The Atrio Ventricular (AV) node is the slowest conducting structure with the longest refractory period in the hea. Cardiac tissue conduction velocity (fastest --> slowest): Purkinje system --> Atrial muscle --> Ventricular muscle --> AV node The slowest conduction velocity is in the AV node Allows time to the atria to contract Allows complete ventricular filling The fastest Conduction velocity is in the Purkinje fibers Allows the ventricles to contract at the same time simultaneously Cardiac tissue Conduction: The heabeat originates in a specialized cardiac conduction system and spreads this system to all pas of the myocardium. The structures that make up the conduction system are the sinoatrial node (SA node), the internodal atrial pathways, the atrioventricular node (AV node), the bundle of His and its branches, and the Purkinje system. The factors related to the speed of conduction are the Magnitude and rate of rise of the action potential Diameter of the fibres (AV node radius of the fibres 7 um and purkinje fibres is close to 50 um). The conduction also depends on the tight junctions: The AV node conduction is slow due to diminished gap junctions between cells, taking ~ 0.09 sec and 0.04 sec, respectively; there is an abundance of gap junctions in intercalated discs between Purkinje fiber cells, therefore traversing entire expanse of ventricle only takes 0.03 sec after depolarization front exits the septum. AV nodal delay: There is delay of 0.1 s before the excitation spreads to ventricles. This delay is shoened by stimulation of the sympathetic nerves to the hea and lengthened by stimulation of the vagi. Tissue Conduction speed (m/s) Sino atrial node 0.05 Atrial pathways 1 Atrio ventricular node 0.05 Bundle of His 1 Purkinje system 4 Ventricular muscle 1 Ref: Cardiovascular physiology, By William R Milnor, Page 88.