Contribution of fast and slow conducting myelinated axons to single-peak compound action potentials in rat spinal cord white matter preparations

Unlike recordings derived from optic nerve or corpus callosum, compound action potentials (CAPs) recorded from rodent spinal cord white matter (WM) have a characteristic single-peak shape despite the heterogeneity of axonal populations. Using a double sucrose gap technique, we analyzed the CAPs recorded from dorsal, lateral, and ventral WM from mature rat spinal cord. The CAP decay was significantly prolonged with increasing stimulus intensities suggesting a recruitment of higher threshold, slower conducting axons. At 3.5 mm conduction distance, a hidden higher threshold, slower conducting component responsible for prolongation of CAP decay was uncovered in 22 of 25 of dorsal WM strips by analyzing the stimulus-response relationships and a normalization-subtraction procedure. This component had a peak conduction velocity (CV) of 5.0 ± 0.2 (SE) m/s as compared with 9.3 ± 0.5 m/s for the lower threshold peak (P < 0.0001). Oxygen-glucose deprivation (OGD), along with its known effects on CAP amplitude, significantly (P < 0.015) shortened the CAP decay. The hidden higher threshold, slower conducting component showed greater sensitivity to OGD compared with the lower threshold, faster conducting component, suggesting a differential sensitivity of axonal populations of spinal cord WM. At longer conduction distances and lower temperatures (9.8 mm, 22-24°C), the slower peak could be directly visualized in CAPs at higher stimulation intensities. A detailed analysis of single-peak CAPs to identify their fast and slow conducting components may be of particular importance for studies of axonal physiology and pathophysiology in small animals where the conduction distance is not sufficiently long to separate the CAP peaks.