1. Sci 2 Lect. 2 Membrane Potential ©Dr Bill Phillips 2002
Balance of electrical charge
Ion channels
Concentration gradients - chemical driving force
Electrical forces on ions
Ionic currents across the membrane
The Smarties game / Nernst Potential
Relative permeability to ions

3. Balance of electrical charge Depends upon:
Number of positive charges (cations) inside minus negative charges (anions) inside
Number of cations outside - anions outside

4. The Membrane Potential is the
electrical potential difference
of the cytoplasm compared to the
charge in the extracellular fluid

5. Balance of electrical charge Depends upon:
Number of positive charges (cations) inside minus negative charges (anions) inside
Number of cations outside - anions outside

6. Balance of electrical charge

7. Balance of electrical charge
For membrane potential to change cations or anions must SELECTIVELY cross the membrane
Only a tiny movement of ions is needed
Movement of charge across the membrane is called an ionic current
Ion channels permit ionic currents

8. Types of ion channels

9. Concentration gradients - chemical driving force

11. Electrical forces on ions
Cations attracted to areas of net negative charge
Anions attracted to areas of net positive charge
In a resting (not signalling) neuron, the inside is more negative than outside. We say the membrane potential is negative or polarised. Resting membrane potential.

12. Ionic currents across the membrane Movement of charge- Convention
In physiology we always speak of currents in terms of movement of positive ions.
An “inward current” could be caused by cations moving in (OR by anions moving out)

13. The Smarties game

14. Nernst/equilibrium Potential

15. Nernst/equilibrium Potential
Nernst Potential for K+ ~ -75mV
Nernst Potential for Na+ ~ +55mV
Nernst Potential for Cl- ~ -60mV
REMEMBER: Nernst Potential for an ion depends upon the concentration gradient in the cell in question.

16. Relative permeability to ions
When the membrane is permeable only to K+, the membrane potential (Vm) will be be equal to the Nernst Potential for K+ (EK )
When the membrane is permeable only to Na+, the membrane potential (Vm) will be be equal to the Nernst Potential for Na+
When the membrane is permeable to both K+ and Na+, then EK < Vm < ENa

17. The resting membrane is about 20 times more permeable to K+ than to Na+.
This is why the resting membrane potential is close to EK

18. Membrane Potential: Revision
Concentration gradients of K+ and Na + remain fairly stable (topped up by Na +/K + ATPase).
Nernst Potential of an ion is determined by its concentration gradient.
At rest, the membrane is ~20X more permeable to K + than Na +
Relative permeability determines Vm