Mechanism of muscular contraciton :

The Sliding filament theory explains the movement of the muscles. The theory was given by A.F. Huxley

According to this theory actin and myosin filaments slide past each other with the help of cross bridges to reduce the length of sarcomere and actin filaments which meet in centre of sarcomere.

1. When the stimulus of a motor nerve reaches neuromuscular junction then the vesicles present in the synaptic knobs secrete Acetyl choline which diffuses into sarcomere through membrane channel of muscle fibre.

Na⁺ also flow through channel into sarcomere and results action potential.

2. The muscle fibre which is polarised, after receiving acetyl choline becomes depolarised at that end.

3. The wave of depolarisation spreads due to invagination of sarcolemma called T tubes and Z lines.

4. Due to depolarisation, the sarcoplasmic reticulum secretes Ca⁺² ions which bind to specific site present on troponin component of actin filament. Conformational changes occur in Troponin molecule and active sites of F actin molecules are exposed where the cross bridge of myosin filaments gets joined to actin filaments.

5. The cross-bridges take energy from ATP and get rotated inward. Thus sarcomeres contract as Z-lines come closer.

6. Finally whole muscle contracts and H-lines disappear.

7. ADP molecules take energy from creatine~ phosphate and get converted into ATP.

8. The cross-bridges which have been rotated inward may rotate more after taking more energy from ATP molecule. Thus it brings more contraction of sarcomere.

Relationship between the actin and myosin myofilaments in the (A) relaxed and (B) contracted states