episode45

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

Leslie Samuel IBTV, Physiology, The Circulatory System 170 Comments

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.

187 comments
Sara
Sara

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 ^_^

Eimear Short
Eimear Short

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

Aaron Bell
Aaron Bell

Amazing, in human physiology this semester so this is helpful.

Kathy Le
Kathy Le

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.

Lex
Lex

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

Akos Agyire
Akos Agyire

Can I borrow your brain for my exam tomorrow? Lol thanks for the review!

JJvideoman
JJvideoman

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.

zafarz8
zafarz8

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

Malibu95843
Malibu95843

Very well explained! Thank you so so much!

captainicehockey
captainicehockey

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).

suraj ali
suraj ali

very outstanding teacher love u kisssss u

Gileiys680
Gileiys680

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

EmTheGeeky
EmTheGeeky

Thanks I think I finally understand this :)

dekalbwrestler
dekalbwrestler

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.

WhatU GonnaDo
WhatU GonnaDo

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

ZAR1411
ZAR1411

yes that's right.. because of the equilibrium

ZAR1411
ZAR1411

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 :)

Tommy Moore
Tommy Moore

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!!

chicmistique
chicmistique

thank you so much for posting these videos, they're all very very helpful

Justin Forester
Justin Forester

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 :P

mwilso9123
mwilso9123

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?

greenlightwarfare .
greenlightwarfare .

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!

steph10892
steph10892

Another helpful video! Thank you!!

Daniel O
Daniel O

Very informative I love your videos, Keep them up

HaneenA91
HaneenA91

perfect! thanks for this effort.

SilvieOlgan
SilvieOlgan

i have an exam tomorrow and this is great! :D

Marcela Martinez
Marcela Martinez

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

Gabrielle C.
Gabrielle C.

Thank you!! You made it so easy to understand!

Kyle Danyluk
Kyle Danyluk

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

artem bluntzki
artem bluntzki

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.

HorseLvrArtwork
HorseLvrArtwork

Thanks so much for putting this up. Appreciate it so much. Bless you.

TheGreatNerdHerd
TheGreatNerdHerd

thank you soo much for this heart series as i'm really bad at it as my mock proved :D

Kristin Wells
Kristin Wells

Amazing videos. Thank you so much. Making my classes so much easier to understand!

omega13594
omega13594

A great vedio .. thanks sir (lub-dup:))