Role: The Trigger (Entry)
Mechanism: Voltage-gated channels on the Sarcolemma that open upon depolarization to let a small amount of calcium IN.
Role: The Amplifier (Release)
Mechanism: Calcium-gated channels on the Sarcoplasmic Reticulum (SR) that release a massive flood of stored calcium.
Role: The Effector (Contraction)
Mechanism: Located on Actin filaments. Binds calcium to move tropomyosin, allowing myosin to bind actin.
Role: The Restorer (Relaxation)
Mechanism: An antiporter on the Sarcolemma that uses the sodium gradient to push calcium OUT of the cell.
Role: The Modulator (Signaling)
Mechanism: A cytosolic calcium sensor that activates kinases (like CaMKII) to regulate heart rate/contractility.