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Leslie explains how action potentials are generated by the cardiac cells of the heart and how the release of calcium can generate heart contraction.
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Transcript of Today’s Episode
Hello and welcome to another episode of Interactive-Biology TV where were making Biology fun. My name is Leslie Samuel and in this episode, Episode 47, I’m going to be talking about action potentials and contraction in cardiac muscle cells. So, let’s get right into it.
I’m looking at the heart. We’ve looked at a number of things related to the heart. In the previous episode, we spoke about the SA node, which is what we see here, number one and, we spoke about the AV node, which is this part here, number two, and we spoke about these Purkinje fibers. I’m just going to write PF for now. So, this is the AV node, the SA node and the Purkinje fibers. You can go back to the previous episode to learn more about those, in case you’re not sure what they do; in case you’re not sure how they function.
There are a number of things that I want you to know here. We said that the SA node functions as the pacemaker. There’s an important feature about the heart muscle cells that you need to be aware of. That is the fact that these cells are all electrically connected. So, all of the muscle cells in the ventricle are electrically connected, all of the muscle cells in the atria are also electrically connected.
What that means is that if one of the cells in the ventricle gets stimulated, that signal is going to travel to all of the other cells in the ventricle. Not only that, but, if the SA node starts a signal, that signal is going to spread. This is why we get the heart contracting in response to the signal that’s generated by the SA node. Then, when it reaches the AV node and it spreads via the Purkinje fibers, that signal spreads to all of the muscle cells in the ventricles, causing the ventricles to contract.
There are some other important details that you need to know. When the signal is generated in the SA node and it spreads to the atria, the conduction velocity is one meter per second (1 m/s). So, the signal spreads at a speed of 1 m/s here. At the AV node, it slows down to where it’s somewhere around 0.04m/s. Then, in the Purkinje fibers, it speeds up significantly, and we get a conduction velocity of 5 m/s.
So, what this means is that we have a signal that starts here and spreads throughout the atria relatively quickly at 1 m/s but then, it slows down at the atrioventricular node to 0.04 m/s. So, there’s a delay here, and then, after it passes the atrioventricular node, that signal spreads rapidly to the ventricles. Now, why do we want this? As we mentioned before, the blood first goes to the atria and then, the atria contracts, sending the blood from the atria to the ventricles.
You don’t want the atria and the ventricles contracting at the same time. That would cause problems. You want the ventricles to get filled with the blood from the atria first and then, you want the ventricles to contract sending all that blood to the rest of the body and to the lungs. So, that’s how that works and that is why it’s good that we have this slowing down at the atrioventricular node.
Now that we know that and now that we understand that the muscle cells are all connected electrically, let’s move on and look at what happens inside the muscle cells.
We have a stimulus that comes from the AV node or the SA node and that spreads to the muscle cells. In response to that, what’s going to happen is that the membrane potential of the cardiac muscle cells is all of a sudden going to depolarize very quickly. So, we have that initial depolarization. When the muscle cells depolarize, as with skeletal muscles, we’re going to have calcium being released from the sarcoplasmic reticulum. For a refresher of how that works, you can go back to Episode 42 where I talked about calcium release and how that causes muscle contraction.
Once the calcium is released from the sarcoplasmic reticulum, that’s going to prevent the repolarization that normally happens rather quickly. With a normal neuron, the action potential lasts less than a millisecond. However, in cardiac muscle cells, we have calcium that’s being released that slows down the repolarization process and we get a phase that’s referred to as the ‘plateau.’ The membrane potential does not repolarize as quickly. Then, at a certain point, calcium gets pumped back into the sarcoplasmic reticulum, potassium also leaves as usual, and we get the repolarization of the cardiac muscle cells.
As you can see, the time scale that we have here shows that this action potential can last as much as 300 milliseconds as opposed to the one millisecond or less than one millisecond that we get with a neuron. That’s because of the calcium released. That’s because of this plateau phase.
Let’s see what that does for muscle contraction. Yes, we’re going to have a depolarization but then, we’re going to have the calcium released and that is going to cause the muscle cells to contract just like I showed in Episode 42. Once again, you can always go back at Episode 42 to revisit that concept.
This is what we’re going to do. I’m going to plot the tension in the cardiac muscle cells. So, we’re looking at the cardiac muscle and here, nothing is happening. But, as soon as calcium starts being released, that’s going to cause the muscle cells to contract. This is what’s going to happen. This is the tension and then, once calcium starts being pumped back into the sarcoplasmic reticulum, the muscle cell is going to relax and go back to its resting state.
So, we have the action potential lasting significantly longer than we’ve seen before, because of the calcium that’s released from the sarcoplasmic reticulum and that calcium then causes the muscle cells to contract and we get this tension in the muscle cells.
