Relaxed skeletal muscle showing sarcomeres & nucleus

Transmission electron micrograph of myofibrils within an uncontracted skeletal muscle fibre (a single muscle cell) of a guinea pig. One of the cell's nuclei is also visible. Each myofibril is made up of sarcomeres (contractile units) A sarcomere extends from one heavy black line (Z line) to the next. The actin filaments extend from the Z line towards the centre of the sarcomere. The myosin filaments form the dark band in the centre of the sarcomere. These 2 types of filaments slide over each other to cause the muscle to contract.