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 (PVCs) are heartbeats that occur earlier than they should. These early beats briefly interrupt the heart's rhythm. A PVC may feel like a skipped heartbeat or a flutter. PVCs are the most common type of change in heart rhythm. They are common in children and teens as well as in adults.