Question

This Ca2+ reuptake enables the tropomyosin to:

A) Block the myosin binding sites on actin

B) Uncover the myosin binding sites on actin

C) Stabilize the sarcomere structure

D) Initiate muscle contraction

Answer 1

Ca2+ reuptake into the sarcoplasmic reticulum causes tropomyosin to block the myosin-binding sites on actin, hence causing muscle relaxation after contraction.

Step-by-step explanation:

The reuptake of Ca2+ into the sarcoplasmic reticulum (SR) is essential for muscle relaxation. During the excitation-contraction coupling process, calcium ions are released from the SR, which then bind to troponin. This causes a conformational change that moves tropomyosin away from the myosin-binding sites on actin filaments. As a result, myosin heads can attach to the actin, forming cross-bridges and initiating muscle contraction. When Ca2+ is actively pumped back into the SR, its concentration in the sarcoplasm decreases. In turn, calcium detaches from troponin, prompting tropomyosin to return to its resting position, which blocks the myosin-binding sites on actin and leads to muscle relaxation.