Skeletal muscle. Light micrograph of a longitudinal section through skeletal muscle fibres. Muscle fibres are extremely long, cylindrical cells (running from top to bottom). The fibres contain numerous elongated nuclei (black, top right). Running the length of each fibre are thin threads called myofibrils (not seen). The cross striations (purple 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, hence it is also called voluntary muscle.