A built-in “do not disturb” window
Right after a heart cell fires, it cannot be made to fire again, no matter how strong the next signal is. This recovery window is the refractory period. For most of it the cell is completely unresponsive; near the end it can respond, but only to an unusually strong stimulus. Only once it has fully repolarized is it ready to fire normally again.
Remember the long plateau from the last guide? It is what makes the cardiac refractory period so long. Because the cell stays depolarized for a couple of hundred milliseconds, it stays refractory for nearly the whole length of its own contraction. The electrical lock-out and the mechanical squeeze almost completely overlap.
Why this protection matters
Skeletal muscle can be driven into a sustained, locked contraction if you stimulate it fast enough. The heart must never do that — a heart frozen mid-squeeze cannot fill and pump. The long refractory period makes a locked-up heart impossible: by the time a cell can answer again, it has already relaxed and refilled. So the heart is physically protected from being driven too fast to do its job.
The refractory period also keeps the wave moving in one direction. As the excitation front sweeps forward, the tissue just behind it is refractory, so the wave can’t double back on itself. This is one of the body’s defenses against a signal looping around in a circle — the kind of self-sustaining loop that underlies many forms of arrhythmia.