Myofibril is a long contractile fiber found in striated muscles. It is composed of overlapping thick and thin myofilaments and two muscle regulatory proteins, troponin, and tropomyosin. The thick filaments are composed of the protein myosin and the thin filaments are composed of the protein actin. The thick and thin filaments are arranged in alternating overlapping bands, which give skeletal muscles and cardiac muscles their characteristic striped appearance.