As a field expert in neuroscience, I can explain the process of depolarization during an action potential. During the depolarization phase of an action potential, the first change to occur is the opening of voltage-gated sodium (Na+) channels in response to a threshold stimulus. This opening allows Na+ ions, which are more concentrated outside the cell, to rush into the cell down their concentration gradient. The inward movement of these positively charged ions causes the membrane potential to become more positive (less negative), as it shifts from the resting membrane potential (typically around -70 millivolts) towards zero. This rapid change in voltage is what characterizes the depolarization phase of the action potential. read more >>

What is the first change to occur in response to a threshold stimulus? During the depolarization phase of the action potential, open Na+ channels allow Na+ ions to diffuse into the cell. This inward movement of positive charge makes the membrane potential more positive (less negative).read more >>