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.
There are many possibilities. Some of them include: A) Diseases of heart have to be ruled out, e.g. problems with heart muscle or heart valves are a common cause of both PAC's and PVC's. B) Systemic causes (not related to heart) have to be considered also, e.g. anemia, thyroid overactivity. C) Autonomic and neuro-cardiogenic causes can also have same manifestation, e.g. increase in heart rate with standing because of drop in BP.