The autonomic nervous system is divided into three parts: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system. The autonomic nervous system controls smooth muscle of the viscera (internal organs) and glands.
P wave is the atria “firing”/depolarizing, the QRS complex is the ventricles depolarizing (and the atria are depolarizing while this happens), and the T wave is the ventricles depolarizing. Depolarizing is when it contracts, and repolarizing is when it relaxes and refills.