Question

Which specific ion movement across the cardiac cell membrane is primarily responsible for the rapid depolarization phase of the action potential?

Accepted Answer

The rapid depolarization phase of the cardiac action potential, specifically in non-pacemaker cells like atrial and ventricular myocytes, is primarily caused by the fast influx of sodium ions (Na+) through voltage-gated sodium channels. An action potential is a sudden change in the electrical charge across a cell membrane. Depolarization refers to the process where the inside of the cell becomes less negative, moving toward a positive charge. Voltage-gated sodium channels are specialized proteins in the cell membrane that open automatically when the electrical charge inside the cell reaches a specific threshold. Once these channels open, sodium ions rush into the cell because they are highly concentrated outside the cell and are attracted to the negative charge inside the cell. This massive movement of positively charged sodium ions into the cell rapidly shifts the membrane potential from a resting state of approximately -90 millivolts toward a more positive value, creating the steep upward spike seen on an action potential graph. This rapid sodium entry is known as Phase 0 of the cardiac action potential.

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Which specific ion movement across the cardiac cell membrane is primarily responsible for the rapid depolarization phase of the action potential?