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Qu. How K+ contribute to the development of an electrical potential across the cell

membrane?

 There is a difference in K+ concentration inside (ICF intracellular fluid) and outside (ECF

extracellular fluid) of the cell The cell membrane is selectively more permeable to K+

 

Diffusion of K+ down its concentration gradient (the chemical force) drives the chargeseparation as the impermeable intracellular protein anions cannot pass through the cell

membrane

Qu. How membrane depolarization occurs following the closure of K+ channels (e.g.

ATP-sensitive K+ channels)?

 When the membrane conductance to K+ decreases, the K+ efflux becomes less

 With the continuing Na+ influx down the electrochemical gradient, the initial membrane potential gradually becomes less negative (i.e. being depolarized, as more of Na+ ions enter

the cell than K+ ions leaving)

Qu. Why extracellular [K+] has a major impact on the resting membrane potential?

What is the effect of hypokalaemia or hyperkalaemia?

 Under resting condition, the cell membrane is more permeable to K+. With a high K+

conductance, only a small K+ driving force (i.e. VM-EK ) is needed to produce an efflux of

K+ (IK ) that is sufficient to balance the Na+ influx (I Na). As a result, the resting membrane

 potential (VM) is very much drawn to the equilibrium potential of K+ (EK ) A small change in extracellular [K+] leads a bigger change in its ratio to intracellular [K+]

(e.g. when compared with [Na+]), and hence the EK  and VM 

 

Hypokalaemia results in membrane hyperpolarization as there is bigger concentrationgradient (chemical force) to drive the charge separation

 Hyperkalaemia results in membrane depolarization as there is a reduced concentration

gradient (chemical force) to drive the charge separation (i.e. decrease in hyperpolarizingcurrent). It is NOT that there is an increased influx of extracellular K+ to depolarize the cell

membrane (i.e. NOT an increase in depolarizing current)

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Qu. What is the impact of 5 mM increase in extracellular [K+]o or [Na+]o on theequilibrium potential?

Ion Original [ion]o/[ion]I 

Revised [ion]o/[ion]I 

 Na+ 12:1 12.5:1

K+ 1:30 1:15

Ion Original Eion  Revised Eion 

 Na+ +66 mV +67 mV

K+ -90 mV -72 mV

 For the same absolute change (i.e. 5mM) in extracllualar [K+] or [Na+], [K+]o have higher

impact on equilibrium potential than [Na+]o due to:  A large change in [ion]o/ [ion]I ratio for K+ than Na+

  Higher membrane permeability to K+, with membrane potential more determined by

EK+, than E NA+