Animation of a muscle contracting and relaxing. Muscles are made up of basic units called sarcomeres, which are composed of long, fibrous proteins that slide past each other as the muscles contract and relax. The two proteins are myosin at bottom, straight and actin at bottom, coiled. The M_line M, grey is at the centre of the myosin and the Z_line Z, green forms the borders of each sarcomere. Other sections include the I_band I, blue, for isotropic where there is only actin filaments and the A_band A, red, for Anisotropic for the entire length of a myosin filament. The H_zone H, white is the section of myosin filaments not superimposed by actin filaments. Master same as original.