Chapter 04 : Physical chemistry of transport phenomena (Nernst Law)

IV.5.Nernst equation and reversal potential

The Nernst equation defines the electrochemical equilibrium between a concentration gradient and an electrical potential, for one particular ion. The electrical potential that exactly balances a concentration gradient is called the reversal potential. The Nernst equation reveals how the reversal potential of an ion depends on the inside and outside concentrations of this ion:

What can you say about the reversal potentials of N a+ , Cl−, and K+?c

IV.5.1.Example K⁺:

 

 n                Figure 48:  High K⁺ concentration inside

                       

                                Figure 49 : K+ ions exchange.

IV.5.2.Example Na⁺:

 

 

                                     Figure 50 : High Na⁺ concentration outside!

 

                                               

 

                                   Figure 51: Na⁺ ions exchange.

IV.5.3.Example Cl⁻ and Ca²⁺:

IV.5.4.Differential sensitivity to extracellular concentration

What is the effect of increasing extracellular concentration of either K+ and Na+ by 5 mM?

 

IV.5.5.Maintaining low extracellular K+

In the central nervous system (CNS), the resting potentials of neurons  depend on low extracellular K+ concentration. To ensure this, the blood-brain barrier prevents the entry of K+ into the extracellular fluid. In addition, specialized glia cells (astrocytes) absorb extracellular K+ and distribute it over a large area (so that it can be pumped back into neurons).