The whole living cell and in particular the nerve cells have electrical potential difference (DDP) between the inner and outer faces of your cell membrane.
This DDP is generated by the difference in ion concentration inside and outside the cell. As cytoplasm contains proportionally fewer positive ions than external fluid, the inner surface of the membrane is negative relative to the outer one.
Rest potential is the difference in electrical potential that the inner and outer faces in the membrane of a neuron that is not transmitting nerve impulses. The resting potential value is of the order of -70mV (milliVolts). The negative sign indicates that the interior of the cell is negative relative to the exterior.
The existence of the resting potential is mainly due to the difference in ion concentration of sodium (Na+) it's from potassium (K+) inside and outside the cell. This difference is maintained by a mechanism of active ion pumping through cell membranes, in which sodium is forced out of the cell and potassium in.
Despite the name maintaining the resting potential demands energy expenditure by the cell, since ion pumping is an active transport process that consumes ATP.
The cell membrane has numerous protein structures that function as "gates" for the passage of sodium and potassium ions. These doors are normally closed in a resting neuron, opening when it is stimulated.
When an appropriate stimulus reaches the neuron, the sodium passageways open immediately in the area of the membrane that has been stimulated: sodium ion, being in greater concentration in the external cellular environment, quickly penetrates through these membrane openings. The sudden influx of positive loads causes the membrane potential, which was about -70mV (resting potential), to increase to approximately + 35mV. This change in potential is called depolarization.
This abrupt transition of electrical potential that occurs during depolarization, and whose amplitude is on the order of 105 mV (from -70mV to +35 MV), is the action potential.
In the area affected by the stimulus, the membrane remains depolarized, only 1.5 ms (thousandth of a second). Soon the potassium doors open, allowing this ion, which is in greater concentration inside the cell, to escape. Thus, the membrane repolarization occurs, which returns to the resting condition.