Name the ion responsible for unmasking of active sites for myosin for cross-bridge activity during muscle contraction.

A neural signal reaching neuro-muscular junction releases a neurotransmitter (Acetyl choline) which generates an action potential in the sarcolemma. Action potential spreads through the muscle fiber and causes the release of calcium ions into the sarcoplasm. Increase in Ca++ level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin.