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.

 

310 comments
crystal
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!

Ahmed Maher Hawila
Ahmed Maher Hawila

how can the nerve respond to a stimulus during -ve after potential ..isn't this response requires soduim influx .. isn't its conc reversed ..? please, answer my question .. thank you .

Schipper Remco
Schipper Remco

thanks so much. It really really helps me and gives me hope I can really get through this study.

Eargasm75
Eargasm75

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

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hasib nafis

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Lizzette Rojas
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.

Shane Cronin
Shane Cronin

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

Pierre Marcelin
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?

Germy E
Germy E

Very precise, informative, and clear video! Thank you sir ^_^

T Esco
T Esco

Brilliant. Your explanation and method of instruction is in par, thanks!!!!

Sheyna Hoitsma
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!

mrgetrealpeople
mrgetrealpeople

So is potassium excitatory meaning it would increase resting threshold or is it potassium inhibitory?

SuperKaneification
SuperKaneification

Thanks for taking time out of your day to make this it really helped me out

Tay Cole
Tay Cole

This is going to help me out on my next test. Thank you for posting this video!!

Alex Seb
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.

daras1871
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)?

sam aryan
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

sam aryan
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)

Laurie Bergman
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.

MrTinkasaurus
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 :)

4kezzah
4kezzah

why does the membrane potential not reach the equilibrium potential for Na+

4kezzah
4kezzah

how does the stimulus cause the membrane potential to become more positive?

nazrull haziq
nazrull haziq

Thanks ! i never understand Action Potential thing, but now i can really understand it 100 %!

hiral san
hiral san

very clear explaination. thnk you very much.

Lara Romero
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! :-)

Kalyca Becktel
Kalyca Becktel

Thank you! How would we label IPSP and EPSP on a graph such as yours.

deballah1
deballah1

your work is truly appreciated,thank you

embids
embids

Thank you! This video helped me understand Action Potential a lot better.

BlueV Jones
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)