Relaxed skeletal muscle. Transmission electron micrograph (TEM) of a longitudinal section through relaxed skeletal, or striated, muscle. The dark vertical lines mark the borders of sarcomeres, the smallest functional units of striated muscle. The horizontal striations are due to the muscle fibrils, which run left to right. These are made up of the proteins actin and myosin. The lighter regions are stretches of actin, the darker regions are actin and myosin. Muscles contract when actin slides over the myosin. The energy for the contractions comes from mitochondria (small dark ovals). Skeletal muscle is under conscious control. Magnification: x16,200 when printed at 10 centimetres wide.