CVS Practice Test 2025 – The All-In-One Guide to Exam Success!

The primary ion involved in establishing the resting membrane potential during phase 4 of the non-pacemaker action potential is potassium. In this phase, the cell is at rest, and the membrane potential is primarily influenced by the permeability of the membrane to potassium ions.

Potassium ions are more concentrated inside the cell compared to outside, and the cell membrane is relatively permeable to potassium due to potassium channels that remain open during this phase. As potassium ions tend to move out of the cell down their concentration gradient, this outward movement creates a negative charge inside the cell, thereby establishing a negative resting membrane potential.

This resting potential is critical for the cell's excitability and ability to generate action potentials in response to stimuli. Understanding the role of potassium in this context is essential because any alterations in potassium concentrations or permeability can significantly affect the electrical activity of the heart and other excitable tissues.

While sodium, calcium, and chloride also play roles in cellular excitability, they are not the primary determinants of the resting membrane potential in this specific phase. Sodium tends to increase the membrane potential during depolarization due to action potentials, calcium is involved in excitation-contraction coupling, and chloride ions generally have a negligible effect on the resting membrane potential in cardiac tissues under