Why is the inside of a resting neuron about 70 mV negative? · Grey Matter

The pump keeps potassium about thirty times more concentrated inside the cell than outside, and at rest the membrane is permeable mostly to potassium through leak channels.

Why is the inside of a resting neuron about 70 mV negative?

The pump keeps potassium about thirty times more concentrated inside the cell than outside, and at rest the membrane is permeable mostly to potassium through leak channels. Potassium drifts out down its gradient, carrying positive charge with it, until the negative charge left behind pulls back as hard as the gradient pushes; for 140 mM inside and 5 mM outside that balance is near −84 mV. A small leak of sodium inward pulls the real value up to around −70 mV. The pump itself adds only a few millivolts directly (it moves three sodium out for two potassium in); its main contribution is keeping the gradients that the leak then turns into a voltage.