Muscle fiber is constructed of bundles small long organelles called what?

Muscle fibers are constructed of bundles of small, long organelles called myofibrils. These myofibrils are responsible for the contraction and relaxation of muscles. Composed of proteins called actin and myosin, myofibrils contain repeating units called sarcomeres that give muscles their striated appearance. Within these sarcomeres, actin and myosin filaments slide past each other, leading to muscle contraction. Thus, myofibrils play a crucial role in muscle functioning and movement.