Cardiac Action Potential, Animation.

Cardiac Action Potential, Animation.

The heart is essentially a muscle that contracts
and pumps blood. It consists of specialized muscle cells called
cardiac myocytes. The contraction of these cells is initiated
by electrical impulses, known as action potentials. Unlike skeletal muscles, which have to be
stimulated by the nervous system, the heart generates its OWN electrical stimulation. In fact, a heart can keep on beating even
when taken out of the body. The nervous system can make the heartbeats
go faster or slower, but cannot generate them. The impulses start from a small group of myocytes
called the PACEMAKER cells, which constitute the cardiac conduction system. These are modified myocytes that lose the
ability to contract and become specialized for initiating and conducting action potentials. The SA node is the primary pacemaker of the
heart. It initiates all heartbeats and controls heart
rate. If the SA node is damaged, other parts of
the conduction system may take over this role. The cells of the SA node fire SPONTANEOUSLY,
generating action potentials that spread though the contractile myocytes of the atria. The myocytes are connected by gap junctions,
which form channels that allow ions to flow from one cell to another. This enables electrical coupling of neighboring
cells. An action potential in one cell triggers another
action potential in its neighbor and the signals propagate rapidly. The impulses reach the AV node, slow down
a little to allow the atria to contract, then follow the conduction pathway and spread though
the ventricular myocytes. Action potential generation and conduction
are essential for all myocytes to act in synchrony. Pacemaker cells and contractile myocytes exhibit
different forms of action potentials. Cells are polarized, meaning there is an electrical
voltage across the cell membrane. In a resting cell, the membrane voltage, known
as the RESTING membrane potential, is usually negative. This means the cell is more NEGATIVE on the
INSIDE. At this resting state, there are concentration
gradients of several ions across the cell membrane: more sodium and calcium OUTSIDE
the cell, and more potassium INSIDE the cell. These gradients are maintained by several
pumps that bring sodium and calcium OUT, and potassium IN. An action potential is essentially a brief
REVERSAL of electric polarity of the cell membrane and is produced by VOLTAGE-gated
ion channels. These channels are passageways for ions in
and out of the cell, and as their names suggest, are regulated by membrane voltage. They open at some values of membrane potential
and close at others. When membrane voltage INCREASES and becomes
LESS negative, the cell is LESS polarized, and is said to be DE-polarized. Reversely, when membrane potential becomes
MORE negative, the cell is RE-polarized. For an action potential to be generated, the
membrane voltage must DE-polarize to a critical value called the THRESHOLD. The pacemaker cells of the SA node SPONTANEOUSLY
fire about 80 action potentials per minute, each of which sets off a heartbeat, resulting
in an average heart rate of 80 beats per minute. Pacemaker cells do NOT have a TRUE RESTING
potential. The voltage starts at about -60mV and SPONTANEOUSLY
moves upward until it reaches the threshold of -40mV. This is due to action of so-called “FUNNY
currents present ONLY in pacemaker cells. Funny channels open when membrane voltage
becomes lower than -40mV and allow slow influx of sodium. The resulting DE-polarization is known as
“pacemaker potential”. At threshold, calcium channels open, calcium
ions flow into the cell further DE-polarizing the membrane. This results in the rising phase of the action
potential. At the peak of depolarization, potassium channels
open, calcium channels inactivate, potassium ions leave the cell and the voltage returns
to -60mV. This corresponds to the falling phase of the
action potential. The original ionic gradients are restored
thanks to several ionic pumps, and the cycle starts over. Electrical impulses from the SA node spread
through the conduction system and to the contractile myocytes. These myocytes have a different set of ion
channels. In addition, their sarcoplasmic reticulum,
the SR, stores a large amount of calcium. They also contain myofibrils. The contractile cells have a stable resting
potential of -90mV and depolarize ONLY when stimulated, usually by a neighboring myocyte. When a cell is DE-polarized, it has more sodium
and calcium inside the cell. These positive ions leak through the gap junctions
to the adjacent cell and bring the membrane voltage of this cell up to the threshold of
-70mV. At threshold, FAST sodium channels open creating
a rapid sodium influx and a sharp rise in voltage. This is the depolarizing phase. L-type, or SLOW, calcium channels also open
at -40mV, causing a slow but steady influx. As the action potential nears its peak, sodium
channels close quickly, voltage-gated potassium channels open and these result in a small
decrease in membrane potential, known as EARLY RE-polarization phase. The calcium channels, however, remain open
and the potassium efflux is eventually balanced by the calcium influx. This keeps the membrane potential relatively
stable for about 200 msec resulting in the PLATEAU phase, characteristic of cardiac action
potentials. Calcium is crucial in coupling electrical
excitation to physical muscle contraction. The influx of calcium from the extracellular
fluid, however, is NOT enough to induce contraction. Instead, it triggers a MUCH greater calcium
release from the SR, in a process known as “calcium-induced calcium release”. Calcium THEN sets off muscle contraction by
the same “sliding filament mechanism” described for skeletal muscle. The contraction starts about half way through
the plateau phase and lasts till the end of this phase. As calcium channels slowly close, potassium
efflux predominates and membrane voltage returns to its resting value. Calcium is actively transported out of the
cell and also back to the SR. The sodium/potassium pump then restores the
ionic balance across the membrane. Because of the plateau phase, cardiac muscle
stays contracted longer than skeletal muscle. This is necessary for expulsion of blood from
the heart chambers. The absolute refractory period is also much
longer – 250 msec compared to 1 msec in skeletal muscle. This long refractory period is to make sure
the muscle has relaxed before it can respond to a new stimulus and is essential in preventing
summation and tetanus, which would stop the heart from beating.

