A: Muscle contraction is a physico chemical phenomenon. Sliding filament
Hypothesis explains the physical changes. It was proposed by Hugh Huxley and
Hanson (1965). Sliding filament hypothesis states that during muscle contraction,
thin filaments slide over the thick filaments resulting in shortening of sarcomere.
1. Stimulation of muscle: When a nerve impulse reaches the neuromuscular
junction, sarcolemma of muscle fibre gets depolarised. It extends into the T
- tubule. Then cisternae of sarcoplasmic reticulum releases Ca++ ions into sarcoplasm.
2. Contraction of muscle:
a. Formation of cross bridges:
i. The Ca++ ions bind to the TnC of troponin of thin filament.
ii. Thus troponin - tropomyosin complex is shifted and hence, active sites are
exposed in thin filament.
iii. Using energy released from ATP, the head of myosin now binds to the
exposed active site to form a crossbridge (actin - myosin complex). It
releases inorganic phosphorus.
b. Power stroke:
i. The cross bridge swings towards H - zone. This is called power stroke.
ii. The Z - lines attached to thin filaments are pulled inwards over the thick
filaments on both sides causing shortening of sarcomere, that is
contraction.
During contraction
★ A - Band remains same.
★ I - Band shortens.
★ H - zone also shortens or some times disappear.
c. Recovery stroke:
i. The power stroke is followed by the release of ADP.
ii. A new ATP is binds to the head of myosin and thus cross bridge is
broken.
iii. The new ATP is again hydrolysed into ADP and inorganic phosphorus.
Thus head of myosin swings to its original position and then attached to
a new active site and the process repeats.
d. Relaxation of muscle:
i. When motor impulses stop, the Ca++ ions detach from the thin filament
and diffuse into the cisternae of sarcoplasmic reticulum.
ii. Troponin permits the tropomyosin to cover the active sites of thin filaments.
Thus heads of myosin are unable to attach the active sites.
iii. These changes cause the return of Z - lines to their original places. Then
sarcomere is said to be relaxed.
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