What is required to initiate the coupling of myosin to actin?

Explanation:

The actin and myosin are the myofilaments which are involved in the contraction and relaxation of the muscle. The actin is thin filaments and contains the attachment site for the myosin but the site is blocked by the tropomyosin and troponin proteins.  

When calcium ions present in the muscle binds to the troponin molecules, it changes the conformation of the tropomyosin. This change exposes the attachment site of the myosin and then the myosin binds the troponin and causes Powerstroke and leads to contraction of a muscle.

Thus, Calcium ions is correct.

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a compound light microscope is used to magnify objects in steps.  

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