Muscle fibers of the heart have to be harmoniously contracted, so that the heart can work effectively as a pump. Electric pulses from the group of cells in the right atrium – that is, in a natural “pacemaker” – are controlling the cardiac (heart) contractionts; impulses travel to the muscle fibers in all four chambers. Sometimes, the signals transmitted to the heart from other parts of the body (through external nerves) often have harmful effects on these electric impulses. These ”harmful” signals can change the impulses that travel through the “paths”, and can damage the path itself by disturbing the heart rate (pulse). If this, or if something else, causes the disruption, the proper rhythmic heart rate will also be disrupted and disorders may occur.