Final answer:
The contraction of ventricular tissue is triggered by the depolarization of the ventricles, represented by the QRS complex on an electrocardiogram.
Step-by-step explanation:
The contraction of ventricular tissue is normally triggered by depolarization of the ventricles, which is the correct option from the given choices.
Depolarization refers to the electrical change that occurs when cells shift from a resting negative charge to a positive charge during the action potential phase. This electrical event starts with the sinoatrial (SA) node in the right atrium, initiating the impulse that triggers the heart's contractions. The wave of depolarization rapidly spreads across the atria and then down to the ventricles. Once the ventricles depolarize, this leads to their contraction, effectively pumping blood to the lungs and the rest of the body.
On an electrocardiogram (ECG), this ventricular depolarization and subsequent contraction are represented by the QRS complex. Following ventricular contraction, there is a repolarization phase signified by the T wave on an ECG, which allows the ventricles to recover and prepare for the next cardiac cycle.