【正确答案】Definition: Action potential is a rapid, reversible, and conductive change of the membrane potential after the cell is stimulated. [2.5`] Forming mechanisms: (1) Reduction in membrane potential (depolarization) to "threshold" level leads to opening of Na+ channels, allowing Na+ to enter the cell. [1.5`] (2) Interior becomes positive, reversed polarization [1.5`] (3) The Na+ channels then close automatically followed by a period of inactivation. [1.5`] (4) Next K+ channels open, K+ leaves the cell and the interior again becomes negative (repolarization). [1.5`] (5) Na+- K+ ATPase serves as a pump to recover normal ionic distribution in or out the membrane. [1.5`]