Final answer:
The electrical events in the heart, involving conductive cells and ion movement, precede and trigger the mechanical events, which involve contractile cell contractions that pump blood. This can be visualized on an ECG, with blocks potentially disrupting this process.
Step-by-step explanation:
The heart's function is controlled by both electrical and mechanical events that are critical for the coordination of effective blood circulation. Conductive cells initiate and propagate electrical impulses through distinct pathways within the heart, beginning with the sinoatrial (SA) node and moving through the atrioventricular (AV) node, passing through the bundle of His, and traveling along the bundle branches until reaching the Purkinje fibers. This sequence establishes the heart rate and initiates contraction of the heart muscle. The contractile cells receive these electrical signals and respond by contracting, thereby pumping blood through the heart and to the rest of the body.
The cardiac cycle starts with an electrical event -- a wave of depolarization -- that occurs first and is attributed to the ion movement across the membranes of the conductive cells, characterized by an influx of Na+ followed by Ca2+ and an efflux of K+. This electrical impulse is followed by a mechanical event, the contraction of the heart muscle, to pump blood. The action potentials can be seen on an electrocardiogram (ECG), where the P wave indicates atrial depolarization, the QRS complex denotes ventricular depolarization, and the T wave means ventricular repolarization.
Any disruption along the conduction pathways, known as blocks, can interrupt the cardiac cycle and impair the heart's ability to pump effectively. The key components observed in an ECG correspond to specific events in the cardiac cycle, and analyzing these components can provide insights into the functioning and rhythm of the heart. Hence, the electrical events always precede the mechanical events, and together they define the cardiac cycle.