Actin and Myosin are contractile proteins of muscle while Troponin and Tropomyosin are regulatory proteins. Myosin is a polymer of Meromysoin units and this is the main protein of Dark bands. Myosin is a polymer made up of Meromyosin monomers. The molecule consists of Head and Tail that are attached by cross bridges. Head consists of Actin binding site and ATPase enzyme. This enzyme is needed for muscle contraction. This Light bands consist of Actin, Troponin and Tropomyosin. Actin protein consists of two double stranded polypetides while troponin is attached on these strands at intervals. Actin shows presence of Myosin binding site and when myosin attaches to this site the muscle shows contraction due to formation of cross bridges. However this cross bridge formation is not seen when muscle relaxes as this site is masked by Troponin molecule. At intervals on actin, trimeric molecule Tropomyosin is also seen. This includes TpC, TpT and TpI.