Contribution of Fast and Slow Conducting Myelinated Axons to Single-Peak Compound Action Potentials in Rat Spinal Cord White Matter Preparations

Abstract

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.

Changes in gap junction expression and function following ischemic injury of spinal cord white matter

“…In control aCSF, CAPs recorded from spinal cord white matter had an average amplitude of 8.5 mV Ϯ 1.9 mV (Fig. 2), which is comparable to amplitudes in the submaximal range cited previously by our laboratory with single (Eftekharpour et al 2005;Fehlings and Nashmi 1997;Nashmi and Fehlings 2001;Sinha et al 2006) or double (Velumian et al 2010(Velumian et al , 2011 sucrose gap techniques. Adding 300 M 1-octanol or 100 M carbenoxolone for 20 min to aCSF had no significant effect on CAP amplitude compared with control by the end of the 20-min period of drug administration ( Fig.…”

“…To record compound action potentials (CAPs), we used a sucrose gap technique, which provides higher-amplitude signals with better signal-to-noise signal resolution compared with other extracellular recording techniques (Mert 2007;Peasley and Shi 2002;Utzschneider at al 1991;Velumian et al 2010Velumian et al , 2011. The sucrose gap apparatus modeled after previous designs published by our group (Fehlings and Nashmi 1997;Nashmi and Fehlings 2001;Sinha et al 2006) was modified and custom designed to contain three Lexan polycarbonate chambers.…”

Traumatic Brain Injury in Young Rats Leads to Progressive Behavioral Deficits Coincident with Altered Tissue Properties in Adulthood

“…N1 amplitudes of CAP reflects excitability of fast-conducting myelinated axons (Fig. 8), and represents the sum of all individual action potentials (Bolton and Carter, 1980;Velumian et al, 2011). The average N1 amplitudes in CCI animals were significantly reduced compared to responses recorded in sham-operated animals ( p < 0.005; Fig.…”

A Novel Approach for Studying the Physiology and Pathophysiology of Myelinated and Non-Myelinated Axons in the CNS White Matter

“…In the CNS, "nerve-like" electrophysiological studies have been extensively used on the optic nerve [34–40,44,47,69] and on spinal cord white matter by others [47,70–77] and by our group [51,78–83] but never applied to other white matter structures.…”