sarcoplasmic reticulum (stores Ca) releases calcium ions (keys) once a nervous impulse has been received
the calcium ions bind to troponin (lock) which causes the troponin-tropomysin complexes move away from the myosin active site on actin
once this happens contraction can begin
myosin heads contain a site for ATP binding
ATP is hyrolyzed into ADP freeing up a phosphate molecule, this energizes the myosin head (power stroke)
myosin attaches to the myosin active site on actin and release the free phosphate group
phosphate release by myosin triggers the actual power contraction stroke
myosin releases the ADP and energy
as the head rotates it moves in a ratchet motion and pulls the thin filaments closer to the centre of the M long of the sarcomere
when the ratchet motion is over the myosin awaits another ATP so it can be released from the actin active site
is Ca or ATP is available the myosin detaches and begins another stroke of the cycle - contraction continues