another internet source also said that “Immediately after an action potential, the neurone enters a refractory period”. So does the absolute refractory period occurs before or after an action potential is reached(+40mV)?

  9. InteractiveBiology

    Thank you for watching the video. Unfortunately, Leslie is busy at the moment working a lot on improving the site. He is unavailable to answer any more questions. In the meantime, please go to our site directly for more Biology videos. New episodes will be uploaded very soon!

  10. InteractiveBiology

    @skinnimelon Thank you for watching the video. Unfortunately, Leslie is busy at the moment working a lot on improving the site. He is unavailable to answer any more questions. In the meantime, please go to our site directly for more Biology videos. New episodes will be uploaded very soon!

  11. InteractiveBiology

    Thank you :) Glad to know you appreciate this. There are more Biology videos in the website that you might want to check out. Stay tuned for more new videos! :)

  12. InteractiveBiology

    @UniGirl9008 Unfortunately, Leslie has a lot of things to do at the moment, getting busy with the site, his work, and personal life. He’s not had the time lately to check posts and get back to them. Please do stay tuned though as he’ll be uploading more videos soon that may clarify your question. Thank you!

  13. InteractiveBiology

    Unfortunately, Leslie has a lot of things to do at the moment, getting busy with the site, his work, and personal life. He’s not had the time lately to check posts and get back to them. Please do stay tuned though as he’ll be uploading more videos soon that may clarify your question. Thank you!

  14. InteractiveBiology

    Glad we were able to help. Please stay tuned for more Biology learning and fun. We have more Biology videos to be uploaded very soon!

  15. InteractiveBiology

    Unfortunately, Leslie has a lot of things to do at the moment, getting busy with the site, his work, and personal life. He’s not had the time lately to check posts and get back to them. Please do stay tuned though as he’ll be uploading more videos soon that may clarify your question. Thank you!

  16. LucifersCounterpart

    the day just smiled at me :D this is the best channel
    i hav my physiology exam in 2 days n i couldnt understand certain things before finding this channel but as i said grrrrrr8 channel ;D

  17. yashsaxena1217

    @UniGirl9008 Nah, hyperpolarization is a phase not a phase. When the mV drops below -70mV its called hyperploarization, and when it goes back up to -70mV its called polarization. Refractory period is the time it takes for a Na+ channel to open again. This makes sure the action potential moves in one direction.

  18. yashsaxena1217

    @UniGirl9008 Nah, hyperpolarization is a phase not a phase. When the mV drops below -70mV its called hyperploarization, and when it goes back up to -70mV its called polarization. Refractory period is the time it takes for a Na+ channel to open again. This makes sure the action potential moves in one direction.

  19. yashsaxena1217

    Nah, hyperpolarization is a phase not a phase. When the mV drops below -70mV its called hyperploarization, and when it goes back up to -70mV its called polarization. Refractory period is the time it takes for a Na+ channel to open again. This makes sure the action potential moves in one direction.

  20. yashsaxena1217

    Nah, hyperpolarization is a phase not a phase. When the mV drops below -70mV its called hyperploarization, and when it goes back up to -70mV its called polarization. Refractory period is the time it takes for a Na+ channel to open again. This makes sure the action potential moves in one direction.

  21. yashsaxena1217

    Nah, hyperpolarization is a phase not a phase. When the mV drops below -70mV its called hyperploarization, and when it goes back up to -70mV its called polarization. Refractory period is the time it takes for a Na+ channel to open again. This makes sure the action potential moves in one direction.

  22. fireandice5969

    @UniGirl9008 Refractory period = Repolarization. Hyperpolarization are the K+ ions going further down towards the -90mV range before the pump kicks in to return the Na+ and K+ ions to their resting membrane potential.

  23. fireandice5969

    @UniGirl9008 Refractory period = Repolarization. Hyperpolarization are the K+ ions going further down towards the -90mV range before the pump kicks in to return the Na+ and K+ ions to their resting membrane potential.

  24. fireandice5969

    Refractory period = Repolarization. Hyperpolarization are the K+ ions going further down towards the -90mV range before the pump kicks in to return the Na+ and K+ ions to their resting membrane potential.

  25. fireandice5969

    Refractory period = Repolarization. Hyperpolarization are the K+ ions going further down towards the -90mV range before the pump kicks in to return the Na+ and K+ ions to their resting membrane potential.

  26. colacasados

    Hello, and THANK YOU for your videos!!! They truly simplify information. Would you happen to have videos on ALL of the characteristics of Action Potential (including “nondecremental” & “irreversible”)?

