CVS Practice Test

The resting cell membrane potential (RMP) is predominantly determined by the permeability of the cell membrane to potassium ions (K+). At rest, the cell membrane has a higher permeability to K+ compared to sodium ions (Na+), calcium ions (Ca++), and chloride ions (Cl-). This is primarily due to the presence of leak channels that allow K+ to exit the cell more freely than other ions can enter or exit.

As K+ ions leave the cell, they carry positive charges with them, which makes the inside of the cell more negatively charged relative to the outside. This movement of K+ establishes a negative resting membrane potential, typically around -70 to -90 mV in neurons. The key factor here is that the resting membrane potential is largely influenced by the concentration gradient and permeability characteristics of K+, making it the primary ion that contributes to the RMP.

In contrast, while Na+, Ca++, and Cl- do play roles in cellular physiology, their permeability at rest is not as significant as that of K+, which is why K+ is the correct answer when considering which ion the resting cell membrane is more permeable to.