K1vi: Outline factors contributing to generation & maintenance of Resting Membrane Potential

  • RMP = the steady state potential which exists across the cell membrane

→ The potential refers to the intracellular relative to extracellular RMP for myelinated peripheral n. = -70mV

Development of Rmp

  • K equilibrium potential
  • Electrogenicity of 3Na/2K pump (ATPase)
  • Gibbs-Donnan effect (small)

Different ionic [  ] ICF & ECF:

Plasma (mol/L)

Intracellular (mol/L)

Na+

145

10

K+

4

155

Cl–

110

3

Proteins

10

45

1) Na/K/ATPase sets up a chemical diffusion gradient

  • Pumps Na+ out of cell
  • Pumps K+ into cell
  • BUT cell membrane Is x 100 more permeable to K+
  • K+ leaks down its [ ] gradient out of cell
  • ∴loss of +ve charge inside cell → NET NEGATIVE CHARGE INTRACELLULARLY

2) Na/K/ATPase sets up an electrogenic gradient

  • 3Na+ out for 2K+ in
  • ∴net +ve charge lost & → NET NEGATIVE CHARGE INTRACELLULARLY

3) Gibbs-Donnan Effect

  • Intracellular proteins & inorganic phosphates → carry a large -ve charge
  • Those are not diffusible across cell membrane
  • ∴small ions that can diffuse will → NET NEGATIVE CHARGE INTRACELLULARLY

The actual RMP can be calculated at any time from the Goldman equation:

Goldman Equation

i/o = intracellular/extracellular, P = permeability of the ion

  • Main extracellular anion Cl– can freely pass the cell membrane & is similar to K+
  • Other cations (Na+) are impermeable & ∴have little effect on RMP
  • Ions that contribute to RMP most have:
    • High [ ] present
    • Large transmembrane gradient
    • Large membrane permeability
  • K+ has all these properties & is the major contributor to RMP
  • Actual RMP does not differ much from equilibrium potential for K+ (as determined by Normst equation)