How is action potential generated?

Action potential may be generated either to threshold or supra-threshold stimuli on a muscle cell. It involves three main steps – depolarization, overshoot, and repolarization. 

On initiation of an action potential, an electrical current flows through the gap junction, which depolarizes the adjacent cells. If the membrane potential is more positive than the threshold after depolarization, self-propagating action potentials are generated in the adjacent cells too. This is responsible for initiating muscle contraction. Muscle contraction would not be possible in the absence of action potential.