Neuron Structure and Signal Propagation (3A) Practice Test

A lab compares conduction along two peripheral axons of equal diameter: Axon 1 is myelinated with long internodes and regularly spaced nodes of Ranvier; Axon 2 is unmyelinated. Both are stimulated with identical current injections at the axon initial segment, and both express similar densities of voltage-gated Na+ channels at spike initiation sites. Which factor most influences the speed of signal propagation in these neurons?

In a study of demyelinating disease, an axon segment loses myelin but retains intact nodes of Ranvier. Recordings show that small depolarizations spread farther along the demyelinated region but decay more quickly in time, and some APs fail to propagate through the demyelinated zone. Which factor most influences the speed and reliability of signal propagation in this case?