Myosin heads and thick myofilaments cross bridge with actin and thin myofiliments. Ca+ ions released from sarcoplasmic reticulum bind to troponin. Troponin moves tropomyosin away from myosin head so that actin can bind. Thin myofiliments pulled toward each other and muscle contracts. Process ends when ca+ is actively transported back to sarcoplasmic reticulum. Troponin returns to original shape and tropomyosin covers myosin binding site on actin. Sarcomere contraction: sliding filaments (myofilbril contraction) muscle contacts actin filaments slide along myosin filaments pulling z bands close shortening sarcomere. Muscle fascicles muscle fibers myiofibril sarcomere. Runners all have similar # of muscle fibers sprinters just have larger muscle fibers. Hypertrophy: getting larger and hyperplasia: not in skeletal muscle (divides and makes more muscle) Multiple: string of sarcomeres , ordered array of proteins, interconnected by intermediate laments= protein. Dystrophin: in the myotendinous junction connects myofibrils to connective tissue of the cell wall.