List the 4 events of cross bridge cycle.
The correct answer and explanation is:
The four events of the cross-bridge cycle in muscle contraction are:
- Cross-Bridge Formation: The cycle begins when the myosin head binds to an actin filament. This is facilitated by the presence of calcium ions, which bind to troponin, causing a shift in tropomyosin that exposes the myosin-binding sites on the actin filament. The myosin head then attaches to the exposed binding site on actin, forming a cross-bridge.
- Power Stroke: Once the cross-bridge is formed, the myosin head undergoes a conformational change. The energy stored in the myosin head, which was previously obtained from the hydrolysis of ATP into ADP and inorganic phosphate, is used to pull the actin filament toward the center of the sarcomere. This action is referred to as the power stroke. It causes the thin actin filaments to slide over the thick myosin filaments, resulting in muscle contraction.
- Cross-Bridge Detachment: After the power stroke, a new ATP molecule binds to the myosin head, causing a reduction in the affinity of the myosin head for actin. As a result, the cross-bridge is broken, and the myosin head detaches from the actin filament.
- Reactivation of the Myosin Head: The ATP bound to the myosin head is hydrolyzed into ADP and inorganic phosphate. This hydrolysis provides the energy needed to “cock” the myosin head back into its high-energy state. The myosin head is now ready to bind to actin again, and the cycle repeats as long as calcium remains present to keep the binding sites on actin exposed.
The cross-bridge cycle is essential for muscle contraction and relaxation, and the continuous interaction between myosin and actin fibers drives the shortening of the sarcomeres, resulting in overall muscle movement.