<p> Origin and Conduction of the Heart Beat Answer Sheet</p><p>The sinus venosus of lower vertebrates has become incorporated into the right atrial walls and is known as the pace - maker or (sino-atrial node [SA node]) in mammals. This structure contains a group of nerve cells near the junction of </p><p> right atrium and superior </p><p> vena cava known as the</p><p> pace- maker or SA node with an intrinsic rhythmic rate of 40 to 60 beats per minute. This is the area of heart beat initiation. A wave of nervous excitation in the SA node causes the atria to contract , forcing blood through the atrio-ventricular valves [AV valves]. The time for all the left and right muscles to contract is approximately 0.09 seconds, effectively stimulating all the atrial muscles at the same time. As this wave of nervous excitation approaches the atrio-ventricular junction, it collects to form the atrio- - ventricular node [ AV node ], located in the septum of the heart. From this, fibres pass along the septum forming the atrio- ventricular bundle or bundle of His , which divides into left and right branches. Each branch gives rise to a network of nervous conducting fibres called</p><p>Purkinje fibres which are made up of cells high in glycogen. Nerve impulses from the Purkinje fibres pass down the septum separating the left and right ventricles, then up the sides to the apex. The delay rate from start to finish is approximately 0.06 seconds, effectively stimulating all the ventricular muscle at the same time. Instantaneous excitation of both atria and ventricles is important in the proper functioning of the heart, allowing maximal blood flow. Any errors in this nervous transmission can lead to a 20-30% decrease in the effectiveness of the ventricles.</p>