During extreme dehydration, plasma K+ can increase to as high as 10 mM. (1) What

Question

During extreme dehydration, plasma K+ can increase to as high as 10 mM. (1) What would the membrane potential of this neuron be under these conditions? (Assume there are no other changes in ion concentrations.) (2) What would happen to the ability of the neuron in the preceding question to generate action potentials during extreme dehydration? Why might this be problematic?

HINTS: You can assume that PK is 100 times the other Ps. Look up average AP threshold, in textbook or use value from next question to answer part B.

Be careful how to handle [Cl–] because of its negative charge.

Explanation / Answer

Increased extracellular potassium levels result in depolarization of the membrane potentials of cells due to the increase in the equilibrium potential of potassium. This depolarization opens some voltage-gated sodium channels but also increases the inactivation at the same time. Since depolarization due to concentration change is slow, it never generates an action potential by itself instead, it results in accommodation. Above a certain level of potassium, the depolarization inactivates sodium channels, opens potassium channels, thus the cells become refractory. This leads to the impairment of neuromuscular, cardiac, and gastrointestinal organ systems. Of most concern is the impairment of cardiac conduction which can result in ventricular fibrillation or asystole.

When extracellular K+K+ concentration increase by a certain amount, excitability of cells is higher because the resting potential shifts toward a higher equilibrium potential of K+, therefore causing depolarization.

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