One of the first diagnostic tools used at the hospital was an electrocardiogram (EKG or ECG), which reflects the electrical activity of the cardiac muscle. We know that the atria contract first (the P wave) and then, after a brief delay, the ventricles contract (the QRS complex). Given that the heart does not have any nerves to stimulate the cardiac muscle cells, how is the timing of contraction coordinated? How do action potentials get from muscle cell to muscle cell? If the EKG shows a long delay between the P wave and the QRS complex, which type of cardiac tissue might have been damaged?