How does pumping calcium back into the sarcoplasmic reticulum cause muscle relaxation?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram showing the structure of human striated or skeletal muscle, with labeled cross-sections from whole muscle to fascicle, muscle fiber, myofibril, and sarcomere, highlighting actin and myosin filaments. — muscle The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

Muscle relaxation occurs when the sarcoplasmic reticulum reabsorbs calcium ions from the sarcoplasm, returning their concentration to a low resting level.1

During contraction, elevated calcium levels cause troponin and tropomyosin to shift, which allows actin to bind with myosin. By pumping calcium back into the sarcoplasmic reticulum, the cell removes the trigger for this interaction. Without calcium present to maintain the shifted conformation of these regulatory proteins, troponin and tropomyosin return to their inhibitory positions, blocking the interaction between actin and myosin and preventing the development of muscle tension.1