Final answer:
Calcium ions cause a prolonged plateau phase of approximately 175 ms and an extended refractory period totaling about 250 ms in cardiac contractile cells, culminating in a total action potential duration of 250-300 ms.
Step-by-step explanation:
The actions of calcium ions (Ca²⁺) are essential for the stabilization of cardiac membranes. These ions contribute predominantly to the plateau phase of the cardiac action potential by entering through slow calcium channels. This phase is crucial for ensuring the heart muscles contract properly to pump blood effectively. The plateau phase attributed to Ca²⁺ influx is sustained, lasting approximately 175 ms, followed by a rapid repolarization of approximately 75 ms. Combining the plateau phase with the repolarization phase yields an action potential duration of about 250-300 ms for cardiac contractive cells.
The influx of Ca²⁺ during the action potential leads directly to muscle contraction by interacting with troponin in the troponin-tropomyosin complex, allowing the myosin heads to form cross bridges with actin, leading to muscle contraction. Noteworthy is the absolute refractory period, which lasts around 200 ms, and the relative refractory period, lasting about 50 ms. These refractory periods, totaling 250 ms, are critical to ensure that the heart muscle's contraction follows the electrical events and effectively pumps blood.