Factors contributing to resting membrane potential (RMP)

1. Na⁺-K⁺ pump
   • It actively pumps 3 Na⁺ ions out of the cell for every 2 K⁺ ions pumped in.
   • Thus, there is a net loss of one positive charge from the cell, producing intracellular negativity.
   • It also maintains high intracellular K⁺ and high extracellular Na⁺ concentrations.
2. Potassium diffusion through K⁺ leak channels
   • At rest, the membrane is much more permeable to K⁺ than Na⁺.
   • K⁺ diffuses outward down its concentration gradient, leaving behind negatively charged intracellular proteins and anions.
   • This is the major factor determining RMP. The K⁺ diffusion potential is about −94 mV.
3. Slight sodium diffusion inward
   • A small amount of Na⁺ leaks into the cell because the resting membrane has slight Na⁺ permeability.
   • This inward movement of positive ions makes the membrane potential less negative.
   • Combined K⁺ and Na⁺ diffusion produces a potential of about −86 mV.
4. Electrogenic effect of Na⁺-K⁺ pump
   • Since the pump moves 3 Na⁺ out for every 2 K⁺ in, it adds approximately −4 mV to the membrane potential.
   • Therefore, the potential predicted by K⁺ and Na⁺ diffusion plus pump action is about −90 mV. Other ions, particularly Cl⁻, also influence the final value.