As a neurophysiology expert, I can explain the depolarization phase of the action potential in detail. During the depolarization phase of the 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 to diffuse into the cell down their concentration gradient. The inward movement of these positively charged ions makes the membrane potential more positive (less negative), which is referred to as depolarization. This phase is crucial for the propagation of the action potential along the neuron's axon and is the primary event that leads to the initiation of an electrical signal that can travel rapidly through the neuron. 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 >>