Question about the threshold potential
Below is a figure showing the sequence of events during an action potential (AP). While the resting potential is characterized by an equilibrium, during an AP this balance is markedly 'disturbed'. The action potential is characterized by a specific timed series of events, namely the opening of Na+ channels in the initial depolarization phase, followed by an opening of K+ channels in the subsequent repolarization phase. During these phases, the Na+ and K+ currents are anything, but balanced.    


Fig. 1. Series of events during an AP. source: Brigham Young University-Idaho