Final answer:
Muscle contraction involves myosin interacting with actin to shorten the sarcomere using ATP energy and requires the presence of calcium ions. Contraction halts when calcium is removed and actin-binding sites are protected.
Explanation:
During muscle contraction, myosin heads attach to actin binding sites, leading to the sliding of filaments and shortening of the sarcomere. This process is facilitated by the hydrolysis of ATP that provides energy for the myosin heads to pull actin towards the center of the sarcomere.
As long as calcium ions (Ca++) are present on the actin filament, and troponin binds to Ca++, the muscle contraction continues. However, contraction stops when Ca++ is removed, and the actin-binding sites are shielded.
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