Stages in muscle contraction?

[bLub]

Hi guys,

I'm a bit confused about the stages in muscle contraction... could you maybe have a look at the attached document and tell me the answer to my question (at the bottom of the document)?

Thanks!!

Stages in muscle contraction.doc

[sweetnsimple786]

Animations really helped me understand this.

http://highered.mcgraw-hill.com/novella/MixQuizProcessingServlet

It explains the action pretty well. To answer, your question, it can depend on when you're starting your cycle. For example, if you say that there are calcium ions and ATP readily available, then you can say the first step [after the binding site on actin is exposed] starts with the heads already attached with a cross bridge in place that causes the actin to be pulled toward the Z line, making the fiber shorter. Then ATP attaches to the myosin head and the cross bridge unforms and the head detaches. Then the ATP is hydrolyzed [an H₂O molecule is added to break it into the ADP and the inorganic phosphate], which is where the energy in the myosin head comes from. When it's hydrolyzed, it allows the myosin head to cock back into place. Or you could say that the myosin head starts unattached to the actin and then forms a cross bridge with the actin and it goes from there. I assume that the myosin head is not originally attached to the actin when the calcium ions first cause the movement of tropomyosin to reveal active sites. Does that help?

[bLub]

Thank you so much, the animation explains everything

Thanks!!

[Survival Robot]

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