Which one of the following statements regarding muscle contraction and relaxation is correct?

Muscle contraction and relaxation are primarily regulated by the movement of calcium ions (Ca2+). When a muscle cell is stimulated by a motor neuron, an action potential triggers the release of Ca2+ stored within the sarcoplasmic reticulum (SR) into the cytoplasm. These calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from myosin-binding sites on actin filaments, allowing cross-bridge formation and contraction. For relaxation to occur, the signaling ends and Ca2+ ions are actively pumped back into the sarcoplasmic reticulum by Ca2+-ATPase [1]. This reduction in cytoplasmic calcium concentration causes tropomyosin to reshield the actin binding sites, ending the contraction cycle. While ATP provides the necessary energy for these processes, the specific ionic mechanism defining the contraction-relaxation cycle is the release and re-sequestration of calcium by the SR.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > ATP > p. 88