According to the sliding filament theory of muscle contraction, which of the following statements is true?

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Muscle contraction occurs when the thin filaments shorten in response to a nerve impulse.

The two Z-lines of a sarcomere move closer together during muscle contraction.

During contraction, the myosin heads remain stationary while actin filaments slide past them.

Calcium ions bind to tropomyosin, leading to the exposure of myosin binding sites on actin filaments.

answer is D.

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Detailed Solution

During muscle contraction, myosin cross-bridges bind to the thin actin filaments. These cross-bridges then ‘walk’ along the actin filaments, pulling them toward the center of the sarcomere. As the myosin cross-bridges pull the actin filaments inward toward the center of the ‘A’ band, this region shortens. As the actin filaments are pulled inward, the ‘Z’ lines to which they are attached also move closer together. This inward movement of the ‘Z’ lines is a key indicator of muscle contraction and represents the shortening of the sarcomere.

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