Final answer:
The depolarization phase of an action potential is caused by the opening of voltage-gated Na+ channels, which allows sodium ions to enter the cell and change the membrane potential.
Step-by-step explanation:
The depolarization phase of an action potential results from the opening of voltage-gated Na+ channels. During a stimulus to a neuron, these channels allow Na+ ions to rush into the cell, which changes the cell's membrane potential from a resting state of around -70 mV to a more positive value, reaching the threshold of excitation at approximately -55 mV. This rapid influx of sodium ions causes the inside of the cell to become more positive, leading to depolarization and the propagation of an action potential along the neuron. Once the threshold is reached, a large number of Na+ channels open, leading to a spike in membrane potential. Eventually, these Na+ channels close and voltage-gated K+ channels open, allowing K+ to leave the cell and initiating the repolarization phase.