The plasma membrane of the neuron when at rest maintains an electric potential difference between its external and internal surfaces. This voltage is called resting potential. The resting potential about –70 mV indicates that the interior is more negative than the exterior (negative polarization). This condition is maintained by transport of sodium and potassium ions across the plasma membrane.
The membrane is permeable to potassium ions but not to sodium ions. At rest the positive potassium ions exit the cell in favor of the concentration gradient since within the cell the potassium concentration is higher than in the extracellular space. The positive sodium ions cannot however go into the cell. As positive potassium ions exit the cell with not enough compensation of positive ions entering the cell, the intracellular space becomes more negative and the cell stays polarized.
