Human heart Actin and myosin, whose interaction causes muscle contraction, occur in many other cells. Actin is attached to the inner surface of the plasma membrane. The interaction of cytoplasmic myosin and this actin causes contraction of the cell, such a the coordinated contractions of intestinal cells to absorb nutrients.

    Some fish have modified muscles that discharge electricity. These fish have electric organs consisting of modified muscles known as electroplates. The South American electric eel has more than 6000 plates arranged into 70 columns. Maximum discharge is 100 watts.

    Vertebrates move by application of the principles of the lever. Levers amplify or increase the force or velocity of motion. The amount of amplification depends on the length of the lever. There are three types of skeletal system, all interact with muscles using the lever.