Final answer:
Cross-bridges are formed during muscle contraction when myosin heads on thick filaments bind to actin on thin filaments, with myosin and actin being the primary proteins involved. The process is called cross-bridge cycling, crucial for muscle contraction, and involves the repeated attachment, pulling, and release of myosin to and from actin, powered by ATP.
Step-by-step explanation:
Cross-bridges are critical connections between the filaments inside muscle fibers that enable them to contract. These bridges are formed when the myosin heads of the thick myosin filaments attach to the actin in the thin filaments. The proteins involved in forming cross-bridges are primarily myosin and actin. The process of cross-bridge cycling, which includes this formation, is essential in muscle contraction.
The cycle begins when calcium ions (Ca2+) bind to actin, which exposes binding sites for the myosin heads. Following this, the myosin heads, which are energized by ATP, bind to the actin, forming a cross-bridge. The release of adenosine diphosphate (ADP) and inorganic phosphate (Pi) that are bound to myosin results in the myosin head pulling the actin filament toward the center of the sarcomere, known as the power stroke. Afterward, a new molecule of ATP attaches to the myosin head, causing it to release from actin and to its original position, ready to form another cross-bridge. This sequence repeats itself, allowing the muscle to contract.