The resting nerve fibre has more ions outside the membrane while it has more ions inside the membrane. Due to different concentration of ions on the two sides of the membrane, sodium ions tend to passively diffuse into the nerve fibre and potassium tend to diffuse out of nerve fibre down their concentration gradients. The membrane of resting nerve fibre is, however, more permeable to potassium that to sodium. Because of this selective permeability of the membrane, potassium leaves the nerve fibre faster than the sodium enters it. Due to this faster permeability of potassium, outside of the membrane becomes more positively charged while inside becomes more negatively charged. This condition is said to be polarisation of nerve fibre.
Any disturbance caused to integrity of membrane by a stimulus results in leakage of sodium ions down the concentration gradient and decreases the potential difference. As trans membrane potential decreases the membrane becomes more permeable to ions. Due to increased influx of , the membrane potential changes from negative towards positive and the membrane is said to be depolarised.