Heart muscle. Coloured scanning electron micrograph (SEM) of a freeze-fracture of muscle from the heart. The muscle is made up of myofibrils, which run vertically. The red bundle of myofibrils has had its lining (sarcolemma) removed, and appears banded due to the individual sarcomeres of which it is composed. Each sarcomere is a contracile unit of the proteins actin and myosin. Unlike skeletal striated muscle, which contracts after nerve impulses, cardiac muscle contracts regularly of its own accord. Energy for the contractions is produced by the many mitochondria (pink blobs) embedded in the muscle.