1. action potential in motor neuron activates muscle cell= axon knobs release NT acetylcholine into neuromuscular junction (synapse between neuron and muscle cell)
2. acetylcholine binds receptors on sarcolemma- causes depolarization, and (if stimulus is strong enough) an action potential that spreads across muscle cell membrane
3. depolarization of membrane- spreads from sarcolemma to transverse tubules, and then to SR
4. Membrane permeability of SR is altered and CA++ is released (lot of Ca++ is stored in SR) and binds to troponin and alters its shape
5. Change in troponin alters configuration of tropomyosin. Movement of tropomyosin from actin exposes region on actin that can bind to myosin head.
6. Myosin head binds ATP and hydrolyzes it. the ADP and P remain attached. This alters the shape of the myosin molecule, allowing it to make a cross-bridge with actin. This stimulates the release of ADP + P. When those are released from myosin molecule, the myosin head swivels, and pulls the actin filament toward the center of the sarcomere.
7. myosin head binds another ATP. This is necessary in order for the myosin/actin cross bridge to break, returning tone myosin head to its original position (like the na/K pump, myosin is also an ATP are enzyme that can hydrolyze ATP)
8. Myosin head can attach to new actin monomer and repeat the process as long as CA++ and ATP are available
9. Each of the many cross-bridges will break and reform many times per second, producing a smooth sliding movement
10. When stimulus ceases, CA++ returns to SR by active transport (requires ATP)