Define sliding filament theory of muscle contraction.
The sliding filament theory explains the mechanism of muscle contraction. It states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.
The sarcomere is the functional unit of muscle. It has a central 'A' band made of thick myosin filaments and two half 'I' bands made of thin actin filaments on either side, marked by 'Z' lines. During contraction, the myosin head binds to actin forming a cross bridge. These cross bridges pull the actin filaments towards the centre of 'A' band, causing them to slide over myosin filaments. The 'Z' lines are pulled inwards, shortening the sarcomere. During this process, the 'I' bands get reduced while the 'A' bands retain their length.
Explanation
This answer covers the key aspects of the sliding filament theory as described in the NCERT textbook. The theory was proposed to explain how muscles contract at the molecular level. The key concept is that actin (thin) filaments slide over myosin (thick) filaments without either filament changing length. The cross bridge formation between myosin head and actin is the molecular mechanism that drives this sliding. The structural changes in sarcomere (shortening of I band, unchanged A band) provide evidence for this theory.