As told earlier, the fluid which lies outside the cell is called extracellular fluid and the fluid which lies inside the cell membrane is called intracellular fluid. The cell wall is a semi-permeable membrane. So it will allow only some ions to pass and restrict others. The extracellular fluid contains large quantities of sodium ions (Na+) and small amount of potassium (K+), whereas the intracellular fluid has more concentration of potassium when compared to sodium ions.
As a result of phenomenon known as sodium-potassium pump, more sodium ions are transported to the extracellular fluid (ECF) and comparatively less potassium ions are transported in to the intracellular fluid (ICT). So potassium is pumped into to the cell and sodium is pumped out of the cell. As the rate of sodium pumping is more when compared to the potassium pumping, it results in a difference of ion concentration creating an electrical potential. The cell is said to be ‘polarized’. The inside of cell is less positive than outside and cell is said to be in negative potential state called “resting potential”. This potential is about -70mV to -90mV.
When stimulated at resting potential, sodium pump suddenly stops and sodium ions begin to enter the cells slowly. It is because upon stimulation, the nature of cell wall changes and it forces the sodium ions pass into the cell and potassium ions come out of the cell. This causes the inside of the cell to be more positive than outside. This is called action potential and it is about +20mV to +40 mV.
The cell showing a resting potential is called polarized cell and when it is generating an action potential is called a depolarized cell. There is always a refractory period during which the cell becomes repolarized. In this period the cell is resistant to another depolarization.