Final answer:
The rapid depolarization phase of a contractile cell is caused by the influx of sodium ions which shift the membrane potential from negative to positive.
Step-by-step explanation:
The ion movement that changes the membrane potential in a contractile cell from negative to positive during the rapid depolarization phase is the influx of sodium ions (Na+). During depolarization, sodium channels in the cell membrane open, allowing Na+ to rush into the cell. This inrush of Na+ neutralizes the negative charge inside the membrane, causing the membrane potential to shift towards a positive value, typically peaking around +30 to +40 mV. This process leads to the depolarization of the cell and initiates the action potential that is necessary for muscle contraction.
After this rapid depolarization, the cell soon begins repolarization as potassium ions (K+) flow out of the cell, and the sodium channels become impermeable again, returning the cell to its resting potential.