045 The Pacemaker Potential of the SA Node and the AV Node

In this episode, Leslie talks about how a pacemaker potential can cause a heart to beat automatically. Details about how it is generated is discussed in this video. Just how does this happen, our heart beating again and again?

Watch to learn more. Have fun and enjoy!

Transcript of Today’s Episode

Hello and welcome to another episode of Interactive-Biology TV where we’re making Biology fun. My name is Leslie Samuel and in this episode, Episode 45, I’m going to be talking about the pacemaker potential of the S.A. node and the A.V. node. We’re basically going to look at how this results in the heart beating automatically. So, let’s get right into it.

Let’s first talk about the S.A. node. The S.A. node stands for the sinoatrial node and you can see it in this figure over here, it is number one. That’s this cluster of cells. It is basically a specialized group of cardiac muscle cells that don’t contract which is kind of strange. They’re muscle cells and they don’t actually contract.

But, what’s special about these cells is that they are adapted to automatically generate impulses. So, it can automatically cause signals that can spread throughout the heart, causing the heart to beat. The S.A. node functions as the pacemaker of the heart. Yes, we have the A.V. node and some other stuff that we are going to talk about but, these generates signals faster than any of the others so, it sets the pace for the heartbeat.

As you can see, it is located in the right atrium. So, now let’s talk about the A.V. node.

The A.V. node is number two. So, it’s this cluster of cells here and it stands for the atrioventricular node. It is similar in function to the S.A. node in that it automatically generates impulses and it is located between the atria and the ventricles hence the name, atrioventricular node. Let’s go back to the S.A. node and see how this results in the pacemaker potential.

Before we look at that, I just want to point out that we have, in addition to the S.A. node and the A.V. node, we have some fibers that extend from the A.V. node and spread throughout the ventricle and those fibers are called Purkinje fibers. These are also very important in that they spread that signal throughout the rest of the ventricle. Let’s talk about the S.A node.

We said that that functions as a pacemaker. So, we are going to look at the pacemaker cells that we have in the S.A. node. What is special about these cells is that normally, there’s a significantly higher conductance for sodium than there is for potassium. Now, if you go back to Episode 006, I talk about Donnan equilibrium and driving force and I show how there’s normally a driving force for sodium to rush into the cell. I also show that potassium wants to leave the cell.

Because the cell is much more permeable to sodium, we’re going to have a situation where there’s much more sodium coming in than potassium leaving. Because we have more positives going in than leaving, what we’re going to get is a pacemaker potential where the cell normally depolarizes. Then, when it reaches the threshold, something interesting happens. Yes, we have the sodium rushing in and some potassium leaving but, now that we’ve reached the threshold, voltage-gated calcium channels open and calcium is going to rush into the cell.

So, we’re going to get this rapid depolarization. In other words, we’re going to get an action potential. At the peak, we’re going to get a different situation where, yes, we have sodium coming in and potassium leaving but, voltage-gated potassium channels are going to open so that the conductance for potassium increases significantly and potassium is going to rush out of the cell repolarizing the membrane.

At that point, we still have the sodium that’s coming in and the voltage-gated potassium channels close so, we have the initial situation where sodium is rushing into the cell, causing this depolarization then, the same thing happens. It reaches the threshold, voltage-gated calcium channels open depolarizing the cell membrane once again, causing that impulse. Voltage-gated potassium channels open causing potassium to rush out of the cell again.

This process continues over and over and over. What ends up happening is we have this automatic signal that’s generated constantly resulting in the contraction of the heart. This causes the heart to beat. It’s really that straightforward but, the main idea is that the cells in the S.A. node have a significantly higher conductance for sodium so it continuously depolarizes causing that impulse that causes the heart to beat.

That’s really all I want to talk about in this video. As usual, you can visit the website at Interactive-Biology.com for more Biology videos and other resources to help make Biology fun.

This is Leslie Samuel. That’s it for this video and I’ll see you on the next one.



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  • in Vanders Human physiology,it is written that Calcium channel open only briefly and it is an important depolarizing boost to pacemaker potential.
    nice video!(thumbs up)

  • That’s correct. When the pacemaker potential reaches threshold, the Calcium channels open briefly causing the depolarization. That’s what is illustrated in the video. Glad you like it.

    All the best!

    Leslie

  • @tiarafazlin17 I LOVE saving lives with Biology πŸ˜€ – Stay tuned for MANY more, and make sure to share the site with EVERYONE you know who might benefit πŸ˜‰

  • i got 87.5 on my first lecture exam , i used your information and i guess it works out really well…. you don’t need be a rocket scientist to understand this.:)

  • Do you have any videos that are more specific with the mentioning of funny channels/T-type channels/L-type channels and when those, specifically, come into the picture, etc?

