What ion movement changes the membrane potential in a contractile cell from negative to positive during the rapid depolarization phase?

The influx of calcium ions.

Explanation:

The influx of calcium ions at a certain point produces phase of the action potential also called depolarization which results in the ion movement changes the membrane potential from negative to positive inside the cell.

In action potential, calcium ions may control gene transcription, cell excitability, and neurotransmitter release. In chemical synapses, the influx of calcium ions leads to vesicles filled with neurotransmitters move to the surface of the cell, and into the synaptic cleft release their contents.

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