Final answer:
Actin molecules form the thin myofilament, which, along with myosin-made thick filaments, comprises the myofibril. Troponin, attached to the thin filament, regulates muscle contraction by responding to Ca²+ ions.
Step-by-step explanation:
The actin molecule, thin myofilament, myofibril, and troponin are all related structures involved in muscle contraction. An actin molecule is a globular protein that polymerizes to form F-actin, which intertwines with another F-actin strand to create a thin myofilament. Thin myofilaments are helical structures composed of actin but also contain two regulatory proteins, tropomyosin and troponin.
Troponin is a complex of three subunits, each binding to actin, tropomyosin, or Ca²+ ions. Numerous thin myofilaments alongside thick myofilaments, comprised of myosin, combine to form a myofibril, the contractile thread in muscle fibbers.
During muscle contraction, the thin myofilaments slide past thick myofilaments, a process regulated by the presence of calcium ions binding to troponin, which causes tropomyosin to uncover the binding sites on actin for myosin attachment.