  • Do you have any videos that are more specific with the mentioning of funny
    channels/T-type channels/L-type channels and when those, specifically, come
    into the picture, etc?

  • when the level of potassium is high the heart will start beating irregularly , or leads to Cardioplegia,, why is this so,, what is the mechanism that cause the heart to be in this condition

  • when the level of potassium is high the heart will start beating irregularly , or leads to Cardioplegia,, why is this so,, what is the mechanism that cause the heart to be in this condition

  • @zamirahbasher All questions are answered in the Interactive Biology community forums from now on. Go to the website in the description and then visit the community. This is to make it as efficient as possible as we have multiple people over there to help answer questions.

    All the best

  • I like your series of videos but in this case, you really should explain how the Na+/K+ pump works in the pacemaker cell first to set up the potential gradient before the voltage sensitive K+ channels close while the Na+ is still pumped out. At this negative membrane potential, the Na+ starts to flow into the cell against the concentration gradient by diffusion, thus activating the potential of the cell which is the cause leading to -40mV where Ca+2 ions start flooding in.

  • @zackboomer Unfortunately, Leslie is busy at the moment with more work to do for the site. He is unable to answer any questions. But, do stay tuned because more biology videos are coming very soon!

  • Unfortunately, Leslie is busy at the moment with more work to do for the site. He is unable to answer any questions. But, do stay tuned because more biology videos are coming very soon!

  • @petercourt The Calcium-induced calcium release by is sequestered back into the SR. The remaining calcium is pumped out of the cell by the Sodium Calcium Exchanger. The sodium is then swapped back out for potassium.

  • The Calcium-induced calcium release by is sequestered back into the SR. The remaining calcium is pumped out of the cell by the Sodium Calcium Exchanger. The sodium is then swapped back out for potassium.

  • The Calcium-induced calcium release by is sequestered back into the SR. The remaining calcium is pumped out of the cell by the Sodium Calcium Exchanger. The sodium is then swapped back out for potassium.

  • All these videos are great. Please keep them coming. You are making
    learning a lot easier!

  • i dont know how u do it but it is clear that u hve a BIg heart….giving free lecture to others.Im a veterinarian and this helps me a lot to recollect the things which i have forgtn

  • i dont know how u do it but it is clear that u hve a BIg heart….giving free lecture to others.Im a veterinarian and this helps me a lot to recollect the things which i have forgtn

  • @mlalramhluna Thank you. It’s what Leslie main goal is, to be able to share his knowledge to those who need them. Glad that you’re finding value in his videos. Stay tuned for more Biology fun!

  • @mlalramhluna Thank you. It’s what Leslie’s main goal is, to be able to share his knowledge to those who need them. Glad that you’re finding value in his videos. Stay tuned for more Biology fun!

  • Thank you. It’s what Leslie’s main goal is, to be able to share his knowledge to those who need them. Glad that you’re finding value in his videos. Stay tuned for more Biology fun!

  • Thank you. It’s what Leslie’s main goal is, to be able to share his knowledge to those who need them. Glad that you’re finding value in his videos. Stay tuned for more Biology fun!

  • Thank you. It’s what Leslie’s main goal is, to be able to share his knowledge to those who need them. Glad that you’re finding value in his videos. Stay tuned for more Biology fun!

  • The Calcium gate open before membrane reach threshold potential

    not till reach threshold potential..

  • @SHFOBA

    I believe this is the correct sequence of events:

    The T-type Calcium channels open after the funny channels closes, continuing depolarization. This brings the cell potential to the threshold which triggers the L-type calcium channel to spring open, allowing large amounts of Ca++, causing the action potential. I think he just merged both T and L type Ca++ channels together.

  • I believe this is the correct sequence of events:

    The T-type Calcium channels open after the funny channels closes, continuing depolarization. This brings the cell potential to the threshold which triggers the L-type calcium channel to spring open, allowing large amounts of Ca++, causing the action potential. I think he just merged both T and L type Ca++ channels together.

  • I believe this is the correct sequence of events:

    The T-type Calcium channels open after the funny channels closes, continuing depolarization. This brings the cell potential to the threshold which triggers the L-type calcium channel to spring open, allowing large amounts of Ca++, causing the action potential. I think he just merged both T and L type Ca++ channels together.

  • Shouldn’t there be a plateau phase? Seems like it is more of a skeletal muscle of action potential. Still very helpful though.

  • Thank you. This really helps.. as I struggle with class lecture. This puts it all together for me. God Bless.

  • Hi loved the video, is there any chance that this video is in spanish? I need it to present it in class, thanks!

  • what is the cause the voltage-gated calcium channels open when we’ve reached the threshold?

  • so the Na keeps building up inside the cell and K keeps coming out? Where does it come from?

