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.
Premature Atrial Contractions (PACs) are amongst the most common forms of arrhythmias. It is due to the premature discharge of an electrical impulse in the atrium, causing a premature contraction. Therefore, it is named premature atrial contraction, or PAC. A PAC is premature, because the it occurs earlier than the next regular beat should have occurred. Most often, patients with PACs complain of palpitations.