Electrical Signaling in Neurons (4C) Practice Test

In a focused experiment on action potential propagation, a single myelinated motor neuron is stimulated at the axon hillock. Resting membrane potential is $V_m=-70\ \text{mV}$ and threshold is $-55\ \text{mV}$. Voltage-gated Na$^+$ channels open rapidly when threshold is reached, and voltage-gated K$^+$ channels open with a delay to repolarize the membrane. The axon is partially demyelinated over a 2-mm segment, increasing membrane capacitance in that segment from $C_m=1.0\ \mu\text{F}/\text{cm}^2$ to $2.5\ \mu\text{F}/\text{cm}^2$ while leaving ion channel densities unchanged. Assume intracellular and extracellular ion concentrations remain constant during a single spike and that the neuron is otherwise healthy. Which outcome is most consistent with the effect of the demyelinated segment on action potential propagation along the axon?

A researcher models resting membrane potential changes in a single neuron as extracellular K$^+$ is increased. Assume the membrane at rest is primarily permeable to K$^+$ via leak channels, and $V_m$ approximately follows the Nernst potential for K$^+$: $$E_K=\frac{RT}{zF}\ln\left(\frac{K^+\text{out}}{K^+\text{in}}\right)$$ with $z=+1$, $R=8.314\ \text{J/mol·K}$, $T=310\ \text{K}$, and $F=96485\ \text{C/mol}$. Initially, $K^+\text{in}=140\ \text{mM}$ and $K^+\text{out}=4\ \text{mM}$; then $K^+\text{out}$ is raised to $12\ \text{mM}$ while $K^+\text{in}$ is unchanged. Which change is most consistent with this manipulation?