Excitation–contraction coupling Outside the fibreCa²⁺Ca²⁺Ca²⁺Ca²⁺Ca²⁺Ca²⁺Ca²⁺Ca²⁺Ca²⁺Ca²⁺Ca²⁺Ca²⁺Ca²⁺Ca²⁺Ca²⁺Ca²⁺ATPATPCa²⁺Ca²⁺Ca²⁺Ca²⁺SarcolemmaT tubuleTerminal cisternae (SR)Sarcoplasmic reticulumMyofibrilCa²⁺Ca²⁺Ca²⁺ADP·PADP·PADP·PADP·PADP·PThin filamentThick filament (myosin heads)Tropomyosin covers the active sitesActive sites exposed: cross-bridgesCa²⁺Ca²⁺TroponinTropomyosinActin, active site1Action potentialinto the T tubules2Calcium releasedfrom the SR3Calcium bindstroponin4Active sites exposed;cross-bridges form5Relaxation: calciumpumped back
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