A characteristic feature of the muscle fibre is the presence of large number of parallely arranged filaments in the sacroplasm called myofilaments or myofibrils.
Each myofibril has alternate dark and light bands on it. The light band contains actin whereas the dark band contains myosin. Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively. Each myosin filament is a polymerised protein. Many monomeric proteins called meromyosins constitute one thick filament. Each meromyosin has two important parts, a globular head with a short arm and a tail, the former being called the heavy meromyosin (HMM) and the latter, the light meromyosin (LMM).