Final answer:
The long plateau phase in cardiomyocytes' action potential is due to the prolonged presence of calcium ions (Ca²+) in the cytosol, which ensures effective heart contractions.
Step-by-step explanation:
The long plateau in the action potential observed in cardiomyocytes is related to calcium ions (Ca²+) staying longer in the cytosol. The influx of Ca²+ through voltage-gated calcium channels during the action potential causes a prolonged depolarization phase, or plateau, which is essential for enabling the cardiac muscle to contract effectively. The Ca²+ influx also contributes to the long refractory period, preventing premature contractions and ensuring the heart pumps effectively.