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.
Ventricular Arrhythmias. Premature ventricular contractions (PVCs) are the most common cause of irregular heart rhythms. The heartbeat is created by an electrical signal that originates in an area of specialized cells in the heartâs upper right chamber, the right atrium. premature ventricular contraction (PVC) is a too-early heartbeat that originates in the ventricles and disrupts the heartâs normal rhythm.