Answer
In order for muscles to contract myosin-actin cross bridges must be formed. ; the myosin head must bind to an active site on a thin actin filament and execute the power stroke. This requires energy and the energy is derived from the hydrolysis of ATP molecules by ATPase.
In the excitation-contraction coupling calcium binds to troponin connected to tropomyosin. This shifts the position of the tropomyosin so that the active sites it covered are unblocked . The myosin picks up an ATP molecule which is hydrolysed to ADP+Pi under the influence of the myosin enzyme, myosin ATPase; this cocks the myosin head -- it assumes a position for binding with an active myosin site. The myosin head binds with an actin site to form the complex( myosin-actin cross-bridge complex. The ADP+Pi is released from the the myosin head which flexes and the thin filament slides over the thick filament in the so-called power stroke; this generates tension. It should be noted that the muscle fiber contracts but the filaments do not shorten. Next the myosin- actin complex releases the ADP-Pi . The myosin head remains bound to the actin filament until it meets a new ATP molecule.
Work Step by Step
When the myosin head binds a new ATP molecule, this destabilizes and breaks the myosin-actin cross-bridge. myosin will h hydrolyze the new ATP to ADP+Pi and and recover its ability to attach to a new site farther down the actin filament. This is the recovery stroke.
These are energy dependent processes and the energy is derived from the hydrolysis of ATP ( ATP>ADP+Pi).
Relaxation requires the release of actin from myosin-actin cross-bridge. When a new ATP binds to the cross-bridge, the cross-bridge is destabilized ; this prepares the myosin to repeat the power stroke .
