Myofibrils are made from two types of protein filaments (myofilaments), the thinner actin filament and the thicker myosin filaments. The monomer G-Actin (globular) forms a linear polymer F-Actin (filamentous) which forms two helices that spiral round each other. Actin filaments have myosin binding sites which are blocked by rod-shaped proteins called tropomyosin. Tropomyosin is held in place by the Troponin complex. This is a globular protein with 3 subunits. The I subunit binds to actin and the T subunit binds to tropomyosin. The C subunit binds to calcium ions.