Final answer:
The cardiac action potential has a distinctive plateau phase due to calcium influx, followed by an extended refractory period allowing full heart contractions before the next electrical stimulus. Variations in this potential are reflected in the ECG's P wave, QRS complex, and T wave, corresponding with atrial and ventricular activity.
Step-by-step explanation:
The typical shape of the cardiac action potential in contractile cells includes a plateau phase, which is longer compared to other muscle cells, due to the influx of calcium ions. This extended plateau phase is followed by an extended refractory period, allowing the heart muscle cells to fully contract before the possibility of another electrical event. Unlike skeletal muscle, the cardiac action potential has the following phases: a rapid depolarization where voltage-gated sodium channels open, then a plateau phase due to the opening of slow calcium channels and simultaneous closure of some potassium channels, and finally a repolarization phase where calcium channels close and potassium channels open, restoring the resting membrane potential.
In the typical ECG recording, this action potential sequence corresponds with the P wave for atrial depolarization, the QRS complex for ventricular depolarization, and the T wave for ventricular repolarization. The entire cardiac cycle includes atrial and ventricular systole and diastole and is controlled by conductive tissue in the heart, such as the SA node and AV node, leading to rhythmic contractions that propel blood effectively through the body.