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Step 1: The brain sends a nerve impulse through the motor unit down to a Neuromuscular junction
Step 2: nerve impulse known as action potential causes voltage gated calcium ion channels to open which allow calcium ions to enter the neuron. This increase in Calcium stimulates the release of the neurotransmitter, acetlycholine, which is released  into the cleft.
Step 3: ACh binds to a ACh receptor sight and activates voltage gated sodium channel pumps Na+ is pumped into the muscle cell and K+ is pumped out. This process is called depolarization as more Na+ is pumped into the cell than K+ is pumped out which causes the interior of the cell to become negative
Step 4: Initial depolarization spreads to adjacent areas, opening voltage gated sodium channels and eventually initiating an action potential that moves long the lengths of the sarcolemma
Step 5: Na+ concentrations caused by depolarization cause a change in membrane permeability by opening voltage gated K+ pumps which allow K+ to diffuse across the sarcolemma out of muscle fibers. This process is called repolarization.

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Step 2: Transmission of an action potential along the sarcolemma leads to the release of calcium ions from SR and the sliding of myofilaments. Action potential along the T tubules come adjacent sarcoplasmic reticulum (SR) to release Ca2+ which flows into the cytosol.

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Step 4: Cocking of the myosin head. As ATP is hydrolyzed to ADP and P, the myosin head into high-energy state.

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Step 3: Cross bridge detachment. After ATP attaches to myosin, the link between myosin detaches. (the cross bridge "breaks").

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Step 2:The power stroke, ADP and P are released and the myosin head pivots and bends, changing to it's bent, low-energy state. As a result it pulls the actin filament towards the m line.

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step 1: Cross bridge formation, Energized myosin head attaches to actin myofilament, forms a cross bridge

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Step 3: Ca2+ combines with troponin (Protein located on actin) and removes the blocking action of tropomyosin by changing the shape of the actin so that the myosin binding site are exposed.
Step 4: Myosin binds to actin forms cross bridges and contraction begins

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Step 1:  The action potential moves down the sarcolemma (muscle cell membrane) and down the T tubules (pertrusion of the sarcolemma that protrude deep into the muscle cell).