 Dear students, in this topic we shall discuss the transmission or spread of excitation over the heart. Dear students, the cardiac cells are coupled electrically through gap junctions. These gap junctions are the regions of low resistance between the cells. These gap junctions allow the current to flow from one cell to the next. The pacemaker's electrical activity spreads over the entire heart because of these gap junctions. Dear students, we shall have an overview of the path of transmission of excitation over the heart. The excitation that is generated in the sinoatrial node is first transmitted to the atrial myocardium. From atrial myocardium, through the junctional fibers, it is transmitted to the atrioventricular node. From here, the excitation is transmitted to the atrioventricular bundles. From the bundle or bundle of hairs, the transmission occurs to the bundle branches, right and left bundle branches. These bundle branches transmit excitation to the purkinje fibers. These purkinje fibers then transmit excitation to the ventricular musculature. After this brief overview, we shall now discuss the transmission of cardiac impulses through these parts in detail. First of all, we shall discuss the transmission of cardiac impulses through atria. Dear students, the sinoatrial nodal fibers connect directly with the atrial muscle fibers. So the wave of excitation spreads over both the atria in a concentric way. The velocity of conduction in most of the atrial fibers is about 0.3 m per second. However, conduction is rapid in smaller bands of atrial fibers that is about 1 m per second. Dear students, the spread of excitation from atrial musculature to atrioventricular node occurs through small junctional fibers. These junctional fibers form inter nodal pathways. These inter nodal pathways include an anterior interatrial band and three small bands which curve through the anterior, middle and posterior atrial walls. Dear students, the velocity of wave of excitation in junctional fibers is very low, that is about 0.05 m per second. This slowness causes a delay in the conduction of impulse to the ventricles. This delay is very significant because it allows atrial contractions to precede ventricular contractions. Before ventricular contractions, the atrial contractions become complete and it allows the time for blood to move from atria into the ventricles. Now we shall discuss the conduction of impulse or excitation through the bundle of his and the bundle branches to the purkinje fibers and to the myocardial cells of ventricles. The bundle of his, its branches and purkinje fibers deliver the wave of excitation to all regions of ventricular myocardium. The conduction is rapid. The conduction is rapid through the bundle of his and purkinje fibers that is about 4 to 5 meter per seconds.