Final answer:
The intracellular concentration of calcium ions (Ca2+) is strongly correlated with myocardial contractility. These ions influence both the plateau phase of the cardiac action potential and the contraction of cardiac muscle by initiating cross-bridge cycling.
Step-by-step explanation:
Myocardial contractility is best correlated with the intracellular concentration of calcium ions (Ca2+). Calcium ions play two crucial roles in the physiology of cardiac muscle. They are responsible for the prolonged plateau phase of the action potential and affect the contractility of the cardiac muscle by binding to troponin, leading to myosin and actin cross-bridge formation.
Calcium ions influx through slow calcium channels during the plateau phase of the action potential and accounts for approximately 20 percent of the calcium required for contraction, with the remainder released from the sarcoplasmic reticulum. The pattern of spontaneous depolarization observed in the SA node, which is responsible for setting the heart rate, is influenced by the movement of calcium ions into the cell after the initial influx of sodium ions. Moreover, in cardiac contractive cells, the action potential differs from other muscle and nerve cells due to the role of calcium in depolarization and repolarization phases.
Maintaining calcium homeostasis is vital for the normal excitability and contractility of the heart muscle. On the other hand, ions like sodium (Na+), potassium (K+), and magnesium (Mg2+) also play roles in cardiac function, but calcium is the most directly correlated ion with myocardial contractility.