Skeletal muscle. Coloured transmission electron micrograph (TEM) of a section through skeletal (striated) muscle. The muscle myofibrils (orange) run from upper left to bottom right. These myofibrils are divided into contractile units called sarcomeres by the thick discontinuous Z lines (red). A sarcomere contains protein filaments of myosin and actin which can slide over each other, allowing the muscle to contract. Mitochondria (red ovals), such as at lower right, provide the large amounts of energy required by skeletal muscle. Skeletal muscle is under the conscious control of the brain. Magnification: x21,430 at 6x4.5cm size.