Skeletal muscle. Light micrograph of a longitudinal section through skeletal muscle fibres. Muscle fibres are long, cylindrical cells (running left to right). Running the length of each fibre are thin threads called myofibrils (not seen). The cross striations (stripes) are due to the regular arrangement of contractile proteins in these myofibrils. These proteins (actin and myosin) slide past each other when a muscle contraction takes place, shortening the fibre. Skeletal muscle is under voluntary control. Magnification x250 when printed 10cm size.