Question

During muscle contraction, what could be the correct sequence?
A. Release of calcium ions into the sarcoplasm.
B. Release of Acetylcholine.
C. Sliding of actin filaments over each other.
D. Break down of ATP to form a cross bridge.

• A. A D B C
• B. A B D C
• C. B A C D
• D. B A D C

Answer 1

Answer: (D)

Steps involved in muscle contraction are the following:
A. The central nervous system (CNS) sends a signal via a motor neuron of a motor unit.
B. The signal reaches the neuromuscular junction $\to$ Release of neurotransmitter (Acetylcholine) $\to$ Generation of the action potential in the sarcolemma.
C. Spread of Action potential through the muscle fibre and causes the release of Ca ions into the sarcoplasm.
D. An increase in Ca⁺² level within sarcoplasm $\to$ Calcium binds with a subunit of troponin on actin filaments $\to$ Active sites for myosin are exposed.
E. 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 these actins is also pulled inwards resulting in a shortening of the sarcomere (or contraction).
F. The myosin enters the relaxed state releasing the ADP and Pi. New ATP binds resulting in the breaking of the cross-bridge. Hydrolysis of ATP by the myosin head again results in the cross-bridge formation.
G. Repeated formation and breaking of cross-bridges result in further sliding of filaments over each other. Thus contraction occurs.