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?

The heart has an intrinsic conduction system that causes electrical activity in the heart muscles causing them to contract. The intrinsic conduction system is made up specialized cells, that contain nerve and muscular characteristics.
The muscle cells in the heart are linked together by gap junctions, allowing cardiac action potentials to travel from one muscle cell to another.
Atrioventricular (AV) node. The damage to the AV node causes the electrical signals traveling from the upper chambers to the lower chambers to be impaired causing an AV block.

Explanation: