The adrenal medulla develops from neural tissue and secretes two hormones, epinephrine and norepinephrine. These two hormones are secreted in response to stimulation by sympathetic nerve, particularly during stressful situations. A lack of hormones from the adrenal medulla produces no significant effects.
While the sinoatrial node typically regulates the heartbeat during normal sinus rhythm, PACs occur when another region of the atria depolarizes before the sinoatrial node and thus triggers a premature heartbeat.
Clinical Significance of Premature Atrial Complex (PAC) 1  PACs are a normal electrophysiological phenomenon not usually requiring investigation or treatment. 2  Frequent PACs may cause palpitations and a sense of the heart âskipping a beatâ. In patients with underlying predispositions (e.g. left atrial enlargement, ischaemic heart disease, WPW), a PAC may be the trigger for the onset of a re-entrant tachydysrhythmia â e.g. AF, flutter, AVNRT, AVRT.