In electrophysiological studies, the substitution of Na+ is used as a method in the extracellular environment. N-methyl-D-glucamine (megulamine; NMG) is an excipient because of its pharmacologically inactive nature, which can block the Na currents at a cellular level. In this study, we investigated alterations in the contributions of fiber groups to compound action potential (CAP) during NMG replacement. The replacement resulted in a significant decrease in both the amplitude and the area of the CAP for each replacement group. Full replacement did not decrease the CAP area compared to partial replacement. Different replacement ratios of Na+ in the extracellular medium with NMG have been shown to cause changes in the activities of some nerve fibers, as well as blocking the conduction. The findings were obtained by the specific distribution of the nerve conduction velocity calculation method. The partial replacement of extracellular Na affects the fast-conducting fiber groups, whereas full replacement affects the slow-conducting fiber groups.