A sodium-potassium pump turns 2500 times…

At rest the membrane is more permeable to K+ than to Na+, and the pump moves $3\ \text{Na}^+$ out and $2\ \text{K}^+$ in. The outside is positive and the inside is negative. A stimulus makes the membrane freely permeable to Na+, the polarity reverses, and this is the action potential. K+ then leaves and the resting potential returns. For one ion, $E = \dfrac{RT}{zF}\ln\dfrac{[\text{out}]}{[\text{in}]}$, and $RT/F = 26.7\ \text{mV}$ at 310 K.