David Anderson

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100 thoughts on “Cardiac Action Potential, Animation.

  1. Alila Medical Media says:

    If this video is helpful to you, please consider supporting our next projects. As a token of our appreciation, we also offer early video access and free image downloads in return, please check us out here:

  2. onyinye nkemchor says:

    this video was very helpful, thank you, however I would like to see a video on refractory period and excitable contraction coupling

  3. Concerned says:

    Don't know why I bother paying for a university course when this is so much better

  4. drbaby chitra says:

    very good animation very much useful to the medical students.

  5. fatima maqbooll says:

    wooww its just waooo ths vedio clearifies alooot of points fr me good wrk gys

  6. Mustafa Ahmed says:

    Thanks for such great effort ❤❤

  7. Debojit Dey says:

    All topics of biochemistry… Of first year mbbs exam..

  8. Aaron Nixon says:

    great video!!

  9. Namrta Desai says:

    Soo good thank u soo much it helped me a lot 😄🎉

  10. Sl pharma academy says:


  11. SREEDHAR ATTLA says:


  12. Ana Herrera says:

    Excellent thank you

  13. Alexander Beny Susanto says:

    Wow… this is amazing

  14. 72295 aq says:

    Thank you sooooooooooooo much

  15. xx ziyad says:

    God help me💯💯😢

  16. Robert Wu Runtian says:

    But there is something wrong here. Where is the second and third step of cardiac repolarization?and in the second step of cardiac repolarization, both the entrance for Ca2+ and K+ should be opened

  17. Lee sue rYe says:

    Thank you so much 💜💜💜💜💜💜💜💜💜💜💜💜💜💜💜💜💜💜💜💜💜💜💜💜💜💜

  18. Arthur Migliazza says:

    Very helpful video, thank you!

  19. sewcrazed says:

    please explain how hyper and hypokalemia cause arrhythmias

  20. Mehedi Hasan says:


  21. zynab1234 says:


  22. zynab1234 says:

    awesome ! lets hope I pass cardio exam !

  23. Teresa McNee says:


  24. Scuola Scuola says:

    Thank you

  25. Norhan Ali says:

    it's an amaaaaaaazing animation ..keep on …we need animation on physiology of the kidney and autonomic nervous system

  26. Donna White says:

    Calmadulin is replaced with tropomyosin in cardiac myocytes

  27. kuhataparunks says:

    Phenomenal animations that make the material very clear, thank you so much.

  28. Sagar Hemnani says:

    Alila medical u made studying very easy n interesting…Thank you


    Amazing video. …. it helps us to understand the subject clear and quickly. … thank you so much. ….

  30. Shree Veena says:


  31. Precious211 says:

    I understood everything thank you…I would’ve liked to hear about the muscarinic receptors though

  32. Mazin Juvale says:

    Can you please explain pharmacology also

  33. Ibrahim F07 says:

    It’s DE-polarizing!!!

  34. vaidehi patel says:

    Could you make videos on all the cvs drugs there mechanism of action and also for other systems like ans, cns, rp system


    I lov it…
    Pls upload jugular venous pressure video

  36. Alita Huga says:

    You are the best 😍

  37. Nathan Young says:

    bruh I bang with this

  38. ISHANI BISHT says:

    ECG basics

  39. Rose L'luma says:

    A good elaborative teachings about action potential in the cardiac muscle cells. Am inspired.

