Nerve excitability properties in lower‐limb motor axons: Evidence for a length‐dependent gradient

Abstract

In this study, nerve excitability protocols were adapted for lower-limb recordings in 25 healthy subjects to enable comparison of excitability parameters between proximal and distal recording sites of the same nerve and between different nerves. Excitability parameters (stimulus-response curves, strength-duration properties, threshold electrotonus, a current-threshold relationship, and the recovery cycle) were recorded from tibialis anterior, extensor digitorum brevis, and abductor hallucis. Excitability recordings were technically possible from each site, and normative values were established for lower-limb nerves. In this process, inter- and intranerve differences in excitability properties were demonstrated: stimulus intensity and rheobase were reduced in recordings from proximal sites; the relative refractory period and late subexcitability were increased; superexcitability was reduced; and a relative "fanning-in" occurred for threshold electrotonus curves recorded from proximal sites. Such a length-dependent gradient in nerve excitability may underlie the greater tendency for ectopic activity to arise from the proximal segments of motor axons and may contribute to the length-dependent involvement of motor axons in the development of peripheral neuropathy.

Evaluation of Na+/K+ pump function following repetitive activity in mouse peripheral nerve

“…The threshold changes to polarizing currents in mouse tibial motor axons were smaller than those reported for human tibial nerves (Krishnan et al, 2004) and cat tibial nerves (Moldovan and Krarup, 2004b) but similar to those reported in rat caudal motor nerve (Yang et al, 2000). The comparisons were complicated by the fact that we set the polarization current relative to the 0.2 ms threshold and not relative to 1 ms threshold as in previous studies.…”

Assessment of cortical excitability using threshold tracking techniques

“…3,18 In this method, a constant target MEP amplitude is tracked online by a test stimulus using different conditioning-test paradigms. Although threshold tracking has been validated in electrical studies of peripheral ax-ons in healthy controls 23,24,30,35 and in the investigation of disease pathophysiology in a wide array of neurological disorders, 5,6,8,[25][26][27][28]31,33 limited data exist for the application of threshold tracking to TMS to study cortical excitability. Consequently, the aim of the present study was to determine the adaptability of threshold tracking to TMS techniques to investigate the processes of intracortical inhibition and facilitation.…”

Normal data on axonal excitability in Koreans

“…17 In this context, previous studies have demonstrated that even at the same site of the same nerve, the excitability properties of motor axons can differ significantly according to the muscles that they innervate. 8,13,22 Conversely, it has also been reported that axonal excitability in the same muscle recording can …”