At equilibrium, or far from it?
Often when we think of membrane potential, it is in the context of your nervous system. So starting now, we’re going to leave cells in your femur, fingertip, and islet of langerhans behind, and focus on neurons.
OK, take a deep breath…
Most materials that you encounter in daily life are either conductors (like copper wire) or insulators (like plastic wrapped around the wire). Cell membranes are different, in that they can rapidly change, depending on their membrane potential — and nowhere more so than in your nervous system.
The reason these changes can happen rapidly is that, unlike K+, not all of the ions are kept in equilibrium. Exhibit A, the sodium ion:
When the cell is resting, sodium ions are kept out-of-equilibrium and out of the cell by “gated” channels in the membrane. These gated sodium channels block sodium from entering the cell. Otherwise, sodium ions would flood in, since they would be moving down their concentration gradient AND, when the cell is at its resting membrane potential, down a voltage gradient. Both gradients want to push Na+ INTO your neurons, and both are blocked by the gated channel. So, sodium is FAR FROM EQUILIBRIUM in the resting cell.