ANATOMY

Detailed explanation-1: -Once the myosin-binding sites are exposed, and if sufficient ATP is present, myosin binds to actin to begin cross-bridge cycling. Then the sarcomere shortens and the muscle contracts. In the absence of calcium, this binding does not occur, so the presence of free calcium is an important regulator of muscle contraction.

Detailed explanation-2: -Muscle contraction occurs when sarcomeres shorten, as thick and thin filaments slide past each other, which is called the sliding filament model of muscle contraction. ATP provides the energy for cross-bridge formation and filament sliding.

Detailed explanation-3: -B: With contraction, movement of the thin filaments toward the center of the sarcomere occurs, and because the thin filaments are anchored to the Z disks, their movement causes shortening of the sarcomere.

Detailed explanation-4: -Explanation: During muscular contraction, the myosin heads pull the actin filaments toward one another resulting in a shortened sarcomere. While the I band and H zone will disappear or shorten, the A band length will remain unchanged.

Detailed explanation-5: -A concentric contraction occurs when the muscle length is shortened during a contraction, e.g., a biceps curl. An eccentric contraction occurs when the muscle length is increased during the contraction, i.e., the “negative” contraction.