Final answer:
The parallel thick and thin filaments in myofibrils are connected by crossbridges, which are formed when myosin heads on the thick filaments bind to actin on the thin filaments, a process crucial for muscle contraction.
Step-by-step explanation:
Most of the time, the parallel thick and thin filaments of the myofibrils are connected by crossbridges that span the space between myosin and actin molecules. The thick filaments are primarily composed of the protein myosin, which has heads that bind to actin molecules present in the thin filaments, facilitating muscle contraction. The thin filaments consist of actin, along with regulatory proteins such as tropomyosin and troponin. When a skeletal muscle fiber contracts, myosin heads attach to actin creating cross-bridges, enabling the thin filaments to slide over the thick filaments, which results in the shortening of sarcomeres and ultimately muscle contraction.
The process requires calcium ions, which bind to troponin, resulting in a conformational change that moves tropomyosin away from the myosin-binding sites on actin filaments, allowing for the formation of cross-bridges.