What type of cardiac action potential is characterized by rapid depolarization maintained by calcium?

The correct choice is the type of cardiac action potential characterized by rapid depolarization maintained by calcium is the contractile action potential. This specific action potential occurs in the ventricular and atrial muscle cells, where the rapid upstroke of the action potential is primarily due to the influx of sodium ions. Following this phase, depolarization is maintained by calcium ions, which enter the cells through L-type calcium channels. This influx of calcium is crucial for the contraction of the heart muscle, as it triggers the release of more calcium from the sarcoplasmic reticulum, facilitating the strong, coordinated contractions needed for effective blood pumping.

In contrast, autorhythmic action potentials, which occur in pacemaker cells (such as those in the sinoatrial node), have a different mechanism, mainly relying on a gradual depolarization driven by sodium and calcium channels that allow for spontaneous pacing. Refractory potentials pertain to the period during which the heart muscle cannot be re-stimulated, ensuring proper timing between beats. Intermediate action potentials do not accurately describe the established categories of cardiac action potentials in standard physiology. Thus, the key characteristics of contractile action potentials highlight calcium's essential role in sustaining the depolarization phase for muscle contraction.