Explain the process muscle contraction?

According to sliding filament theory, the contraction of muscle fibres occurs when the thin filaments slide over the thick filaments.

Depolarisation of Sarcolemma:

• A signal sent by the central nervous system via motor neuron reaches the neuromuscular junction or motor end plate which causes the release of acetylcholine.
• Release of acetylcholine sets the action potential in the sarcolemma.

Release of Calcium Ions:

• Sarcolemma transmits its action potential to sarcoplasmic reticulum to release calcium ions in the sarcoplasm.

Conformational Changes in Actin Filaments:

• Released calcium ions binds to troponin and tropomyosin on active filaments.
• This changes the three dimensional shape of the actin-troponin-tropomyosin complex and the active site for myosin present on the actin filament is exposed.

Activation of Myosin Heads:

• Due to the release of calcium ions, myosin head also get activated and they release energy from ATP.
• Release of energy by the hydrolysis of ATP results in binding of myosin heads to the active sites present on the actin filaments; forming a cross-bridge.

Sliding of Actin Filaments over Myosin:

• As soon as the cross-bridge is formed, the myosin head rotates to pull the actin filament towards the centre of A-band i.e. H-zone.
• The pulling of actin filaments from opposite ends results in the contraction of sarcomere due to which the I-band shortens and A-band retains its length.

Repolarisation of Sarcolemma (Relaxation of Muscle):

• When sarcolemma again retains the resting potential, calcium ions are pumped back into the sarcoplasmic reticulum due to which troponin and tropomyosin become free and they mask the active site for myosin present on the actin filament. This results in breaking of the cross-bridge.
• As soon as the cross-bridge is broken, the actin filaments return to their original position.
• Sarcomeres relax, I-band retain their normal length.