The rhythmic contraction of the atria and ventricles is due to the intrinsic system of the heart that is made possible by the presence of nodal tissue, a unique type of cardiac muscle. nodal tissue, which has both and nervous characteristics, is located in two regions of the heart. the is located in the upper dorsal wall of the right atrium. the is located in the base of the right atrium very near the septum. the sa node initiates the heartbeat and sends out an excitation impulse every which causes the atria to contract. when impulses reach the av node, a slight delay allows the to finish contraction before the begin to contract. the sa node is called the because it usually keeps the heartbeat regular. if the sa node fails to work properly, the heart still beats due to impulses generated by the av node but the beat is slower, beats per minute.