During muscle contraction, cross bridges are formed when the thick filaments, which are primarily made of myosin, bind to the thin filaments, primarily consisting of actin. The correct answer is b) myosin actin.
To explain further, when a muscle fiber receives a signal to contract, calcium ions are released inside the muscle cell. These ions bind to troponin, which causes a conformational change that moves tropomyosin away from the binding sites on actin. This exposure allows the myosin heads on the thick filaments to attach to the actin filaments, forming cross bridges.
Once the cross bridges are formed, the myosin heads pull the actin filaments towards the center of the sarcomere, resulting in muscle contraction. This process is important for muscle strength and movement, making the myosin-actin interaction crucial in muscle physiology.