Accommodation to depolarizing and hyperpolarizing currents in cutaneous afferents of the human median and sural nerves

1. To determine whether accommodation to depolarizing and hyperpolarizing s timuli differs for cutaneous afferents in the median and sural nerves, stud ies were performed in normal human subjects using threshold electrotonus. 2. The changes in threshold for compound sensory action potentials of 50 % of maximum were recorded when the nerves were subjected to long-lasting dep olarizing and hyperpolarizing DC. The premise was that the threshold change s largely mirror the underlying electrotonic changes in membrane potential. 3. The maximal threshold changes produced hyperpolarizing and hyperpolarizi ng currents were greater for median afferents, suggesting that the DC produ ced greater changes in membrane potential in these afferents. 4. Median afferents underwent greater accommodation to depolarizing current s than sural afferents and a greater threshold undershoot at the end of the currents, suggesting greater activity of a slow K+ conductance. Median aff erents also underwent greater accommodation to hyperpolarizing currents, su ggesting greater inward rectification. 5. These conductances are voltage dependent, and the differences in accommo dation could be due to greater changes in membrane potential for the median nerve. The changes in threshold produced by long-lasting depolarizing and hyperpolarizing currents of graded intensity were therefore measured. When the threshold changes were matched for the two nerves, median afferents und erwent 22.4% more accommodation to depolarizing currents and 28.7% more acc ommodation to hyperpolarizing currents. 6. We conclude that there is greater expression of two internodally located conductances responsible for accommodation on median afferents. The biophy sical differences identified in this study might contribute to the finding that sural afferents hare a greater tendency to dysfunction than median aff erents.