Recovery of excitability of cutaneous afferents in the median and sural nerves following activity

In acquired polyneuropathies, symptoms and signs are typically distal and s ymmetrical, more prominent in the lower limbs than the upper limbs. This st udy was undertaken to measure the extent of the decrease in excitability pr oduced by single impulses and by impulse trains in cutaneous afferents in t he median and sural nerves, and to compare the resulting changes in excitab ility of these afferents. Threshold tracking was used in 10 healthy subject s to measure the changes in threshold for a compound sensory action potenti al of 50% maximum produced by conditioning stimuli. Following a single supr amaximal conditioning stimulus, the threshold changes occurring during the refractory and supernormal periods were identical for the two nerves, but t here was a greater increase in threshold during the late subnormal period f or median afferents. Following a train of 10 supramaximal conditioning stim uli, threshold increased by similar to 40% for median afferents and by simi lar to 20% for sural afferents. These differences are consistent with diffe rences in a slow K+ conductance. It is suggested that the hypo-excitability produced by brief trains of impulses may be sufficient to disturb conducti on in diseased nerve fibers, and that the lesser expression of slow K+ cond uctances on cutaneous afferents in the sural nerve could render them more s ensitive to depolarizing stresses than median afferents. This could be a fa ctor in the ease with which sural afferents become ectopically active in po lyneuropathies. (C) 2000 John Wiley & Sons, Inc.