Final answer:
The firing of the sinus node, controlled by the heart's internal pacemaker, triggers the start of the cardiac cycle. Pacemaker cells in the SA node send out electrical signals that cause the atria and ventricles to contract in a coordinated manner.
Step-by-step explanation:
The firing of the sinus node, which triggers the start of the cardiac cycle, is controlled by the heart's internal pacemaker. The pacemaker cells in the sinoatrial (SA) node send out electrical signals that cause the atria to contract. The signals then pass through the atrioventricular (AV) node and the bundle of His before reaching the ventricles, causing them to contract as well. This coordinated electrical activity leads to the rhythmic beating of the heart.