CVS Practice Test 2025 – Complete Exam Preparation

The myocardial cell membrane is indeed relatively impermeable to calcium and sodium ions. This permeability characteristic is crucial for maintaining the electrical and chemical gradients necessary for heart muscle function.

In the context of cardiac physiology, the myocardial cells rely on these gradients to create action potentials and facilitate contraction. While myocardial cells do allow the passage of sodium and calcium ions at specific times—particularly through voltage-gated channels during action potential generation—the overall resting membrane permeability to these ions is low. Therefore, under resting conditions, the membrane does not easily allow sodium and calcium to pass through.

This property is essential for the cardiac action potential's phases, ensuring that the heart muscle contracts rhythmically and effectively as part of the cardiac cycle. The relatively low permeability helps in sudden increases in intracellular calcium during the action potential, which is vital for muscle contraction, as well as in regulating sodium that contributes to depolarization.

Other ions mentioned in the incorrect options also play roles in cardiac function, but the permeability characteristics focus specifically on the limited movement of sodium and calcium ions in resting conditions. Thus, highlighting the importance of the myocardial cell membrane's selective permeability is key to understanding the mechanisms underlying cardiac excitability and contractility.