Answer
Andrew is right. ATP is a source of energy required for contraction and relaxation.
Forv contraction to take place myosin mus bind and hydrolyse an ATP to ADP+Pi. The energy released by this reaction activates myosin heads. When the myosin heads care "cocked" (activated) they can bind to actin filaments to form crossbridges, and execute the power stroke.
Recovery: The myisin holds on to the ADP and Pi until it meets another ATP. The binding of the new ATP desabilizes the crossbridge and prepares the myosin to repeat the power stroke
Work Step by Step
Relaxation occurs when no more neural stimuli arrive at the neuromuscular junction, and the synaptic knob stops releasing acetylcholine (ACH).. The ACH that was bound separates from the receptors and is broken into fragments by acetylcholinesterase (ACHe). The molecfular fragments of ACH are then reabsorbed by the synaptic knob for reuse. As nerve stimulation ceases the sarcoplasmic reticulum ceases to release calcium, but continues to absorb it. The binding of large amounts of calcium in the sarcoplasmic reticulum (SR), facilitated by the protein calsequestrin. results in a fall in calcium levels in the sarcoplasm, The loss of sarcoplasmic calcium causes calcium bound to troponin to be displaced without replacement. As a consequence of the loss of calcium from troponin (tropomyosin-trponin complex)n tropomyosin resumes its original position( form) and blocks the active actin sites. This means that the myosin can no longer bind to actin and the muscle fiber loses tension--relaxes.
