Sarcomeresare the fundamental contractile unit of skeletal muscle and are characterized by their highly ordered appearance under a polarizing light microscope Thick filaments Thick filaments in the A band are composed primarily of the protein myosin. Each myosin molecule is composed of six monomers: two protein strands intertwined in a helical arrangement (termed heavy chains) and four smaller, globular proteins (termed myosin light chains). Thin filaments Thin filaments are composed of three primary proteins: actin, tropomyosin, and troponin