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Excitation-contraction coupling in skeletal muscle.

 The top panel shows an action potential in the transverse tubule that causes a conformational change in the voltage-sensing dihydropyridine (DHP) receptors, opening the Ca++ release channels in the terminal cisternae of the sarcoplasmic reticulum and permitting Ca++ to rapidly diffuse into the sarcoplasm and initiate muscle contraction. During repolarization (bottom panel), the conformational change in the DHP receptor closes the Ca++ release channels and Ca++ is transported from the sarcoplasm into the sarcoplasmic reticulum by an adenosine triphosphate–dependent calcium pump.