Action Potential is

All-or-Nothing Principle: Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane if conditions are right, but it does not travel at all if conditions are not right. This principle is called the all-or-nothing principle, and it applies to all normal excitable tissues.The threshold for Initiation of the Action Potential: An action potential will not occur until the initial rise in membrane potential is great enough to create the positive feedback. This occurs when the number of sodium ions entering the fiber becomes greater than the number of potassium ions leaving the fiber. A sudden rise in membrane potential of 15 to 30 millivolts is usually required.Ref: Guyton; 13th edition; Chapter 5; Membrane Potentials and Action Potentials