Excitability properties of median and peroneal motor axons

Threshold tracking was used to compare excitability properties (stimulus-re sponse curves, strength-duration properties, recovery cycle, and threshold electrotonus) of median motor axons at the wrist and peroneal motor axons a t the ankle in 12 healthy subjects. Stimulus-response curves and strength-d uration properties were similar, though higher stimulus intensities were re quired for peroneal axons. However, there were significant differences in t he recovery cycle of excitability following a conditioning stimulus and in threshold electrotonus. In the recovery cycle, median axons had significant ly greater supernormality and late subnormality. In threshold electrotonus, the initial slow threshold changes in response to subthreshold depolarizin g and hyperpolarizing currents (S1) were significantly greater in median ax ons, and there was also greater accommodation to depolarizing currents (S2) and greater threshold undershoot after depolarization. Similar differences in supernormality and the S1 phase of threshold electrotonus were found be tween peroneal axons at ankle and knee, suggesting that these properties ma y be dependent on nerve length. When median motor axons at the wrist were c ompared with peroneal motor axons at the knee, there were no differences in refractoriness and supernormality and only small differences in S1, but th e late subnormality and undershoot were significantly greater in the median axons. These findings suggest that, in addition to any length-dependent di fferences, peroneal axons have a less prominent slow K+ conductance. We con clude that the properties of different motor axons are not identical and th eir responses to injury or disease may therefore differ. (C) 2000 John Wile y & Sons, Inc.