Striated muscle. Coloured transmission electron micrograph (TEM) of a longitudinal section through striated skeletal muscle. The striated banding- pattern of the muscle fibrils is seen. The fibrils run in parallel (from left to right) and between them runs sarcoplasmic reticulum (SR) that trans- mits nerve impulses to the fibrils. Here, the SR contains many mitochondria (brown). Within each fibril are contractile units called sarcomeres, separated by red lines. A sarcomere has protein filaments of myosin (pink) and actin (yellow) that slide over each other, thereby causing the whole muscle to contract. Skeletal muscle is responsible for voluntary muscle movement in the body.