Explain the sliding filament theory of muscle contraction.
Answers
"The sliding filament theory of muscle contraction was postulated by A.F. Huxley, Niedergerke, H. E. Huxley and J. Hanson.
They observed that the sarcomere length shortened with contraction and returned to the former length with relaxation. They also saw that the A, B band was of constant length in both contraction and relaxation. I-band, rather, shortened with contraction. That scrutinization made them to hypothesize that myosin was sliding over the actin filaments during contraction leading to shortening of the sarcomere. The actin is connected to the structures found at Z discs and thus its shortening leads to sarcomere shortening.
Mechanism of Muscle Contraction/ Sliding Filament Theory:
According to sliding filament theory, contraction of a muscle fiber takes place by sliding of thin filaments over the thick filaments.
A neural signal reaching neuro-muscular junction releases a neurotransmitter (acetylcholine) which generates an action potential in a sarcolemma.
Action potential causes release of calcium ions from sarcoplasmic reticulum into sarcoplasm.
Calcium ions bind to troponin causing it to change its shape.
This change in shape moves tropomyosin complex away from myosin binding sites on actin.
Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge.
This pulls the attached actin filaments towards the centre of A- band.
The Z- line attached to these actins are also pulled inwards thereby causing shortening of sarcomere, i.e. contraction.
Myosin released ADP+ Pi, a new ATP binds and the cross bridge breaks. As a result myosin goes back to its relaxed state.
The ATP is hydrolysed by myosin head and cycle of cross bridge formation and breakage is repeated.
The process continues till Ca2+ ions are pumped back to sarcoplasmic reticulum resulting in masking of actin filaments.
This causes return of Z lines back to their original position, i.e. relaxation.
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