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.
Premature ventricular contractions (PVCs, PVC) are premature heartbeats originating from the ventricles of the heart. Premature ventricular contractions are premature because they occur before the regular heartbeat. The heart has four chambers. The upper two chambers are the atria, and the lower two chambers are the ventricles.
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.