The nerve impulse enters the T Tubules and Sarcoplasmic Reticulum, stimulating the release of calcium ions.
Calcium ions combine with Troponin, shifting troponin and exposing the myosin binding sites on the actin.
ATP breaks down ADP + P. The released energy activates the myosin cross bridges and results in the sliding of thin actin myofilament past the thick myosin myofilaments.
The sliding of the myofilaments draws the Z lines towards each other, the sarcomere shortens, the muscle fibers contract and therefore muscle contracts.
ACh is inactivated by Acetylcholinesterase, inhibiting the nerve impulse conduction across the sarcolemma.
Nerve impulse is inhibited, calcium ions are actively transported back into the Sarcoplasmic Reticulum, using the energy from the earlier ATP breakdown.
The low calcium concentration causes the myosin cross bridges to separate from the think actin myofilaments and the actin myofilaments return to their relaxed position.