episode47

047 Action Potentials and Contraction in Cardiac Muscle Cells

Leslie Samuel IBTV, Physiology, The Circulatory System 121 Comments

Leslie explains how action potentials are generated by the cardiac cells of the heart and how the release of calcium can generate heart contraction.

Watch to learn more.

Enjoy!

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.

Comments 121

  1. InteractiveBiology

    @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.

  2. InteractiveBiology

    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.

  3. samia

    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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      Author
  4. Corey Sagstuen

    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

  5. mrmikeymills

    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).

  6. mrmikeymills

    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).

  7. mrmikeymills

    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).

  8. Anumanu1711

    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.

  9. Anumanu1711

    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.

  10. Anumanu1711

    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.

  11. InteractiveBiology

    @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!

  12. InteractiveBiology

    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!

  13. InteractiveBiology

    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!

  14. 19Tranc3r92

    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.

  15. 19Tranc3r92

    – 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.

  16. 19Tranc3r92

    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.

  17. 19Tranc3r92

    - 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.

  18. rounikt

    @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+)

  19. rounikt

    @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+)

  20. rounikt

    ——-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+)

  21. rounikt

    ——-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+)

  22. rounikt

    ——-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+)

  23. wildflower0214

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  25. MrGoodClass

    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?

  26. MrGoodClass

    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?

  27. kimmi

    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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  28. mirrorreflex

    Also if the heart did have tetanic contraction it would fatigue, which we do not want as our heart is needed all the time.

  29. sabah1242

    the videos are really good for revision! thankyou!
    can i just ask tho, in smooth/skeletal muscles, calcium is released aswell from the sarcoplasmic reticulum so why doesnt that result in a plateau phase..? is it to do with the timings, because action potentials are time dependent also?

  30. sabah1242

    the videos are really good for revision! thankyou!
    can i just ask tho, in smooth/skeletal muscles, calcium is released aswell from the sarcoplasmic reticulum so why doesnt that result in a plateau phase..? is it to do with the timings, because action potentials are time dependent also?

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  48. brumified

    @anas lahrichi The bundle of His is a cluster if specialised conduction myocardiocytes, they transmit that electrical energy from the AV node to the Purkenje Fibres; so they function to transmit charge Moreno than concentrate it as in the AV node cardiomyocytes.

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