Final answer:
The sinoatrial (SA) node initiates the heart's electrical impulse, functioning as the natural pacemaker. It controls the rhythm of heart contractions, which is reflected in an electrocardiogram (ECG). The impulse is then delayed at the AV node, allowing the atria to pump blood into the ventricles effectively.
Step-by-step explanation:
The heart's electrical impulse is normally initiated by the sinoatrial (SA) node, which is often referred to as the pacemaker of the heart. It has the highest inherent rate of depolarization and initiates the sinus rhythm or normal electrical pattern of heart contractions. The pattern of prepotential or spontaneous depolarization, which leads to the rapid depolarization and repolarization, starts in the SA node. As the impulse spreads throughout the heart, it moves next to the atria causing them to contract, then it pauses at the atrioventricular (AV) node before continuing to the ventricles. The AV node's delay ensures that the atria can properly contract and pump blood into the ventricles before the ventricles themselves contract. Each component of the cardiac conduction system can generate its own impulse; however, the rates are progressively slower as you move further away from the SA node.
The normal sinus rhythm of the heart can be influenced by the autonomic nervous system through the sympathetic and parasympathetic nerves. Yet, it is the SA node that naturally and autonomously generates the resting heartbeat. An electrocardiogram (ECG) can be used to observe the electrical impulses of the cardiac muscle, which reflect the function of the SA node and subsequent conduction pathway.