The thick filament or the myosin is made up of the heavy chain of myosin and light chain of myosin The myosin molecule is composed of six polypeptide chains- Two heavy chains, each with a molecular weight of about 200,000 and four light chains with molecular weights of about 20,000 each. The two heavy chains wrap spirally around each other to form helix, which is called the tail of the myosin molecule. One each end of these chains is folded into a globular polypeptide structure called myosin head. These heads contain an actin-binding site and a catalytic site that hydrolyses ATP. Thus, there are two free heads at one end of the double-helix myosin molecule. The four light chains are also pa of the myosin head, two to each head. These light chains help control the function of the head during muscle contraction. Ref: Guyton and Hall Textbook of Medical Physiology 13th edition Pgno: 78-79