Differences in responses of cutaneous afferents in the human median and sural nerves to ischemia

Abstract

Recent evidence suggests that two conductances responsible for accommodation to changes in membrane potential (a slow K(+) conductance and inward rectification [I(H)]) are less active on cutaneous afferents in the sural nerve than on those in the median nerve, and it has been suggested that these axons would therefore respond differently to stress, whether natural or due to disease. The present study was undertaken in eight healthy volunteers to determine whether these afferents respond differently to the depolarizing and hyperpolarizing stresses that accompany ischemia for 13 min and subsequent recovery. During ischemia, the decrease in threshold was quantitatively less for the sural afferents, as were the changes in the other indices of axonal excitability, presumably because the ischemic depolarization was less for sural afferents. Following release of ischemia, there was, as predicted, a divergence in the pattern of threshold change. With median afferents there was evidence of a transient depolarization, believed to be due to inward rectification, superimposed on a long-lasting hyperpolarization. The response of sural afferents lacked this transient depolarizing threshold change. Cutaneous afferents in the median and sural nerves behave differently in response to ischemic and postischemic stresses, and it is likely that they will also respond differently to disease processes. In a number of respects the differences between sural and median afferents are analogous to differences between diabetic and normal nerves.