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enter image description here

As can be seen from the figure

  1. if myosins from both sides apply equal force then how does muscle contract? And also how do actins resist tearing?

  2. Is there any kind of coordination between different sarcomeres? How does they achieve it? I know that acetylcholine is released in response to action potential so does the release of acetylcholine controlled in each individual sarcomere?

  3. Or simply there is an increase in the number of myosins as the distance of each sarcomere increases from the neuromuscular junction?

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  • $\begingroup$ Can you focus your question more narrowly? Clearly, muscles do contract, so is the question just "how do sarcomeres coordinate contractions?" $\endgroup$ – kmm Aug 19 '16 at 15:22
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    $\begingroup$ I am waiting for a more physics oriented explanation but until that arrives, Think of all this as a rubber band. If you mark off sections on it to represent sarcomeres, then stretch it, each "sarcomere" will stretch. Then if you let go of one end of the rubber band, every "sarcomere" will contract, and the rubber band will contract toward the held end. When this happened, every "sarcomere" was being pulled in both directions, but the forces were all transferred from one to section another, because one end of the band was unable to move. On the whole, the band contracted towards the fixed end $\endgroup$ – Polisetty Aug 19 '16 at 18:24
  • $\begingroup$ For the second one, Nerves innervate the muscle and activate muscle fibres as a whole. A muscle fibre is a larger unit containing many myofibrils which are sarcomeres attached in series. Which of muscle fibres get excited is a different story. But when a muscle fibre gets excites, the impulse travels along its sarcolemma and when it reaches the the t-tubules leads to release of calcium from the cisternae. Because the contraction is finally mediated by calcium, the process is sychronized $\endgroup$ – Polisetty Aug 19 '16 at 18:32
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I'm answering question 1, I think it'd be better to post these questions separately.

Consider action and reaction: myosins (thick) filament is experiencing force F towards Z line. http://imgur.com/4l6Xl4vl.png

If the muscle is acting on something that is movable (e.g. you're pulling on a drawer which isn't locked), the force F towards Z line is greater than the force that maintains your thick filaments in place, and they'll slide towards Z line. The speed at which they slide is limited by the rate at which myosin heads can detach once they have performed power-stroke.

If the drawer is locked, then force F cannot overcome the external force. This force however is normally lower than the force required to tear actin or z-line, it is limited by two molecular factors: myosin will stall and not effectuate power stroke under too strong an opposing force, and myosin heads will detach after some time and relax the tension they have built up. This is how you get the maximum force under clamped conditions.

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