Sarcomere question

[Pamplemousse123]

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I have a general question about the sarcomere:
do actin filaments go toward each other (decreasing length of the H band) and away from each other (increasing length of the H band) while using ATP?

Or is it that the relaxation or increasing the length of the Hband does not require ATP, while the contraction (decreasing length of the H band) is ATP dependent?

 

[dougkaye]

Good question. ATP (and calcium!) are needed for the muscle to contract, which in the sliding filament model will reduce the length of the H-band. However, you want to consider what happens if there is not ATP or calcium. Understanding this means that you "get" the muscle physiology really well.

To answer your question more directly: ATP is indeed needed for contraction because it "cocks" the myosin head, allows it to attach to actin, and then provides energy for muscle ratcheting. Because the myosin head only detaches from the actin filament and resets in the presence of ATP, another molecule of ATP is also needed for the muscle to "relax" after contraction via detachment of the myosin head. **Consider what happens to a muscle when you have a lack of ATP, and an overabundance or lack of calcium.

Think of it in this way: ATP is needed for the "power stroke" (contraction), but is also needed to 'reset' the mechanism thus allowing the muscle to relax.

Here is a simple model: http://legacy.owensboro.kctcs.edu/GCaplan/anat/images/Image336.gif

 

[Pamplemousse123]

Good question. ATP (and calcium!) are needed for the muscle to contract, which in the sliding filament model will reduce the length of the H-band. However, you want to consider what happens if there is not ATP or calcium. Understanding this means that you "get" the muscle physiology really well.

To answer your question more directly: ATP is indeed needed for contraction because it "cocks" the myosin head, allows it to attach to actin, and then provides energy for muscle ratcheting. Because the myosin head only detaches from the actin filament and resets in the presence of ATP, another molecule of ATP is also needed for the muscle to "relax" after contraction. Consider what happens to a muscle when you have a lack of ATP, and an overabundance or lack of calcium.

Think of it in this way: ATP is needed for the "power stroke" (contraction), but is also needed to 'reset' the mechanism thus allowing the muscle to relax.

Thank you for this explanation.

when the sarcomere extends its length or relaxes, does it need a one time ATP shot for the myosin heads to detach from the actin filaments or a continuous ATP supply? If you can tell me in more detail how the sarcomere relaxes, I'll understand it better. Thanks in advance!

 

[Pamplemousse123]

I did a bit of research and it says that there's a calcium ATP pump on the sarcoplasmic reticulum membrane that actively pumps calcium back so that troponin/tropmyosin recover the actin filament to prevent myosin head groups from binding. And yes, ATP is required to knock off that remaining head from actin. But now my question is this, in order for the sarcomere to extend its length (instead of getting shorter as in contraction) do the myosin head groups travel the other way as opposed to contraction so that the entire sarcomere length increases? How do extensors work?

 

[dougkaye]

I did a bit of research and it says that there's a calcium ATP pump on the sarcoplasmic reticulum membrane that actively pumps calcium back so that troponin/tropmyosin recover the actin filament to prevent myosin head groups from binding. And yes, ATP is required to knock off that remaining head from actin. But now my question is this, in order for the sarcomere to extend its length (instead of getting shorter as in contraction) do the myosin head groups travel the other way as opposed to contraction so that the entire sarcomere length increases? How do extensors work?

For this muscle model, just understand that relaxation involves detachment of myosin head from actin. I think your question about extensors etc. is probably beyond scope of MCAT in terms of things you have to 'memorize'.