  40. raveena kandera says:

    Nice and very helpful video

  41. ashtonhenley says:

    These videos are fantastic!! Would love to see some on hemodynamic principals I.e. Law of LaPlace, Darcy, Pousille etc. Thank you so much

  42. Jano allgood says:

    Very helpful thank you

  43. dominic matur says:

    So helpful!👍❤🙏

  44. TheMerahi says:

    Wow. How dedicated u r to make us understand such complex idea in simple animation. God bless u and keep it upup!!!

  45. Kat2291 says:

    U made this so easy to understand and remember… Thank YOU so much

  46. Booma Saeed says:

    Couldn’t just get it !

  47. mohamed mostafa says:

    thank you so much
    it was really helpful video for me

  48. Maya Sajan Narekat says:

    Thank you so much.Great work

  49. Jon Dutra says:

    Phenomenal vid, but I don’t understand why potassium makes the extra cellular side positive again during repolarization. They NA, CA and K all have positive charges? Can anyone help me out please?

  50. Maria Okon says:


  51. Kathy C says:

    Thank you sooo much!

  52. Parul Sanaik says:

    Thank u so much it is the best vedio I have ever seen on cardiac action potential thanks it's quite easy to understand..

  53. Romaric Agossou TANDJIEKPON says:

    Well explained

  54. Huda هدى Othman says:


  55. Franciscanisha Gomesbukkam says:

    Very nice explaination. Please show magnetic resonance and implications to psychiatry.

  56. Burak DEVELİ says:

    BEST !

  57. Rodri Cantos says:

    Not all heroes wear capes

  58. Peppe says:

    Now I know how to fight against L.T. on next Monday

  59. master pee says:

    i am forced to subscribe, impressive

  60. Golam Mahabub says:

    well explanation

  61. lina sreng says:

    When the membrane voltage drop to -70mv in phase repolarisation then could the sodium channels open again?

  62. Amina Farah says:

    Thank you so much very helpful video!

  63. Amr Kaid says:

    marry me thnx

  64. Ajay Singh says:

    Nicely explained.

  65. Krishna Priya says:

    Thank you madam 😇😇

  66. Ilva Gjoka says:

    Great job!

  67. Kks Ga says:

    Great Video

  68. Krishna Priya says:

    Thank you so much

  69. Toqa Al-rashdan says:

    Thanke you 🌼

  70. Reilly Alexander says:

    Great facts! Is the voltage in the cell AC current or DC current?

  71. QRS says:

    00:18 .. unlike ..

  72. shrishti Singh says:

    Antiarrhythmatic drugs

  73. rjomoss says:

    Love it, thanks. <3

  74. Luana Augusta says:

    Best channel on earth. Pkease never stop producing <3

  75. Mohammad Shormij says:

    very good and helpful

  76. Carl Crus says:

    i thought a plateau is formed when there is rising of calcium ion?

  77. Benjamin Steel says:

    THANKYOU!!! for hurting my brain…

  78. Jayashri Chavan says:

    Can you please make a video on how anti arrhythmic drugs act and how the impulses are corrected.

  79. Prabir Biswas says:


  80. Gerard Hoeltzel says:


  81. انتصار سعيد says:

    Wow great video …… And also nice illustration …… Thanks

  82. Sasha Hanna says:

    Thank you very much!Best video!

  83. Sumit Kumar says:

    negative is upto -90mv

  84. Kannan Kamal says:

    Histology topics, tissue preparation

  85. Cedric J says:

    Wow is all I can say, very well presented and easy to follow!!

  86. MANASI RANA says:

    It's so much interesting really

  87. Abood Abood says:

    really good video, it explained for me everything

  88. h Khan says:

    Uh are love🔥

  89. Zenab Naaz says:

    i love this channel sooo fkn much

  90. Muhammad Kamran says:

    Excellent Job. Comprehensive and affective.

  91. Hasan Çiftçi says:


  92. Hazem El Nahrawy says:

    Greaaaaaat ♥️♥️♥️

  93. Nighthawk681 says:


  94. Julie says:


  95. aRMY But also multi fandom says:


  96. Šwęëțý Ģįřļ says:

    Very helpful .. thanks 🌸

  97. shan. s. says:

    The only bit that confused me a lot is how the pacemaker cells were restored after becoming depolarised. How did the electrolytes make it so there was more potassium on the inside of the cell again?

  98. Dr. Ahmad Salah AWAD says:


  99. AKHILA M says:


  100. Bhagyalakshmi Bapanapalli says:

    Hi do you have pharmacology videos.can u upload please

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