What causes depolarization in action potential?

As a domain expert in neuroscience, I can explain the process of depolarization during an action potential. The action potential is a rapid and temporary change in the electrical potential across the cell membrane of neurons and muscle cells. Depolarization is a crucial part of this process. Depolarization occurs when the membrane potential, which is normally polarized (negative inside, positive outside), becomes less polarized or even positive inside. This is caused by the opening of voltage-gated sodium channels in response to a stimulus that depolarizes the cell to a certain threshold level. When the membrane potential reaches this threshold, voltage-gated sodium channels open, allowing an influx of sodium ions (Na+) into the cell. The inward flow of sodium ions increases the concentration of positively charged cations inside the cell, which causes depolarization. This shift in charge makes the cell's membrane potential more positive, or higher than the cell's resting potential. After depolarization, the cell undergoes a process called repolarization, where the membrane potential returns to its resting state, and the sodium channels close while potassium channels open, allowing potassium ions to flow out of the cell.

As the membrane potential is increased, sodium ion channels open, allowing the entry of sodium ions into the cell. ... The inward flow of sodium ions increases the concentration of positively charged cations in the cell and causes depolarization, where the potential of the cell is higher than the cell's resting potential.