The Beat Goes On

The Beat Goes On 

The Cardiomyocyte: Your heart beats about 72 times per minute, or 100,000 times a day, clocking an average of 2.5 billion times in a lifetime and working harder than any other muscle in your body. After all, it has to pump some 2000 gallons of blood around 60 miles of blood vessels each day. No matter how hard you train, the skeletal muscles of your arms or legs could never keep this up. So spare some love for the cardiac muscle cell, or cardiomyocyte

The Pacemaker: The heart marches to its own tune, unlike the skeletal muscle which gets direct input from a motor nerve. Indeed, the heart would be an unwieldy mess if each individual fiber needed a motor nerve connection.  Instead, every beat of the heart starts within the sinoatrial node (SAN) containing <10,000 pacemaker cells equipped with a built-in clockwork mechanism to fire rhythmically. These electrical impulses spread through the muscle fibers by direct communication from cell to cell via  special channels called gap junctions, that synchronize the contraction. If the pacemaker fails, the ~5 billion working cardiomyocytes don’t get their marching orders and the heart slows down or becomes arrhythmic. Promising new research aims to convert ordinary cardiomyocytes to pacemaker cells by expressing a master regulator gene, Tbx18 to replace those lost by disease or defects.

Sparks and Stripes: Each cardiomyocyte is packed with ordered arrays of thin and thick filaments that slide past each other to make the muscle contract. The thin filaments are made of actin seen as red stripes in the image. The thick filaments are an assembly line of myosin motors that use a rowing motion to pull on the actin filaments. In the absence of an electrical signal, the muscle is relaxed, with the filaments kept apart by a guardian protein called troponin C. The magical molecule that sets the contraction in motion is calcium, seen in the gif as sparks and waves. Each electrical impulse releases a packet of calcium that binds to troponin C, and moves it out of the way to trigger contraction. But the calcium is quickly captured (by calcium pumps and exchangers) and moved back into stores, so the muscle relaxes..before it all begins again.  

Another installment in the   #excyting series on cell types.




REF: Direct conversion of quiescent cardiomyocytes to pacemaker cells by expression of Tbx18. Kapoor et al., 2013

IMAGE: Composite put together by Kevin Staff from and . Thanks, Kevin!



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91 Responses to The Beat Goes On

  1. As always – an awesome animation!

  2. Rajini Rao says:

    Thanks, Ca2+ dynamics are fun to watch 🙂

  3. Calcium, the magical molecule!


  4. Rajini Rao says:

    Strictly speaking, an ion 😉

    But what’s a few electrons between friends, Víktor Bautista i Roca ? 

  5. Jesse H says:

    Amazing post Rajini

  6. Rajini Rao I was just quoting you 🙂

  7. Very interesting, I never really understood how the impulse signal spread throughout the heart muscle in proprer synchronization

  8. Rajini Rao says:

    I know, I feel guilty calling it a molecule for the sake of alliteration 🙂 But calcium the inimitable ion didn’t make the cut.

  9. Tom Lee says:

    Great post as usual Rajini Rao

  10. Rajini Rao says:

    Jeffrey J Davis  (happy birthday!), the gap junctions make each chamber of the heart beat as one..even though they are separate muscle cells, they act like a “syncytium”. 

  11. Rajini Rao says:

    Tom Lee , I did think of you when I linked to Sonny and Cher..what an oldie, I wonder if it was in your uncle’s vinyl collection 🙂

  12. Hopefully it does go on … 😉

    Great post my friend … as usual !!

    Have a nice Sunday and upcoming week Rajini Rao … 🙂

  13. Rajini Rao says:

    Hey, you too Magnus Fahlén ♥!

  14. Thanks dear I’ll do my best Rajini Rao … ;-D

  15. Chad Haney says:

    Great post Rajini Rao. If anyone is curious about the 2000 gallon/day figure comes from (it threw me off for a sec, I need caffeine), the average cardiac output for an adult male is 5.6 L/min, which is 8064 L/day or 2130 gallons/day.

    My boss is working on a contrast agent that “turns on” when there is calcium and is more or less dark when calcium is absent.

  16. Rajini Rao says:

    Super, thanks for the calculations Chad Haney . Sometimes I do a quick “back of the envelope” calculation to check the pop sci claims, but this time I didn’t. Glad it computes, whew! 🙂

    Is this contrast agent extracellular, do you know? Does it get into the cells or highlight tissues?

  17. Chad Haney says:

    I’m sure they have done cellular uptake/binding studies, so I can get the answer to that. My guess is that it is intracellular. The initial idea is to use it in the brain and image response to stimuli when calcium stores are released. I don’t know much about the project but I can find out more.

