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Step 2: Working (power) stroke: myosin head pivots and pulls actin filament toward M line

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

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Step 4: Cocking of myosin head: energy from hydrolysis of ATP “cocks” myosin head into high-energy state

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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: 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 2: Transmission of the action potential along
the T tubules cause the adjacent sarcoplasmic
reticulum (SR) 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
protrude deep into the muscle cell)

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Action potential travels to axon terminal. The nerve impulse opens voltage gated calcium channels causing an increase in calcium ions and the release of acetylcholine release into the cleft. The ACh activates the opening of voltage gated Na+ channels causing sodium ions to flow across the sarcolemma into the cell and potassium ions out. More Na+ ions are pumped in causing the depolarization of the interior of the cell. The membrane permeability changes by opening K+ voltage gated pumps allowing K+ ions to diffuse across sarcolemma out of the muscle fiber also known as repolarization.