Resting Potential - Resting Potentials

Resting Potentials

The resting membrane potential is not an equilibrium potential as it relies on the constant expenditure of energy (for ionic pumps as mentioned above) for its maintenance. It is a dynamic diffusion potential that takes this mechanism into account—wholly unlike the equilibrium potential, which is true no matter the nature of the system under consideration. The resting membrane potential is dominated by the ionic species in the system that has the greatest conductance across the membrane. For most cells this is potassium. As potassium is also the ion with the most negative equilibrium potential, usually the resting potential can be no more negative than the potassium equilibrium potential. The resting potential can be calculated with the Goldman-Hodgkin-Katz voltage equation using the concentrations of ions as for the equilibrium potential while also including the relative permeabilities, or conductances, of each ionic species. Under normal conditions, it is safe to assume that only potassium, sodium (Na+) and chloride (Cl-) ions play large roles for the resting potential:

This equation resembles the Nernst equation, but has a term for each permeant ion. Also, z has been inserted into the equation, causing the intracellular and extracellular concentrations of Cl- to be reversed relative to K+ and Na+, as chloride's negative charge is handled by inverting the fraction inside the logarithmic term. *E_m is the membrane potential, measured in volts *R, T, and F are as above *P_X is the relative permeability of ion X in arbitrary units (e.g. siemens for electrical conductance) *_Y is the concentration of ion X in compartment Y as above. Another way to view the membrane potential is using the Millman equation:

or reformulated

where P_tot is the combined permeability of all ionic species, again in arbitrary units. The latter equation portrays the resting membrane potential as a weighted average of the reversal potentials of the system, where the weights are the relative permeabilites across the membranes (P_X/P_tot). During the action potential, these weights change. If the permeabilities of Na+ and Cl- are zero, the membrane potential reduces to the Nernst potential for K+ (as P_K+ = P_tot). Normally, under resting conditions P_Na+ and P_Cl- are not zero, but they are much smaller than P_K+, which renders E_m close to E_eq,K+. Medical conditions such as hyperkalemia in which blood serum potassium (which governs _o) is changed are very dangerous since they offset E_eq,K+, thus affecting E_m. This may cause arrhythmias and cardiac arrest. The use of a bolus injection of potassium chloride in executions by lethal injection stops the heart by shifting the resting potential to a more positive value, which depolarizes and contracts the cardiac cells permanently, not allowing the heart to repolarize and thus enter diastole to be refilled with blood.