What is the cause of a membrane potential?

The cause of a membrane potential is the distribution of ions across a cell membrane, which creates a difference in electric charge inside and outside the cell. This difference is mainly due to the permeability of the membrane to certain ions (like sodium, potassium, chloride) and the action of ion channels and pumps, such as the sodium-potassium pump, which actively transports ions to maintain concentration gradients. This results in a resting membrane potential, typically around -70 mV in neurons.