Transmission electron micrograph (TEM) of the ultrastructure of a skeletal muscle cell (a muscle fibre) in cross-section. The nucleus is located at the periphery of the fibre and the cytoplasm (sarcoplasm) is filled with mitochondria and myofibrils organised into bundles. Contractile filaments of myofibrils form hexagonal arrays. The membranes between myofibrils is the sarcoplasmic reticulum. When the muscle fibre is stimulated, calcium flows out of the sarcoplasmic reticulum and rapidly initiates contractions of the myofibrils. Magnification: x6,500 when printed at a width of 10cm.