Final answer:
The thick filament in myofibrils is composed of myosin molecules, while actin makes up the thin filament, alongside associated proteins tropomyosin and troponin involved in muscle contraction regulation.
Step-by-step explanation:
The thick filament of the myofibril is composed of myosin molecules, whereas actin is a protein that makes up the thin filaments. Myosin molecules in the thick filaments have a tail region that connects with other myosin molecules and heads that align at the filament's edges. Actin, the primary component of thin filaments, has specific binding sites for the attachment of myosin. In addition to actin, thin filaments also contain regulatory proteins called tropomyosin and troponin. Tropomyosin strands block the actin-myosin binding sites when the muscle is at rest, and troponin has subunits that bind to tropomyosin, actin, and calcium ions, playing a crucial role in muscle contraction.