Final answer:
The thick filament consists of the protein myosin, with tails forming its backbone and heads for cross-bridge formation. The thin filament is primarily actin, regulated by proteins tropomyosin and troponin, which interact with myosin for muscle contraction.
Step-by-step explanation:
The two kinds of chains that make up the thick filament in muscle myofibrils are primarily composed of the protein myosin. Myosin molecules feature a tail region that binds to other myosin molecules, forming the backbone of the thick filament. The heads of the myosin molecules project outward and are responsible for forming cross-bridges with the thin filament during muscle contraction. The thin filaments, in contrast, are primarily made of actin, along with the regulatory proteins tropomyosin and troponin. In muscle cells, the interaction between myosin's heads on the thick filament and the actin on the thin filament enables muscular contraction.