<P> Two of the important proteins are myosin, which forms the thick filament, and actin, which forms the thin filament . Myosin has a long, fibrous tail and a globular head, which binds to actin . The myosin head also binds to ATP, which is the source of energy for muscle movement . Myosin can only bind to actin when the binding sites on actin are exposed by calcium ions . </P> <P> Actin molecules are bound to the Z line, which forms the borders of the sarcomere . Other bands appear when the sarcomere is relaxed . </P> <P> A muscle fiber from a biceps muscle may contain 100,000 sarcomeres . The myofibrils of smooth muscle cells are not arranged into sarcomeres . </P> <P> The sarcomeres are what give skeletal and cardiac muscles their striated appearance, which was first described by Leeuwenhoek . </P>

I band a band h zone and m line