Final answer:
During the plateau phase of the action potential in myocardial cells, calcium-induced calcium release is initiated, and the prolonged plateau phase helps to prevent tetany by ensuring there is time for the heart muscle to fully contract between beats. Therefore, the answer to the question would be 4) A and C.
Step-by-step explanation:
The important effects of events during the plateau phase of the action potential of myocardial cells include initiating calcium-induced calcium release (CICR) and preventing tetany. The influx of calcium ions through slow calcium channels not only supports the contraction by binding to troponin, leading to cross-bridge formation with actin, but also facilitates the extended refractory period which ensures the heart muscle fully contracts before it can respond to a new electrical impulse. This refractory period is crucial in preventing the tetany of cardiac muscle, which could be fatal.
Options enhancing the efficiency of oxygen use by the cells is not directly related to the plateau phase of the action potential.