<P> The electrical conduction system of the heart transmits signals generated usually by the sinoatrial node to cause contraction of the heart muscle . The pacemaking signal generated in the sinoatrial node travels through the right atrium to the atrioventricular node, along the Bundle of His and through bundle branches to cause contraction of the heart muscle . This signal stimulates contraction first of the right and left atrium, and then the right and left ventricles . This process allows blood to be pumped throughout the body . </P> <P> The conduction system consists of specialised heart muscle cells, and is situated within the myocardium . There is a skeleton of fibrous tissue that surrounds the conduction system which can be seen on an ECG . Dysfunction of the conduction system can cause irregular, fast, or slow heart rhythms . </P> <P> Electrical signals arising in the SA node (located in the right atrium) stimulate the atria to contract and travel to the atrioventricular node (AV node), which is located in the interatrial septum . After a delay, the stimulus diverges and is conducted through the left and right bundle of His to the respective Purkinje fibers for each side of the heart, as well as to the endocardium at the apex of the heart, then finally to the ventricular epicardium . </P> <P> On the microscopic level, the wave of depolarization propagates to adjacent cells via gap junctions located on the intercalated disc . The heart is a functional syncytium (not to be confused with a true "syncytium" in which all the cells are fused together, sharing the same plasma membrane as in skeletal muscle). In a functional syncytium, electrical impulses propagate freely between cells in every direction, so that the myocardium functions as a single contractile unit . This property allows rapid, synchronous depolarization of the myocardium . While advantageous under normal circumstances, this property can be detrimental, as it has potential to allow the propagation of incorrect electrical signals . These gap junctions can close to isolate damaged or dying tissue, as in a myocardial infarction (heart attack). </P>

When does electrical activity occur at the av node
find me the text answering this question