  18. Rajini Rao says:

    Thanks, Chad. Calcium imaging in situ, and in the brain would be extremely useful. I’ll be on the lookout for any papers that come out.

  19. Azlin Bloor says:

    Very informative, as always, Rajini. Am I right that human cardiac fibroplasts (even mouse?) can be converted into cardiomyocytes?

  20. Rajini Rao says:

    Azlin Bloor , thanks! On the one hand, fibroblasts (an ordinary sort of cell) can be converted to cardiomyocytes by stem cell technology. This is useful because one can easily get fibroblasts from a patient, convert them to cardiomyocytes for personalized therapy. The other possibility that I mentioned in the post is to convert a cardiomyocyte into a specialized pacemaker cell, which has special beating characteristics. The hope is that this would one day do away with the need for artificial pacemakers. Hope that makes sense. 

  21. They are fascinating! I have a few cardiomyocites ( iCells) beating in vitro. Very nice post !!

  22. Azlin Bloor says:

    Rajini Rao totally thanks! 

  23. Rajini Rao says:

    Christiane Cantin , cardiomyocytes are neat to watch beating away in vitro. The story on the discovery that they need Ca2+ to beat is an interesting one. They were sitting in a solution made with London tap water, beating away, but when the solution was made with distilled water (supposedly better) they stopped! This was traced to calcium in the city water 🙂 

  24. Rajini Rao says:

    Peter Lindelauf , quick diagnosis: temporary arrhythmia due to parasympathetic innervation. Hope he does not need a pacemaker 😉

  25. Chad Haney says:

    Peter Lindelauf stay away from my magnets with that pacemaker.

  26. I’ve just got to find a suit like Sonny Bono’s.

  27. Really interesting and simplified explanation. I just started to love to Cardiomyocyte! 😉

  28. Great reading.  Thanks Rajini Rao 

  29. good attempt to  make it simplified

  30. Rajini Rao says:

    ganga pratiek , good point..there is a lot more to it than this of course. I hope people feel free to ask, if they have question. 

  31. it would be great to know why tumor in heart is not common…+Rajini Rao

  32. Rajini Rao says:

    That’s a great question, ganga pratiek , for which I have no answer. It may have to do with cardiomyocytes rarely dividing, similar to neurons? Most cancers of the brain arise from glial cells. 

  33. Thank you very much for this post – it’s very interesting, I love how info dense it is.

  34. Rajini Rao says:

    Thanks, all! fan tai , to be consumed in small doses, perhaps? 🙂

  35. It takes talent to take a paper/etc and turn it into something that us normal people can read and find interesting! 🙂

  36. Alan Kotok says:

    Thanks Rajini Rao for the informative explanation … My pacemaker needs a little help from a device with the same name made by Medtronic. (Don’t get old.)

  37. Rajini Rao says:

    Nifty little device, Alan Kotok ! The cellular versions are pretty cool too – put prone to problems. 

  38. Rajini Rao says:

    Now you have me intrigued, do tell, Peter Lindelauf ?

  39. Great post!

    The cardiomyocytes march to the fastest and strongest leader, which normally is the SA node. If that fails, the baton gets passed on to other heart cells — such as ectopic foci in the atria, the atrioventricular (AV) node, or Purkinje fibers in the ventricles — which become the new but inherently slower pacemaker. They’re not as good as the SA node, but it’s neat how that backup system works and how other cells automatically take over.

    Another tidbit is that troponin subtypes are used as cardiac biomarkers in blood tests to detect tissue injury (for example, in a heart attack). Instead of troponin C (calcium binding portion), they’re looking at troponin I (inhibitory) and troponin T (tropomyosin binding) components that are part of the complex that prevents actin from binding to myosin in the absence of calcium (the “guardian” you alluded to).

    Just for fun and referencing the Buddy Holly song mentioned above, when people talk about the heart skipping a beat, that’s either premature atrial contraction (PAC) or premature ventricular contraction (PVC). If cells beat (depolarize) too soon, the recovery (refractory) period is extended during which the cells cannot beat again. The delay takes the place of where a normal beat would be expected to occur (thus “skipping” it), and the subsequent resetting “startup” beat feels out of place, leading to the perception of palpitations.

