Overview
This lecture reviews resting membrane potential, focusing on how ion gradients and membrane permeability determine the electrical charge across nerve and muscle cell membranes.
Ion Gradients and Membrane Potential
- Cells have higher potassium (K⁺) inside and higher sodium (Na⁺) outside.
- K⁺ leaving the cell makes the inside more negative; Na⁺ entering makes the inside more positive.
- The movement of ions creates an electrical potential (membrane potential) across the cell membrane.
- At rest, the inside of the cell is negative relative to the outside.
Passive Diffusion and Equilibrium Potentials
- When a membrane is only permeable to K⁺, K⁺ diffuses out, making the inside negative until electrochemical equilibrium is reached.
- Equilibrium does not mean equal K⁺ concentration inside and outside; it means chemical and electrical forces are balanced.
- The equilibrium potential for K⁺ (E_K) is approximately -94 mV.
- When only permeable to Na⁺, Na⁺ moves in, making inside positive until equilibrium, with Na⁺ equilibrium potential (E_Na) at +61 mV.
Resting Membrane Potential and Permeability
- Typical resting membrane potential ranges from -90 to -70 mV, closer to E_K than E_Na.
- Resting potential is closer to the equilibrium potential of the ion with the highest permeability (usually K⁺).
- Membrane permeability is far greater for K⁺ than Na⁺ due to the presence of K⁺ leak channels and the sodium-potassium pump.
The Role of Membrane Permeability
- Greater permeability to K⁺ drives the resting membrane potential toward E_K.
- Increasing Na⁺ permeability shifts the potential toward E_Na; increasing K⁺ permeability shifts it further negative.
- The Goldman equation calculates membrane potential considering all permeant ions and their permeabilities.
- If permeability for an ion is zero, it does not contribute to membrane potential.
Pathological Example: Potassium Chloride Infusion
- Infusing potassium chloride increases extracellular K⁺, reducing the gradient and abolishing resting membrane potential.
- Loss of resting membrane potential stops essential muscles (e.g., heart, diaphragm), leading to death.
Key Terms & Definitions
- Resting Membrane Potential — The electrical charge difference across the cell membrane when the cell is at rest.
- Equilibrium Potential — The membrane potential at which an ion's net flow across the membrane stops due to balanced electrical and chemical gradients.
- Electrochemical Equilibrium — Balance between chemical gradient and electrical force for an ion.
- Permeability — How easily an ion can cross the cell membrane.
- Sodium-Potassium Pump — An active transporter moving 3 Na⁺ out and 2 K⁺ in, contributing to the negative charge inside the cell.
- Potassium Leak Channels — Channels allowing passive K⁺ efflux, increasing K⁺ permeability.
Action Items / Next Steps
- Watch the potassium leak channels video on the course page.
- Use the discussion board to answer and discuss the provided question.
- Review the Goldman equation and its application to membrane potential calculations.