Differences in accommodative properties of median and peroneal motor axons

Objectives-To investigate whether accommodation to depolarising and hyperpo larising currents differs for motor axons of human upper and lower limb ner ves. Methods-The threshold tracking technique was used to measure threshold elec trotonus for median and peroneal motor axons. The threshold current that pr oduced a compound muscle action potential 50% of maximum was measured, and membrane potential was altered using subthreshold polarising currents of 33 0 ms duration but of variable intensity, from +40% (depolarising) to -100% (hyperpolarising) of the unconditioned threshold. Results-The maximal threshold changes (the peak of the S1 phase of threshol d electrotonus) were significantly greater in median axons for both depolar ising and hyperpolarising currents. The subsequent phases of accommodation to depolarising currents (S2) and to hyperpolarising currents (S3) were als o significantly greater in median axons. These findings raised the possibil ity that greater accommodation (S2 and S3) in median axons resulted from gr eater changes in membrane potential. However, regression of S2 against S1 t o depolarising currents disclosed significantly greater accommodation (27.8 %) for median axons, suggesting that slow K+ conductances may be more promi nent on median than peroneal axons. By contrast, the relation between S3 an d S1 to hyperpolarising currents was similar for the two nerves, suggesting that the difference in inward rectification was merely because the conduct ance varies with the extent of hyperpolarisation. Conclusions-Slow K+ conductances are more prominent for median motor axons than for peroneal axons. It would therefore be expected that axons innervat ing the lower limbs have less protection from depolarising stress and could develop ectopic activity more readily.