During which phase of cardiac muscle contraction can an extrasystole be induced?

An extrasystole, or a premature contraction, can be induced during the relaxation phase of cardiac muscle contraction. This is primarily because the heart is momentarily in a refractory state following a contraction, allowing it to respond to an ectopic impulse if one occurs. During relaxation, the cardiac muscle is repolarizing and preparing for the next cycle. If an additional stimulus—such as an ectopic beat—occurs during this phase, it can lead to an extrasystole, since the myocardium may still be ready to respond.

In contrast, during other phases such as contraction or depolarization, the cardiac muscle is either actively contracting or already undergoing a rapid change in membrane potentials, making it less susceptible to responding to additional stimuli. The plateau phase, which occurs in the action potential of cardiac muscle, is also not conducive to inducing an extrasystole as the cells are in a hyperpolarized state, where they are less likely to respond to stimuli. Thus, the relaxed state of the cardiac muscle is crucial for the induction of an extrasystole.