What is the next step in muscle contraction after the actin changes shape?

The myosin filament or more precisely the myosin head can now bind to the actin forming the cross bridges followed by a power stroke during which actin slides over myosin.

Explanation:

The muscle contraction can be explained by sliding filament theory bu Huxley and Huxley. The two muscle proteins which take part in muscle contraction are myosin and actin.

Myosin:  It is a hexameric protein. Each monomer is called meromyosin. Each meromyosin has two important parts, a globular head with a short arm and a tail. The head forms cross bridges with the actin filament. Myosin head acts as ATPase enzyme. When ATP binds, head acts as enzyme hydrolyzing the ATP to produce energy. The head also has the site for binding of actin.

Actin filament: It contains three proteins, filamentous actin, tropomyosin and troponin. Filamentous actin contains active site for myosin binding but at rest, tropmyosin covers the myosin binding site. This prevents the cross bridge formation. Tropomyosin are held in place by troponin molecules.

When calcium is available, the binding of calcium to a TpC sub-unit of troponin causes the shifting of tropomyosin-troponin complex. Now actin can attach to myosin head and slide over myosin.

The actin filaments slide over the myosin filament by the the formation of cross bridges and during this process the I-band gets reduced whereas the A band remain the same. The lengths of actin and myosin filaments remain unchanged.

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