As the calcium gets pumped back into the sarcoplasmic reticulum and the potassium ions leave, that is going to cause the muscle cells to relax and go back to its original state.
And that’s pretty much it. The action potential causes calcium release. Calcium release causes muscle contraction.
That’s all I’m going to cover in this video. As usual, you can head back to the website at Interactive-Biology.com for more Biology videos and for more resources that we’re adding there on a regular basis. So, stay tuned. This is Leslie Samuel. That’s it for this video and I’ll see you on the next one.
About The Author Leslie Samuel
Leslie Samuel is the creator of Interactive Biology. He created this site to help Make Biology Fun and has the goal of making this the biggest and best biology resource on the net.
5. April 2011 at 8:54 am
Thanks for the video! Very helpful. But isn’t the part you’ve labeled Purkenje the left bundle branch of His?
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5. April 2011 at 11:31 am
You are very much welcome.
You are correct. The bundle of His actually branches off into the Purkinje fibers. Those fibers are the little branches you see. Sorry, my line was a bit off when pointing to the purkinje fibers.
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5. April 2011 at 11:31 am
@petercourt You are very much welcome.
You are correct. The bundle of His actually branches off into the Purkinje fibers. Those fibers are the little branches you see. Sorry, my line was a bit off when pointing to the purkinje fibers.
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8. April 2011 at 1:16 pm
Hi Lesli, I am really enjoying your videos, that help a lot on my understanding about hear and function. Do you have a page that I can see all the videos related to the cardiovascular system? I really appreciate. keep up…I wish all the teachers could explain in a easy way like you. thanks
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Leslie Reply:
April 8th, 2011 at 1:27 pm
Hi Samia, yes indeed. Just go to the Video Page and you will be able to see all of the videos that are listed. Glad you are finding value in the videos. All the best!
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10. April 2011 at 10:34 pm
Thank you
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11. April 2011 at 11:05 pm
@cid6000 You’re very much welcome!
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11. June 2011 at 6:13 am
You’re a life saver!
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12. June 2011 at 7:47 am
We love saving lives. Especially with Biology
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12. June 2011 at 7:47 am
@CecilieWhipps We love saving lives. Especially with Biology
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16. June 2011 at 4:04 pm
Leslie
That was a great conformation video, I was wondering if you could expand on the depolariaztion a bit and how it directly effects the contraction of the cardiac muscle fibers. In the cardiac muscle fibers in thier resting state they have a similar membrane to teh electrical cells correct??
With hte release of the Ca ion in hte cells t ocause the depolarisation event to occure and the gap junctions that allow the communication…is it the Ca ion from the electrical cells that crosses thru the gap juntction what sets off the muscle fiber and alters its resting potential..so there si a shift in the Na adn K ions..allowing for the influx of Ca into adn release of Ca from the SR?
thanks for any help
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16. September 2011 at 8:32 pm
Depolarization lasts less than a ms in skeletal muscle cells? I don’t think so…
Also, the calcium is not released from the Sarcoplasmic recticulum until voltage gate Ca2+ channels open, allowing EXTRACELLULAR Ca2+ to enter the cell (this is the nature of the plateau), triggering ryanodine receptors to open the gate of the Sarcoplasmic recticulum. Once this happens, then Ca2+ is released from SR, causing calcium spark and triggering the events of the power stroke (muscle contraction).
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16. September 2011 at 8:32 pm
Depolarization lasts less than a ms in skeletal muscle cells? I don’t think so…
Also, the calcium is not released from the Sarcoplasmic recticulum until voltage gate Ca2+ channels open, allowing EXTRACELLULAR Ca2+ to enter the cell (this is the nature of the plateau), triggering ryanodine receptors to open the gate of the Sarcoplasmic recticulum. Once this happens, then Ca2+ is released from SR, causing calcium spark and triggering the events of the power stroke (muscle contraction).
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23. September 2011 at 12:35 pm
Good job
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28. October 2011 at 12:28 pm
Are you a Trini??
Good stuff!
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28. October 2011 at 2:00 pm
St. Maarten
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28. October 2011 at 2:00 pm
@kerryvp St. Maarten
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1. November 2011 at 11:53 pm
Love your intro, Love your voice, you make Biology fun!!
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15. November 2011 at 1:57 pm
there is also a phase before Plateau phase, and that is Early Rapid Repolarisation, in which the membrane potential reaches to 0 mv, for opening of L type Can channels, which is necessary for the stimulation of SR Ca channels.
