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.
slide 1 of 9. What are PVCs? Premature ventricular complexes are irregular heartbeats caused by the ventricles (the lower heart chambers) contracting earlier than normal. When the heart beats normally, the sinoatrial node in the right atrium (upper chamber) sends an electrical impulse across the atria to the ventricles, causing them to contract.