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Step 4: Cocking of myosin head: as ATP is split into ADP and P, energy from hydrolysis of ATP "cocks" myosin head into high-energy state.

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Step 3: Cross bridge detachment: ATP attaches to myosin head, causing cross bridge to detach.

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Step 2: Working (power) stroke: myosin head pivots and pulls on the actin filaments, pulling it toward M line.

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Step 1: Cross bridge formation: high - energy myosin head attaches to actin myofilaments forming a cross bridge.

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Step 3 and 4: 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 is exposed. The myosin then binds to the acton, forming a cross bridge and contraction begins.

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Step 2: Transmission of the action potential along the T tubules cause the adjacent sarcoplasmic reticulum to release Ca2+ which flows into the cytosol.

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

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The brain sends an action potential through the motor unit down the Neuromuscular junction. The action potential then causes voltage gated calcium ion channels to open, allowing calcium ions to enter the neuron. The increase of calcium stimulates the release of acetylcholine, which is released into the cleft. The ACh binds to the ACh receptor sight and activates voltage gated sodium channels which pumps Na+ in and K+ out, causing the cell inferior to become depolarized. Initial depolarization spreads to adjacent areas, opening voltage gated sodium channels and initiating an action potential that moves along the lengths of the sarcolemma. Na+ concentrations caused by depolarization stimulate a change in the membrane permeability by opening voltage gated K+ pumps, which allow K+ to diffuse across the sarcolemma out of the muscle fiber, repolarizing the cell.