In the heart, are stretched muscles during the diastole more relaxed or are the contracted muscles more relaxed? Depending on the answer, are there then more cross-bridge interactions in cardiac muscle cells when the heart is stretched or contracted?

  • $\begingroup$ Can you clarify your question? What do you mean by "more relaxed". There are a few things to consider here: the contracted or relaxed state of cardiac muscle, as well as the baseline stretch in that muscle. But your use of "relaxed" in your question isn't standard. $\endgroup$ – De Novo Jun 27 '18 at 20:13
  • $\begingroup$ Well that is the question itself. When the heart is contracted, is that when it is more relaxed? Or, is it more relaxed when it is expanded? Which state is the resting state of a heart? $\endgroup$ – John Joe Jun 27 '18 at 20:34
  • $\begingroup$ Relaxed is the opposite of contracted, so when it is contracted it is exactly not relaxed, by definition. Are you referring to the pathological state of stretching that occurs in some forms of heart failure, e.g., dilated cardiomyopathy? $\endgroup$ – De Novo Jun 27 '18 at 20:41
  • $\begingroup$ It's completely possible I have terminology mixed up, I'm not biology expert and different sources gave conflicting descriptions. I'm not referring to any kind of disease, what I want to know is which state, expanded or "smaller", is the heart's resting state when it is not exerting energy to pump blood, and thus, which other state must then be the one that takes more energy to allow more cross-bridge interactions to maintain. Is the heart more relaxed when it is expanded? Or when it is...I guess I can only say "smaller" since contracted could be for either direction. $\endgroup$ – John Joe Jun 27 '18 at 21:13
  • $\begingroup$ Contracted literally means to shrink in size: "contracted" cannot mean "either direction" $\endgroup$ – Bryan Krause Jun 27 '18 at 21:15

The muscular portion of the heart is comprised of cardiac myocytes, elongated cells filled with bundles of actin and myosin called sarcomeres. These cells are similar in many respects to skeletal muscles, but are shorter, have only one nucleus, and slightly different isoforms (versions) of the proteins involved. The purpose of this cell, like the skeletal muscle cell, is contraction, or shortening on activation.

This image of cardiac muscle cells shows how full they are with long bands of proteins (proteins stain pink).

enter image description here

Because the heart is a muscular chamber, instead of simply a band of muscle (like the bicep), the relevant dimension for the organ is volume instead of length. Where your bicep gets shorter when it contracts (and pulls your forearm toward your upper arm, bending at the elbow joint), when your heart muscle contracts, it twists and squeezes against the fibrous part of the organ, decreasing the volume of the chamber.

Since there is no air in a healthy live heart, when the heart contracts, reducing the volume of the container, the blood is forced out of the heart (ejection) through whatever opening is available. This is called systole. When the heart relaxes, the volume of the container increases, and blood enters the container through whatever opening is available. this is called diastole.

enter image description here

Now, how is this related to cross-bridge interactions?

The mechanism for contraction itself, in both skeletal and cardiac muscle, is cross-bridge cycling. Those pink bundles of actin, myosin, and associated proteins in the picture above form bonds, change conformation, break bonds, change conformation, and form new bonds: enter image description here

This cross-bridge cycling occurs during contraction when $Ca^{++}$ enters the cell as part of the cardiac action potential. Relaxation occurs when $Ca^{++}$ concentrations decrease, through the action of the $Ca^{++}$-ATPase, causing cross bridge interactions to decrease.

  • $\begingroup$ Okay, so just to make it clear before I select it as the answer, when the heart is expanded, that is it's natural resting state, and when the cardiac muscles exert force through contraction, it is only to shrink the size of the heart? I just want to make sure on whether it is the expanding or the shrinking that requires the cardiac muscle to contract, and so far it seems like it's the shrinking action. $\endgroup$ – John Joe Jun 27 '18 at 21:30
  • $\begingroup$ You are correct. Contraction causes the volume inside the heart to decrease (the volume inside the chamber shrinks). $\endgroup$ – De Novo Jun 27 '18 at 21:32
  • $\begingroup$ You're putting a lot of work into the question and I appreciate that, but I don't want you to over-invest when I am focused so much on a singular question. I can infer the cross-bridge interactions depending on what the answer is and I am familiar with the theory behind it. $\endgroup$ – John Joe Jun 27 '18 at 21:32

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