A 56-year-old woman with severe muscle weakness is hospitalized. The only abnormality in her laboratory values is an elevated serum K+ concentration. The elevated serum K+ causes muscle weakness because: (A) The Resting Membrane Potential Is Hyperpolarized, (B) The K+ Equilibrium Potential Is Hyperpolarized, (C) The Na+ Equilibrium Potential Is Hyperpolarized, (D) Na+ channels are closed by depolarization
Answer is D. Elevated serum K+ concentration causes depolarization of the K+ equilibrium potential and therefore depolarization of the resting membrane potential in skeletal muscle. Sustained depolarization closes the Na+ channels and prevents the occurrence of action potentials in the muscle.