  27. Cola Casados

    Hello, and THANK YOU for your videos!!! They truly simplify information. Would you happen to have videos on ALL of the characteristics of Action Potential (including “nondecremental” & “irreversible”)?

  28. Cola Casados

    Hello, and THANK YOU for your videos!!! They truly simplify information. Would you happen to have videos on ALL of the characteristics of Action Potential (including “nondecremental” & “irreversible”)?

  29. wahid001

    @UniGirl9008 When voltage-gated Na channels are inactivated, and voltage-gated K channels open. K exits the cell and repolarizes the membrane. At this time, the membrane is in its absolute refractory period.
    When Voltage-gated Na channels change from inactivated to closed. Voltage-gated K channels remain open, causing a hyperpolarization of the membrane. The membrane is now in its relative refractory period.

  30. wahid001

    When voltage-gated Na channels are inactivated, and voltage-gated K channels open. K exits the cell and repolarizes the membrane. At this time, the membrane is in its absolute refractory period.
    When Voltage-gated Na channels change from inactivated to closed. Voltage-gated K channels remain open, causing a hyperpolarization of the membrane. The membrane is now in its relative refractory period.

  31. wahid001

    When voltage-gated Na channels are inactivated, and voltage-gated K channels open. K exits the cell and repolarizes the membrane. At this time, the membrane is in its absolute refractory period.
    When Voltage-gated Na channels change from inactivated to closed. Voltage-gated K channels remain open, causing a hyperpolarization of the membrane. The membrane is now in its relative refractory period.

  32. alphaWAYNE

    Thank you so much for making these videos… You make biology more fun, easy and very understandable… You are a great teacher. A million thank yous.

  33. Haldouda

    Does that mean that depolorization= absolute? I mean during depolorization, there is no way to start a new action potential…am i right?

  34. kazama246

    you said at repolarization another stimulus must be stronger to trigger an action potential BECAUSE of the fewer sodium channels that were reset…why is that???…anyone help

  35. kazama246

    you said at repolarization another stimulus must be stronger to trigger an action potential BECAUSE of the fewer sodium channels that were reset…why is that???…anyone help

  36. Kim

    I’m so glad I found out about this wonderful website!!! Thanks so much Leslie, you sure did make biology more fun. You would be an awesome teacher :) thanks so much.

  37. preet

    wow, your videos are mind blowing.i was sitting in class had no idea what the teacher was talking about.but as soon as i saw a video on you tue about how muscles contarct, everything made so much sense. You really make learning fun, and easy to understand.keep it up.thank you so much for making it available.

  38. GeminalAngel

    I dont think so…
    In refractory period, no stimulus can cause another action potential. till Na gates open up again from their inactivation state. the inactivation gates usualy open up near the RMP after the repolarization has ended. then a strong enough stimulus cna generate another act. potential.
    In hyperpolarization, the membrane is jst depolarized a bit more cuz K channels havent yet closed.
    I maybe wrong :) just sharing wt I know

  39. 4thlineforlife

    guys quick question. after a stimulus when sodium ions rush into the cell and potassium rushes out are these molecules actively transported back to where they were in first place or what?

  40. JBitzz

    Hey I heard ur interview on the Smart Passive Income podcast!! Now I’m studying for my neurophysiology midterm and I coincidentally stumbled to your videos haha. Th

  41. NightWanderer01

    A refractory period simply refers to the time frame in which the generation of further action potentials may be hindered (RRP) or may not occur at all (ARP). Hyperpolarization, in contrast, describes an event where the cell’s membrane potential drops to levels below its resting potential due to excess K+ leaving the cell. However, it would be correct to say that hyperpolarization could occur during the RELATIVE refractory period (RRP). Hope that helps!

  42. baileyp27

    No, the refractory period and the hyperpolarization are separate events that overlap. This graph could use 2 more variables: % of Na channels open; and % of Na channels inactive. You would see that as the AP starts to spike most of the channels go to open, then around the peak most of the channels go to inactive. The relative refractory period actually starts as the voltage is coming back down from the peak as Na channels transition back to closed and ready.

  43. John Guillen

    Thank you for your videos. As I read these chapters its kind of hard to understand what’s going on without some kind of picture/video example. But watching your videos and then reading makes more sense.

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  45. simer sukh

    Yes, refractory period means hyperpolarization. Because when repolarization is happening the K+ are leaving through the potassium channels, and they reach their equilibrium of -93mV(below resting state -70mV) which is called hyperpolarization.

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