Strength-duration properties and excitability of motor and sensory axons across different target thresholds

The present series of studies aimed to investigate the biophysical basis underlying differences in behavior between motor and sensory axons at different target response levels. In 24 healthy individuals, axonal excitability protocols measured strength-duration properties and latent addition across several axonal populations, with target amplitudes set at 10%, 20%, 40%, and 60%. Strength-duration time constants (SDTCs) were typically longer at lower target levels for both motor and sensory axons. Threshold change at 0.2 ms during assessment of latent addition, representing a persistent Na⁺ current (Na_p), was higher in sensory axons. Passive membrane properties were not different across target levels. Significant relationships were evident between the threshold change at 0.2 ms and SDTC across all target levels for motor and sensory axons. These differences were explored using mathematical modeling of excitability data. With decreasing target size, as the internodal leak conductance increased in sensory axons, the Barrett-Barrett conductance decreased, whereas the hyperpolarization-activated cation current (I_h) channels became more depolarized. A similar pattern was observed in motor axons. As such, it was concluded that Na_p was not responsible for the differences observed in SDTC between different target levels, although within specific target levels, Na_p changes contributed to the variability of SDTC. This study provides a comprehensive assessment of Na_p current, SDTC, and outlines key factors operating at different target levels in motor and sensory axons. Findings from the present study may point to the contributing factors of symptom development in human neuropathy.NEW & NOTEWORTHY This study provides a comprehensive assessment concerning the strength-duration behavior of motor and sensory axons at differing target levels of the compound nerve response. Strength-duration time constant was increased at lower target response levels particularly for sensory axons, whereas threshold change at 0.2 ms and passive membrane properties were not different. The results have established templates for axonal behavior in normal human axons, demonstrating altered adaptive responses, presumably secondary to different patterns of nerve activation.