What occurs when multiple stimuli are delivered to the heart at a rate of 20 stimuli per second?

When multiple stimuli are delivered to the heart at a rate of 20 stimuli per second, it is key to understand how cardiac muscle behaves compared to skeletal muscle. The heart's muscle cells, known as cardiomyocytes, possess unique properties, including a long refractory period.

In cardiac muscle tissue, the mechanism of excitation-contraction coupling is such that the muscle cannot undergo wave summation or tetanus. This is primarily due to the longer duration of the action potential and the refractory period, which prevents the heart from sustained contractions that would occur if tetanus were possible. Instead, the heart must contract and relax fully between beats to allow adequate filling of the chambers with blood before the next contraction. This rhythmic contraction and relaxation are essential for effective circulation.

Thus, when stimuli are applied at a high frequency like 20 stimuli per second, the heart's electrical and mechanical properties limit the response to simple twitches, and the muscle fibers do not fuse the stimuli into a sustained contraction as seen in skeletal muscle. Therefore, neither wave summation nor tetanus occurs in this scenario.