Membrane Potential

MEMBRANE POTENTIAL: the difference in the electrical potential between the inside and outside of the cell

SEPARATION OF CHARGES: difference of positive and negative ions in ECF and ICF

  • separation of positive and negative charges has the potential to generate energy/do work

  • extracellular fluid (ECF: outside cell) and intracellular fluid (ICF: inside cell) are separated by the plasma membrane

  • 3 main ions set up resting membrane potential

    • sodium (Na+)

    • potassium (K+)

    • anionic protein (A-)

  • Na is more concentrated in the ECF (outside of the cell) than ICF

  • K is more concentrated in the ICF (inside of cell) than ECF

    • This is maintained by sodium potassium pumps as well as the membrane’s permeability to respective Na and K ions

Concentration and Permeability of Ions Responsible for Membrane Potential in a Resting Nerve Cell (millimoles/litre)

Ion

Extracellular

Intracellular

Relative Permeability

Na+

150

15

1

K+

5

150

50-75

A-

0

65

0

  • higher elative permeability shows that it is easier for K+ to move into the cell, than for Na+ to move out of the cell

  • less sodium leak channels, therefore less relative permeability. more potassium leak channels, therefore more relative permeability

  • anionic protein is mainly concentrated in cellular fluid, it has no relative permeability because of its large size and negative charge

    • therefore no movement, as it is impermeable to the plasma membrane

Effect of K+ Only on Membrane Potential

  • concentration gradient for potassium moves it OUT of the cell (positive charge attracted to negative)

  • when K+ moves, the outside of cell becomes more positive (gaining positive charge), while inside more negative (losing positive charge)

  • electrical gradient as a result of this will tend to move K+ back into the cells

  • only when the inward electrical gradient exactly counters the outward concentration gradient will K+ movement stop

  • equilibrium potential for K = -90 mV

Effect of Na+ Only on Membrane Potential

  • concentration gradient for sodium moves it INTO the cell (positive charge attracted to negative)

  • when Na+ moves, the cell becomes more positive (gaining positive charge), while the ECF will become more negative (losing positive charge)

  • electrical gradient as a result of this will move Na+ back out of the cell

  • only when outward electrical gradient exactly counters the inward concentration gradient will Na+ movement stop

  • equilibrium potential for Na = +60 mV

Resting Membrane Potential

  • in reality, movement of both Na+ and K+ at the same time will determine membrane potential

  • resting membrane potential = -70 mV

    • EP K+ -90 mV + EP Na+ 60 mV = -30, so why -70 mV?

      • due to ion/leak channels; elements permeability to membrane differ

      • anionic proteins are also trapped inside the cell, keeping its negative charge inside

Changes in Membrane Potential

  • Polarization: has charge different than 0 mV, whether positive or negative, has any charge

  • Depolarization: taking on more positive charges, membrane potential will get closer to 0

  • Repolarization: returning to resting membrane potential, becoming more negative after depolarization

  • Hyperpolarization: membrane potential becomes more negative than resting membrane potential