If this cell were permeable only to K+, what would be the effect of reducing the extracellular K concentration from 5 to 2.5 millimolar? Intracellular (mM) Extracellular (mM) 140 K+ 5 K+ 12 Na+ 145 Na+ 5 CI- 125 CI- 0.0001 Ca++ 5 Ca++

If a membrane is permeable to only a single ion Vm is equal to the equilibrium potential for that ion. In this hypothetical cell, EK+ = 88 millivolts. If the extracellular K+ concentration is reduced by half, EK+ = 61 x log (2.5/140) = 107 millivolts, (Hyperpolarisation of 19 millivolts.)