Question

Which of the following stage of cross-bridge is shown in the diagram?

• A. Formation of cross-bridge
• B. Sliding/rotation
• C. Shifting of myosin
• D. Breaking of cross-bridge

Answer 1

Answer: (A)

Steps involved in muscle contraction:
$1.$ Increase in Ca⁺⁺ level within sarcoplasm $\to$ Calcium binds with a subunit of troponin on actin filaments $\to$ Active sites for myosin are exposed.
$2.$ Binding of the myosin head to the exposed active sites on actin to form a cross bridge. This step utilizes the energy from ATP hydrolysis. Attached actin filaments are pulled towards the center of the 'A' band. The 'Z' line attached to the actins are also pulled inwards resulting in a shortening of the sarcomere (or contraction).
$3.$ The myosin enters the relaxed state releasing the ADP and Pi. New ATP binds resulting in breaking of the cross-bridge. Hydrolysis of ATP by the myosin head again results in the cross-bridge formation.
$4.$ Repeated formation and breaking of cross-bridges result in further sliding of filaments over each other. Thus contraction occurs.