ATRIOVENTRICULAR NODE (AV NODE) - part of the electrical control system of the heart that coordinates the top of the heart. It electrically connects atrial and ventricular chambers. AV node is an area of specialized tissue between atria & ventricles of the heart, specifically in the posteroinferior region of the interatrial septum near the opening of the coronary sinus, which conducts the normal electrical impulse from the atria to the ventricles | Wikipedia
Electrical signals fire from multiple locations in the atria, causing abnormal quivering of the atria (1). The atrioventricular node — your heart's natural pacemaker — is unable to prevent all of these chaotic signals from entering the ventricles (2). Your ventricles respond to these extra, chaotic signals by beating faster than normal (3).
The Purkinje fibers, also known as subendocardial braches, are cardiomyocytes that are part of the conduction system of the heart. They lie beneath the endocardium and send impulses from the atrioventricular node to the ventricles causing them to contract. This ensures that the heart maintains its consistent rhythm. The Purkinje fibers were named after Jan Evangelista Purkyne, a Czechoslovakian anatomist that discovered these fibers in 1839.
Red blood cells tightly packed in a capillary from the heart muscle. A small Purkinje fiber (blue) is present on the surface of the capillary. Purkinje fibers are modified cardiac muscle fibers that originate from the atrioventricular node and spread into the two ventricles. They transmit the electrical impulse from the atrioventricular node to the ventricles enabling an almost simultaneous contraction.