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 Ventricular Contractions. Description. Premature ventricular contractions (PVCs) are beats which are initiated in the ventricles or lower chambers of the heart, prematurely. As opposed to PACs, when the SA node (the natural pacemaker of the heart) gets interrupted, PVCs do not interrupt the SA node.