Learning Objective: By the end of this lesson, the learner will be able to explain how resting membrane potential is established and maintained, describe the dominant role of potassium conductance, predict how changes in extracellular ions alter membrane excitability, and apply these principles to clinical scenarios involving electrolyte disturbances and membrane physiology.
Resting Membrane Potential (RMP)
RMP Values in Excitable Tissues
- Nerve cells: approximately –70 mV
- Striated muscle: approximately –90 mV: These values reflect differences in membrane permeability, especially the number of K⁺ leak channels, which dominate the resting ionic conductance.
Role of Potassium in Establishing RMP
Why K⁺ Determines RMP
- Excitable cells have many K⁺ leak channels but very few for Na⁺, Cl⁻, or Ca²⁺.
- Therefore, K⁺ conductance (gK) is the primary determinant of the RMP.
Effect of Extracellular Potassium Changes
Hyperkalemia
- Decreases the gradient for K⁺ efflux → Depolarization
- Clinical effects:
- Increased nerve excitability
- Risk of cardiac arrhythmias
Hypokalemia
- Increases K⁺ efflux → Hyperpolarization
- Clinical effects:
- Decreased nerve excitability
- Increased risk of arrhythmias
Effect of Changing gK
Increased gK
- More K⁺ leaves the cell → Hyperpolarization
- RMP moves from –70 mV toward EK ≈ –95 mV
Decreased gK
- Less K⁺ efflux → Depolarization
Activity
Na⁺/K⁺ ATPase Pump
Key Functions
- Moves 3 Na⁺ out and 2 K⁺ in → Electrogenic effect (net positive charge loss)
- Helps maintain a negative intracellular potential
- Maintains osmotic balance by causing net water efflux
- Restores leaks of Na⁺ into the cell
Effects of Other Ions
Chloride (Cl⁻)
- Resting gCl is low.
- At –70 mV: ↑ gCl → hyperpolarization toward ECl (≈ –76 mV).
- If rEm becomes more negative than –80 mV, ↑ gCl → depolarization.
Sodium (Na⁺)
- Resting gNa is very low.
- ↑ gNa → depolarization (toward +70 mV).
- Changes in Na⁺ concentration do not affect RMP at rest.
Calcium (Ca²⁺)
- Resting gCa is extremely low.
- Only increasing gCa has an effect → Depolarization (toward +125 mV).
