Final answer:
The cardiac cycle involves the flow of excitation through the heart's electrical system, starting from the SA node, and resulting in the contraction and relaxation phases known as systole and diastole. Myocardial contractile cells and myocardial conducting cells work together to ensure efficient blood pumping, with their coordinated actions reflected on an ECG.
Step-by-step explanation:
Understanding the Cardiac Cycle
The heart functions as an electro-mechanical organ with two primary types of muscle cells: myocardial contractile cells and myocardial conducting cells. The conduction pathway begins at the sinoatrial (SA) node, initiating an action potential that spreads through internodal pathways to the atrioventricular (AV) node, the AV bundle of His, bundle branches, and finally the Purkinje fibers. This electrical activity leads to the coordinated contraction of the heart muscle, pumping blood effectively throughout the body. The action potential in conducting cells features a prepotential phase with a slow influx of Na+ followed by a rapid influx of Ca²+ and outflux of K+. The contractile cells respond with a prolonged action potential to allow a full, effective contraction, represented on an electrocardiogram (ECG) by P waves, QRS complexes, and T waves corresponding to atrial depolarization, ventricular depolarization, and ventricular repolarization, respectively.
During the cardiac cycle, relaxation and contraction phases alternate, marking systole and diastole, for both atria and ventricles. The cycle starts with all chambers relaxed, and blood flows from the atria into the ventricles. Atrial contraction follows, pushing more blood into the ventricles, leading to ventricular contraction. This raises ventricular pressure, causing the first heart sound upon the closing of AV valves and the opening of semilunar valves as blood moves to the major arteries. Finally, ventricles relax, pressure drops, leading to the semilunar valves closing (second heart sound), which completes one full cardiac cycle.