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Dear Student
Ans1. 
Mechanism of Muscle Contraction − Sliding Filament Theory

• Muscle contraction is initiated by the signal sent by the CNS.

• Neural signal, on reaching the neuromuscular junction, releases a neurotransmitter (acetylcholine) that generates action potential in the sarcolemma.

• Action potential spreads through the muscle fibre and causes release of calcium ions in the sarcoplasm.

• These calcium ions bind with troponin subunits. Hence, the masking of the active sites of myosin is removed.

• Exposed active sites on actin now binds with the myosin head to form a cross bridge.

• Cross bridge formation pulls the attached actin filament towards the centre of A band. The Z line attached to the actin is also pulled in, leading to muscle contraction.

• Myosin releases ADP + Pi, and relaxes. A new ATP binds and the cross bridge is broken. ATP is again hydrolysed by the myosin head, and cycle of cross bridge formation and breakage is repeated, causing further sliding.

• Process continues till calcium ions are pumped back and actin filaments are masked again. The Z lines return to their original position and the muscle relaxes.
  
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