The resting membrane potential
The cell membrane of nerve cell is said to be polarized when negative potential exists more inside the cell with respect to outside. The potential difference across the cell membrane at rest is called resting membrane potential and it is approx. -65 mV. The resting membrane potential is maintained by active transport of ions against their electrochemical gradient by sodium potassium pump and also by passive diffusion of ions. For active transport, there are carrier proteins located in the cell surface membrane. They are driven by energy supplied by ATP and coupled by removal of three sodium ions from the axon with the help of uptake of two potassium ions. The passive diffusion of ions opposes the active movement of ions. The rate of diffusion depends on the permeability of the axon membrane for the ions. Potassium ions have more permeability than that of sodium ions. Therefore loss of potassium ions is more than the gain of sodium ions. This leads to the net loss of potassium ions from the axon and generation of negative charge within the membrane.

Action Potential or exciting stage
The event of depolarisation initiates a nerve impulse or spike. This nerve impulse is also known as Active potential, generated by change in sodium ion channel. These channels are known as voltage gated channel. At resting stage these channels remain close due to binding of calcium ions. An action potential is generated by a sudden opening of the sodium gates. Opening of gate increases the permeability of membrane for sodium which then enters inside by diffusion. This increase in positive ions inside the axon drops the negative potential inside axon. A change of -10 mV in potential difference from resting membrane potential is known as spike potential, sufficient to trigger a rapid influx of sodium ions; which leads the generation of action potential.
First, the negative resting potential is cancelled out, at this point the membrane is completely depolarised then the potential difference is developed across the membrane. The potential difference at 30 mV is corresponds to the maximum concentration of sodium inside the axon.