4. Membrane polarization in sensory and motor axons: Effects on excitability

Abstract

Introduction In order to define the pathophysiology underlying development of peripheral neuropathies, it is important to understand the excitability effects produced by alterations in membrane potential. Sensory and motor axons display different biophysical properties which are likely to affect their responsiveness to membrane potential changes. Objective To provide a template for the effects of membrane potential changes on sensory and motor axonal excitability. Methods Sensory and motor nerve excitability studies were recorded using threshold tracking techniques and QTracS software in six participants (mean age 31 ± 2 years). The median nerve was stimulated at the wrist, with both CMAPs and CSAPs recorded. A standard axonal excitability protocol was conducted, including assessment of strength–duration properties, threshold electrotonus, recovery cycle and current-threshold relationship. DC currents set to ±50% of the baseline rheobasic current were utilised to ensure comparability between motor and sensory axons. Results As previously reported for motor axons, polarization had significant effects on axonal excitability. The overall pattern of excitability change was similar between motor and sensory axons – with depolarizing currents producing reduced threshold change in threshold electrotonus, upwards shift of the recovery cycle and reduced inward rectification in the current-threshold relationship. Effects on threshold electrotonus were more prominent in motor axons, with more significant reduction in threshold change to depolarizing and hyperpolarizing currents (TEd90ms; Depolarization: Motor: 46 ± 5%; Sensory: 23 ± 3%; P Conclusions These findings provide a template for the differential interpretation of excitability changes associated with membrane potential change in sensory and motor neuropathies.