  • just want to say thank you for making my life SO much easier ’cause I have an exam thats going to be 18 chapters…I don’t know how I will read it all. THANKS a lot πŸ˜€

  • just want to say thank you for making my life SO much easier ’cause I have an exam thats going to be 18 chapters…I don’t know how I will read it all. THANKS a lot πŸ˜€

  • I really just need to verify/clarify something: The sodium ion continuously pumps into the cell and never has a period that it changes course and gets pumped out by Na/K pumps??
    This seems counter-intuitive – sodium can’t just enter the cells forever without putting a huge osmotic pressure on the cell and bursting it??? Can it?

  • I really just need to verify/clarify something: The sodium ion continuously pumps into the cell and never has a period that it changes course and gets pumped out by Na/K pumps??
    This seems counter-intuitive – sodium can’t just enter the cells forever without putting a huge osmotic pressure on the cell and bursting it??? Can it?

  • Great vids. BS in bio, prepping for PA school, love these videos. Really great physio refreshers.

  • You save my life. I’m a working child and I end up missing a lot of class so these videos really really help me. Plus, you make things easy to understand and give a reason for everything which in turn makes it super easy to follow.

  • THANK YOU πŸ™‚ oh this helps me visually . I could not understand what contraction meant (i speak a different language) but seeing the first part now this really helps more than the text book and the lecture audio from my instructor

  • tnx we need more like u thanks so muchhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh so clearrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr

  • thank you soo much for this heart series as i’m really bad at it as my mock proved πŸ˜€

  • Thank you. You just saved me a possible 40 minutes trying to figure out what my textbook is trying to tell me haha, 20 minutes of aimlessly staring into the pages and another 20 trying to read half latin/greek derived words. You display complex effects in simple terms and still manage to include all the crucial information.

  • Thank you so much for this explanation. This has been tremendously helpful and I cannot thank you enough!

  • You are amazing!!!. Do you have any videos in which you explain the blood types and Rh factor?.

  • it helped me alot thank you!!!!! πŸ™‚ u explain it very well πŸ™‚

  • Doesn’t the influx of calcium followed by the opening of voltage gated K channels result in a plateau ? I’m really confused – please help!

  • This is like a skeletal muscles A.P?. Im a little confused my teacher was talking about funny channels, where does that come into the picture?

  • There are two types of cardiac muscles: contractile muscle cells and autorhythmic muscle cells. Contractile cells make up 90% of muscle cells in the heart and autorhythmic cells make up 10% in the nodes. Autorhythmic muscle cells start the action potentials from the SA and AV nodes that disseminate into contractile cells causing them to contract. InteractiveBiology described the action potential of autorhythmic cells, you described contractile cells. Hope this helped πŸ˜›

  • You are an awesome individual to take the time to put all these videos. And you make it so simple and easy to understand without getting lost. Thanks MUCH!!

  • I just love how this particular person gives free lectures plus all the interactive are very easy for me to understand..keep up the good work leslie! You’re making everyone’s life easier πŸ™‚

  • I never understood this fully before, but you make things that much easier to understand. I can’t thank you enough!!!

  • Leslie? Are you like a biology teacher or something? Maybe you just a nerd. Whatever the case thank you so much sir! I can understand you spoof much more than my cardiac teacher. She might be a super smart retired dr. But, there is something to say a out someone like you that can actually make you understand . I love you voice too. It’s soothing and intelligent sounding at the same time.

  • Have you seen MAD Muscle Ripper? (Google it) It is a quick way for you to bulk up fast.

  • Thank you for your help. I’m a paramedic student and you broke down the depolarizing-repolarizing ion involvement very well for me to understand (we are currently beginning to analyze ECGs).

  • why is potassium leaving the cell all the time,, doesnt potassium ever come back into the cell ?

  • when potassium leaves a cell not all of it goes. Its only a percentage change that causes the impulse to occur. So during stages in-between stimulation the potassium does move back in and replenish.

  • Wow. That was so unbelievably clear. Thanks for making such a complicated concept fun and so easy to understand!

  • since there’s a higher concentration of K+ inside the membrane then outside, when K+ channels open, K+ will rush out because ions always move from higher concentrations to lower concentrations. The Na+/K+ pump then uses ATP to pump 2 K+ inside the membrane while pumping 3 Na+ outside so that a high concentration of K+ on the inside, and high concentration of Na+ on the outside will be maintained. This also makes the inside of the membrane negative again and return it to a resting potential.

  • I was taught that there are no functional Sodium ion channels in pacemaker
    cells, is this wrong?

  • OMG
    Thaaaaank u soo much … u helped me :))))
    It seems difficult when my physiology professor say it
    but it’s totally easy when u explain it
    Bless u ^_^

  • This video is amazing!!! It is brief but perfectly explain the cardiac electrical activity! Really thanks !

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