Skeletal muscle fibre. Coloured scanning electron micrograph (SEM) of a freeze-fractured skeletal (or striated) muscle fibre. The fracturing of the fibre has revealed that it consists of a bundle of smaller fibres called myofibrils (red). These are around 1-2 micrometres (millionths of a metre) in diameter. The fibre has a sheath of connective tissue (endomysium) that is seen as the strands scattered around the image. Skeletal muscle, named because skeletal bones are moved with this muscle, is under voluntary control.