    Isolated PACs and PVCs are common, but if PVCs become sustained, they can lead to a deadly arrhythmia (ventricular tachycardia, or v-tach) and eventually cardiac arrest. PVCs are caused by many things, some of which include hypoxia and epinephrine (or stimulants like caffeine, cocaine, and amphetamines). So perhaps the kiss literally took his breath away (hypoxia) or maybe he got a little too excited (release of adrenaline) 😉

    As far as “love hurts”, although rare, there actually is a “broken heart syndrome” called Takotsubo cardiomyopathy where heart muscle is weakened due to significant and prolonged emotional stress (e.g., a traumatizing break-up or death of a loved one). Part of the heart wall that no longer contracts bulges out, which by appearance is why it’s called tako tsubo (“octopus pot”) in Japan where it was first identified (not to be confused with tako in Swahili, which means buttocks… but I digress. Pain in the butt can be discussed another time :P)

  40. Rajini Rao says:

    Fantastic added value, ending with a hearty laugh, thank you for that Johnathan Chung ! 

  41. Peter Lindelauf Didn’t mean to interrupt. By all means, please do continue 🙂

    Rajini Rao You’re welcome. I was excited and pumped to share such interesting info.

  42. Rajini Rao says:

    I’m glad you were pumped, Johnathan Chung . These short posts lose a lot of detail in the contractions .  You brought fresh blood to the post 🙂

    We teach our students about mutations in the delayed rectifier K channels that result in slower openings, and dominant mutations in the Na channels that keep them open longer. Either way, the cells are depolarized longer, and arrhythmia sets in, with the dangerous torsade de pointes and sudden death. 

  43. Amazing stats Rajini Rao !

  44. Mary T says:

    Truly an excellent post Rajini Rao :-).

  45. TA G says:

    Love this post. Thank you.

  46. Vinay Y S says:

    Amazing stuff! Makes me want to go back to school :-p

  47. Rajini Rao says:

    No exams, I promise, Vinay Y S 🙂

  48. Awesome post and animation Rajini Rao! Was curious about Tbx18 since it sounded familiar so I looked it up; the ‘t-box’ genes are a set of transcription factors that are active in early embryonic development, they’re responsible for things as wide ranging as limb, brain and heart development. Very interesting!

  49. Thanks for sharing such and amazing animation and interesting and important info. 

  50. Muddu kishan says:

    And all this creation happened by Chance??

  51. thank you rajini rao ……..u cleared a doubt of mine…

  52. but i have another one if u dont mind ………….when the av node a stops functioning …how come other foci start spontaneously ………..???..can u plzz clear this doubt of mine

  53. Rajini Rao says:

    That’s because all cardiomyocytes have the intrinsic ability to beat spontaneously, ganga pratiek .

  54. kk thnk you …..Rajini Rao

  55. This should go with hindi number ‘dhak dhak karne laga’ as the background 🙂

  56. Rajini Rao says:

    Is that a Bollywood song I have missed, Ramesh Sistla ? 🙂

  57. probably you did. It’s some 10+ yrs old. Noted purely for the dhak dhak – the pulsating heart of a woman supposedly in love 🙂

  58. Rajini Rao when will you be posting again? It has been a while. 

  59. Rajini Rao says:

    Susana M. , thank you for “egging” me on: new post on blue egg color is up 🙂

  60. John Nahorny says:

    Please excuse my adding u to my frends i just wanted 2 go 2 your page faster its so good 2read. Thank u jn

  61. Rajini Rao says:

    It is with great sadness and a heavy heart that I report on the passing away of Kevin Staff who put together this image and many, many more  beautiful gifs that I used in my science post over the last two years. He was much too young.

  62. Chad Haney says:

    I’m sorry to hear that Rajini Rao

  63. Alan Kotok says:

    Thanks Rajini Rao … sad news indeed

  64. Rajini Rao says:

    Thank you, Chad and Alan. I’m so shocked at the transience of life. 

  65. Mary T says:

    I’m sorry Rajini Rao ~

  66. Rajini Rao says:

    Thank you, Mara Rose . It’s good to hear from my friends at such times.

  67. John Nahorny says:

    Rajini sorry for your loss.Sounds

    Iike the kind of guy i wouldn’t mind having a cup of coffee with.


  68. Chad Haney says:

    We have to cherish our loved ones while we can. I certainly cherish our friendship Rajini Rao

  69. Deepak Gupta says:

    Pacemaker(S.A)node generate the impulse in heart chamber travelling through bundle of hiss and purkinje fiber. !,,, it is autonomous thank you for original, ,!

  70. 1 0 says:

    Wow we come to release, it’s now 2016 whayyyyy!

  71. Twwhwygwuhygwsusishyw

  72. Twwhwygwuhygwsusishyw

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