Biology XI: 20 Locomotion & Movement(2 Mechanism of Muscle Contract

Mechanism of Muscle Contraction: Sliding Filament Theory 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.