Muscle structure. Computer artwork showing the protein structure of myofibrils and how they are bound together to form muscle fibres and then muscles. From bottom: Muscle cells contain many myofibrils, organelles that are composed of units (sarcomeres, arrows) of actin and myosin bound together end to end. The myofibrils are secured at either end to the cell membrane. Many cells are then bound together to form a muscle fibre (middle) and many fibres are in turn bound to form muscle tissue (top).