Final answer:
The interior of the cell becomes less negative during depolarization, which is when sodium ions enter the cell. This process transiently moves the membrane potential toward zero from its resting state of approximately -70 mV.
Step-by-step explanation:
The interior of the cell becomes less negative due to an influx of sodium ions during depolarization. This occurs when sodium ion channels open and sodium ions (Na+) rush into the cell, changing the membrane potential toward zero. The resting potential is at approximately -70 mV, indicating a negative charge inside the cell relative to the outside. When sodium enters, it reduces this negative charge, leading to depolarization.
After depolarization, the process of repolarization occurs to return the cell to its resting state. This involves closing sodium ion channels and opening potassium (K+) channels, allowing potassium to exit the cell. Following this, the refractory period happens, during which the cell cannot generate another action potential while it resets its ion concentrations via the sodium-potassium ATPase pump.