The SA node generates the electrical impulse, which spreads through the walls of the atria. The atria contract in unison because the cardiac muscle cells within the atria are electrically coupled. The impulse converges at the atrioventricular (AV) node, another region of specialized tissue. The AV node is located in the wall between the right atrium and the right ventricle. The AV node delays the impulse for about 0.1 second before specialized muscle fibers called bundle branches and Purkinje fibers conduct the signals to the apex of the heart. The current then spreads throughout the ventricle walls. The delay at the AV node ensures that the atria are completely empty before the ventricles contract increasing the efficiency of the system (Campbell 2002).
The impulses that travel through the cardiac muscle during the heart cycle produce electrical currents that are conducted through body fluids to the skin. These currents can be detected by electrodes and recorded as an electrocardiogram (ECG or EKG). The EKG track shows a period of hyperpolarization that prevents the heart from immediately contracting again.