Skeletal muscles, like the biceps brachii, attach to bone via connective tissue called tendons. Muscles are composed of bundles of muscle fibers. Each bundle is separated by connective tissues known as perimysium. Each fasciculus is made up of muscle fibers, which are separated by connective tissue called endomysium. Skeletal muscle fibers or cells are multinucleated and striated in appearance. Muscle cells are composed of subunits called myofibrils. Each myofibril is made up of several myofilaments. The two types of myofilaments (shown in red) are composed primarily of the protein myosin and a thin myofilament (shown in blue) composed mainly of the protein actin. The repeating arrangement of thick and thin myofilaments serves as the fundamental subunit of striated muscle contraction. These subunits are called sarcomeres. A sarcomere contraction is represented by the shortening of the distance between the Z lines. The sarcomere shortens because the thin filaments slide past the thick filaments. In 3D each thick myofilament is surrounded by six thin myofilaments arranged in a hexagonal pattern. The 3D arrangement of sliding myofilaments is the microscopic basis of muscle contraction.