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15. November 2011 at 1:57 pm
there is also a phase before Plateau phase, and that is Early Rapid Repolarisation, in which the membrane potential reaches to 0 mv, for opening of L type Can channels, which is necessary for the stimulation of SR Ca channels.
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16. November 2011 at 7:05 am
@lovelylatina207 Thank you! Stay tuned for more Biology videos coming very soon!
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19. November 2011 at 1:29 pm
what is the “extra event” that occurs in cardiac muscle?
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20. November 2011 at 2:17 pm
@jessicg61 Hi! Unfortunately, Leslie won’t be able to answer specific questions as he is busy with a lot of work. He will definitely get to more systems in the future. He has many to work on at the moment. So stay tuned for more!
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20. November 2011 at 2:17 pm
Hi! Unfortunately, Leslie won’t be able to answer specific questions as he is busy with a lot of work. He will definitely get to more systems in the future. He has many to work on at the moment. So stay tuned for more!
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28. November 2011 at 9:49 am
these viedos are fantastic for review
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28. November 2011 at 4:12 pm
@NeedsAHardOne Thank you!
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28. November 2011 at 4:12 pm
Thank you!
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8. December 2011 at 2:02 pm
Great video, but a couple errors that bother me.
- Yes, the Sarcoplasmic Reticulum (SR) does contain Calcium for the troponin re-organization, but the main source of Calcium is not really from the SR, it’s from the (notice too that the SR in Cardiac muscles is a lot less than in skeletal muscles) external environment of the Cardiac muscle.
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8. December 2011 at 2:02 pm
– At the plateu, I think it’s important to mention that the depolarization is maintained b/c of the Calcium coming INTO the cardiac muscle is equal to the Potassium going OUT of the cardiac muscle, not b/c of the Calcium leaving the SR… then there would no change in the voltage.
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28. December 2011 at 11:43 am
——-absolutely crrct……the early repolarisation phase is missing here……. that produce an inword notch in graph just before onset of plateau …….due toactivation of a trascient outword current carried mainly by k+(efflux of k+)
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28. December 2011 at 11:43 am
@Anumanu1711 ——-absolutely crrct……the early repolarisation phase is missing here……. that produce an inword notch in graph just before onset of plateau …….due toactivation of a trascient outword current carried mainly by k+(efflux of k+)
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28. December 2011 at 11:43 am
@Anumanu1711 ——-absolutely crrct……the early repolarisation phase is missing here……. that produce an inword notch in graph just before onset of plateau …….due toactivation of a trascient outword current carried mainly by k+(efflux of k+)
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28. December 2011 at 11:43 am
——-absolutely crrct……the early repolarisation phase is missing here……. that produce an inword notch in graph just before onset of plateau …….due toactivation of a trascient outword current carried mainly by k+(efflux of k+)
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3. January 2012 at 10:45 pm
I was failing one of my biology classes until I found your channel. You are amazing at explaining everything.
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16. January 2012 at 1:40 pm
WOW…I Love you. Got a test tomorrow and this was the thing i was struggling with for a while.
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9. February 2012 at 2:06 am
This saved me hours of reading and stressing over this topic… Very helpful esp. since I have a test tomorrow.
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17. February 2012 at 2:54 am
It’s 3am,exam at 10:30am. All I can say is Thank You!!!
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29. February 2012 at 11:32 pm
THANK U, THANK U, THANK U!!! 1 HR AGO I WAS DYING TRYING TO UNDERSTAND THE BOOK!! MY BROTHER SUGGESTED TO COME TO YOUTUBE AND FIND A VIDEO AND NOW I FEEL MUCHHHH BETTER AND CONFIDENT ABOUT MY QUIZ TOMORROW!…
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3. April 2012 at 1:01 am
but what is the purpose of the long refractory period of the action potential? I understand how it works, but why does it need 300 milliseconds as opposed to 1 millisecond in a neuron?
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5. April 2012 at 5:13 pm
hello sir physiology was boring before ..but u made it intrstng lk nythng …..got a tst tmmrw on cardiovascular system …thank u so much fr mkng it simple !!!
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Leslie Samuel Reply:
April 8th, 2012 at 10:49 am
Glad to be able to helps
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6. April 2012 at 7:28 am
Very helpful thank you !
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6. April 2012 at 7:28 am
Very helpful thank you !
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9. April 2012 at 1:16 am
gives time for ventricular filling and prevents tetanic contraction
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9. April 2012 at 4:18 am
Thank you
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9. April 2012 at 4:18 am
Thank you
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17. April 2012 at 1:56 am
Also if the heart did have tetanic contraction it would fatigue, which we do not want as our heart is needed all the time.
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17. May 2012 at 12:30 pm
I just watched this video to study for a my test today. Thanks.
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