012 The Absolute and Relative Refractory Periods

012 The Absolute and Relative Refractory Periods

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.

156 comments
simer sukh
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.

mypassioninspiration
mypassioninspiration

Thank you, Mr. Samuel! You make me confident to get an A for my class! I really appreciate it:]

Glenda V.
Glenda V.

Thank YOU, Leslie! God Bless YOU!

suplib
suplib

The voltage axis shows that the depolarization reaches 40 mV. Isn't it 30 mV?

KaYuk Yuen
KaYuk Yuen

what about supranormal period(SNP)?

kobi2187
kobi2187

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EVI I
EVI I

Thank you so much! I am taking this in class and you were incredibly helpful!

Diane
Diane

This is Amazing!

043justdoit
043justdoit

what about M gate and H gate in voltage-gated sodium ion channel?

043justdoit
043justdoit

yeah using ion transporters(pumps) against concentration gradient

Motahare Vamegh
Motahare Vamegh

I totally Understood the concept xD Thank u sooooo muchhhhhhhh :D

Yeeb Xyooj
Yeeb Xyooj

Thank you for the video. This helps a lot :))

Kimberly Shell
Kimberly Shell

yes it would be considered a hyperpolarization effect.

John Guillen
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.

baileyp27
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.

micklo1
micklo1

Superb! A lesson well learned by this video.... by a great teacher.. I must say. Thanks

NightWanderer01
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!

NightWanderer01
NightWanderer01

Yes, Na+/ K+ ATPase restores these ion concentrations back to its resting levels.

JBitzz
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

4thlineforlife
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?

Tasha Tolbert
Tasha Tolbert

Could you do a video an muscle twitches and excitation-contraction coupling??

Daniel Wood
Daniel Wood

I didn't know the guy from shfifty five knew about action potentials.

GeminalAngel
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

eliyunck
eliyunck

Given a tracing of membrane potential over time, identify absolute and relative refractory periods?

preet
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.

Kim
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.

kazama246
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

raxelson1
raxelson1

Wow thank you so much. That was so easy to understand !!!! Thanks again !

raxelson1
raxelson1

Wow thank you so much. That was so easy to understand !!!! Thanks again !

Haldouda
Haldouda

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

Haldouda
Haldouda

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

landigger
landigger

Why Hypocalcemia causes Contraction and Hypercalcemia cause Weakness? hope u can help

landigger
landigger

Why Hypocalcemia causes Contraction and Hypercalcemia cause Weakness? hope u can help

ThePhenom94
ThePhenom94

MAN THANK YOU! That was so easy and you explained it better than my teacher! ThankYou!

ThePhenom94
ThePhenom94

MAN THANK YOU! That was so easy and you explained it better than my teacher! ThankYou!

alphaWAYNE
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.

alphaWAYNE
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.

wahid001
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.

wahid001
wahid001

Or others could contribute to answer some questions while we wait for Leslie.

wahid001
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.