Steps in excitationcontraction coupling

1. The action potential spreads from the cell membrane into the T tubules.

2. During the plateau of the action potential, Ca2+ conductance is increased and Ca2+ enters the cell from the extracellular fluid (inward Ca2+ current).

3. This Ca2+ entry triggers the release of even more Ca2+ from the SR (Ca2+-induced Ca2+ release).

- The amount of Ca2+ released from the SR depends on the amount of Ca2+ previously stored and on the size of the inward current during the plateau of the action potential.

4. As a result of this Ca2+ release, intracellular [Ca2+] increases.

5. Ca2+ binds to troponin C, and tropomyosin is moved out of the way, removing the inhibition of actin and myosin binding.

6. Actin and myosin bind, the thick and thin filaments slide past each other, and the myocardial cell contracts. The magnitude of the tension that develops is proportional to the intracellular [Ca2+].

7. Relaxation occurs when Ca2+ is reaccumulated by the SR by an active Ca2+-ATPase pump.

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