Complete the following statements about the internal control of heartbeat by selecting the appropriate term from each dropdown list. The rhythmic contraction of the atria and ventricles of the heart is due to the presence of nodal tissue, a unique type of cardiac muscle. Nodal tissue, which has both muscular and nervous characteristics, is located in two regions of the heart. The SA node is located in the upper dorsal wall of the right atrium. The AV node 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 0.85 seconds, which causes the atria to contract. When impulses reach the AV node, a slight delay allows the ventricles to finish contraction before the atria begin to contract. The SA node is called the pacemaker because it usually keeps the heartbeat regular.