Sulfur normally exists as an eight-atom molecule, S8, while phosphorus exists as a four-atom molecule, P4

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Z disk 

Thin actin filaments are assembled with their (+) ends embedded in a densely staining structure

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Each sarcomere contains two major types of filaments: 

Thick filaments (myosin II)

Thin filaments (actin and associated proteins)

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Sarcomere

Within each skeletal muscle cell are many myofibrils consisting of a repeating array of a specialized structure

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When the Ca2+ levels drop:

Myosin LC is dephosphorylated by MLC phosphatase, causing muscle relaxation.

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Phosphorylation of MLC 

unfolds the myosin II, active and can assemble into bipolar filaments for contraction

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The CaM*-Ca2+ complex binds and activates MLC kinase, phosphorylates the myosin LC.