Question

To which variable ([Na+]o or [K+]o) do you think membrane potential is more sensitive? Stated another way, do you think small changes in [Na+]o or [K+]o from their normal values would be more harmful to bodily functions? What might those deleterious effects on the body be?

Answer 1

At resting potential, the axonal membrane is comparatively more permeable to potassium ions and nearly impermeable to sodium ions. At rest, the inside of the neuron is negatively charged as compared to outside of the neuron. So the resting membrane potential opposes the diffusion of potassium and sodium ions down their concentration gradients. A change in extracellular concentration of sodium ions, [Na+]0 there will be no change in the membrane potential but if the extracellular concentration of potassium ion, [K+]0 changes, if it increases then the membrane potential will decrease (depolarize the cell) because the reduction in the equilibrium potential for potassium will indicate less potassium will diffuse out of the cell, making the interior more positive than before. That is why the membrane potential is more sensitive to [K+]0.

A change in extracellular potassium potential will lead to a change in the relative volume of the cell altering the membrane potential. This will also lead to changes in concentration of chloride, sodium and other internal ions. With the increase in extracellular concentration of potassium ions, the membrane potential move toward zero but not reach it. But with the change in extracellular concentration of sodium ions, there is not much appreciable change in the membrane potential. On the contrary, decrease in extracellular sodium ion concentration result in smaller action potential.

Increased extracellular potassium level cause depolarization of the membrane potentials of the cell due to increase in the equilibrium potential of potassium and above a certain level it may inactivate the sodium channels and open the potassium channels thus the cells become refractory. This is a condition of hyperkalemia which can cause cardiac arrest, kidney failure, hypoaldosteronism and rhabdomyolysis.