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Solution


Q.

Sliding filament theory can be best explained as               [2015]
1 actin and myosin filaments do not shorten but rather slide pass each other  
2 when myofilaments slide pass each other, myosin filaments shorten while actin filaments do not shorten  
3 when myofilaments slide pass each other actin filaments shorten while myosin filaments do not shorten  
4 actin and myosin filaments shorten and slide pass each other  

Ans.

(1)

During muscle contraction, the laterally projecting heads (cross bridges) of the thick myosin myofilaments come in contact with the thin actin myofilaments and rotate on them. This pulls the thin myofilaments toward the middle of the sarcomere, past the thick myofilaments. The Z lines come closer together and the sarcomere becomes shorter. Length of the A band remains constant. Myofilaments (both actin and myosin) stay the same length. Free ends of actin myofilaments move closer to the centre of the sarcomere, bringing Z lines closer together. I bands shorten and H zone narrows. A similar action in all the sarcomeres results in shortening of the entire myofibril and thereby of the whole fibre and the whole muscle.