Muscle cells

Muscle cells are syncytial (multi-nucleated) up to ~50cm in length formed by the fusion of myoblasts. Muscle cells contain a number of myofibrils each of which consists of a regular repeating array of sarcomeres, the base unit of muscle action. Electron micrographs show alternating light and dark bands with a line (the Z line) down the middle of a light band. Each sarcomere is defined as the region between the Z lines and has one dark band and one half of a light band on either side. The dark band are composed of myosin II filaments and the light band of actin filaments. The darkest region is the region of overlap containing both myosin filaments and actin filaments arranged in a regular pattern. When the muscle contracts the light band disappears as the region of overlap between the myosin and actin filaments increases