Question about the threshold potential

Question

I am a student of Physiology, and I have ended up a bit confused after what I've read today regarding events during a threshold potential. So, while cells are in their resting membrane potential, the inward current of sodium is equal and opposite to the outward current of potassium. The same is true about the threshold potential.

When the threshold potential is reached, the inward sodium current is exactly equal and opposite to the outward potassium current. If this is true, why is there a change in the voltage of the membrane, let's say from -70 mV to -55 mV? I mean, both of the currents are equal and opposite and therefore there shouldn't be any change in the resting potential of the membrane.

Answer 1

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}