Final answer:
The SA node initiates the electrical impulse in the heart, establishing the sinus rhythm and making it the pacemaker of the heart.
Step-by-step explanation:
The SA node, or sinoatrial node, is a specialized cluster of myocardial conducting cells located in the right atrium. It is responsible for initiating the electrical impulse in the heart, establishing the normal cardiac rhythm known as the sinus rhythm. This makes it the pacemaker of the heart. The impulse generated by the SA node spreads to the atria, causing them to contract, and is then delayed at the atrioventricular (AV) node before being passed on to the heart apex and ultimately causing the ventricles to contract.