What are the Molecular Bases of Muscle Contraction? – Answered!

The most important molecular basis of muscle contraction is the repetitive formation and breakage of cross bridges which are the only structures responsible for the contraction of muscle.

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As these cross bridges are formed in between the thick filaments of A band and thin filaments of I band, the two sets of filaments start movement past one another resulting in the shortening of the sarcomere.

It has been established that cross bridges are formed only in the absence of ATP, while they disappear in the presence of ATP.

How sliding in the filaments takes place? The cross bridges which take their origin from the myosin (thick) filaments are supposed to consist of heavy or H-meromyosin which shows ATPase activity and its active enzymatic sites are presumed to be nearer the base of the cross bridges.

Because of the presence of Mg⁺⁺ ions the base and the distal end of the cross bridge are supposed to be negatively charged owing to the presence of ionized ATP formed through a complex with Mg⁺⁺ ions.

The mutual repulsion of these two sets of negatively charge would keep the cross bridges in an extended state, and the ATP away from the active sites on the H-meromyosin.

As the calcium ions are released from the T-sarcotubular system and L- sarcotubular system during activity, form chemical links between the distal end of the cross bridge and the adjacent thin filaments.

On release of calcium ions the negative charge on the distal end is abo­lished with the result mutual repulsion between the two ends of the cross bridge is ended.

As the mutual repulsion is over cross bridges are formed and shortening of the muscle is resulted. Because of the shortening the ATP at distal end is brought close to the active sites at the base of bridge.

The ATP splits by the meromyosin ATPase which may lead the breaking of calcium-actin complex link, but it is rephosphorylated by creatine phosphate (CP).

The ionized ATP is thus reformed and the electrostatic repulsion between the two ends of the bridge is restored. This process goes on again and again until the calcium ions are exhausted.