What is meant by the generation of action potential???

Action potential is the electrical potential difference that develop across the plasma membrane at a site where stimulus is applied. This action potential is also termed as a nerve impulse. When a stimulus is applied at a site , then that polarized site become freely permeable to Na+. The polarity of that site is reversed i.e. the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged and hence depolarized. The next immediate site has a positive charge on the outer surface and a negative charge on its inner surface. As a result, a current flows on the inner surface from first site to next site.In the same way polarity of next site is reversed and action potential is generated at that site.This sequence is repeated along the length of the axon.