The action potential

a. Changes in membrane conductance at a point on an axon as a neural impulse passes.

The membrane first becomes permeable to sodium ions, allowing a large inward rush of Na+. A decrease in the Na+ permeability results and is in turn followed by an outward flow of K+

b. Changes in membrane potential accompanying the permeability changes shown in (a).

As Na+ rushes in, the potential increases and becomes positive. As K+ influx increases, the potential decreases, undershooting the resting potential, before returning to the resting potential.