Long latency inhibition of h-reflex recovery by cutaneous tactile stimulation in man: A cutaneous transcortical reflex

Abstract

The effect of cutaneous tactile stimulation on motoneuron excitability was studied in 20 normal subjects and in patients of hemiplegia ( n = 14) and paraplegia ( n = 15) by plotting H-reflex recovery curves during application of twin pulses alone (“basal” H-reflex recovery curve), and twin pulses synchronized with electrical stimuli evoking tactile sensation in skin over the lateral border of the small toe. The “basal” H-reflex recovery curves from normal subjects showed a significant lateral asymmetry of motoneuron excitability, with an even distribution of subjects showing greater excitability on the left and right sides. However, there was no relation between handedness and the side with greater excitability. The cutaneous stimulation produced a highly significant inhibition of the H-reflex recovery between 600 and 6000 ms, with the maximum inhibition recorded at 1000 and 2000 ms, at which time even a complete inhibition of the test H-reflex was observed in some instances. The effect of cutaneous stimulation before 600 ms was statistically insignificant. The amount of cutaneous inhibition of H-reflex recovery showed a lateral asymmetry. The side with greater motoneuron excitability showed more cutaneous inhibition of the H-reflex recovery. A comparison of the H-reflex recovery at higher frequencies of cutaneous stimulation with that at basal frequency showed a slight but statistically insignificant difference in the amount of cutaneous inhibition of the H-reflex recovery. In hemiplegics, the “basal” H-reflex recovery curves showed greater motoneuron excitability on the affected side as compared to those of the unaffected side or controls, with the late inhibitory phase being completely obliterated. A similar pattern was also observed in paraplegics. Significantly, the lateral asymmetry of motorneuron excitability observed in the control group was absent in paraplegics. The cutaneous stimulation failed to produce any significant effect on the H-reflex recovery curves either in the affected side of hemiplegics or in both sides of paraplegics. The significant long latency inhibition of the H-reflex recovery curve produced by cutaneous tactile stimulation is a new finding. An absence of any effect of cutaneous stimulation before 600 ms is contrary to earlier reports, but is probably due to methodological differences. However, it is suggested that the H-reflex recovery curves recorded by applying twin pulses are probably also affected by cutaneous afferent stimulation in the posterior tibial nerve by the first of the twin pulses. It is also concluded that the present method is an improvement over the earlier method in bringing out the late reflex effects of cutaneous stimulation on motoneuron excitability. The observations in the present study also suggest the possibility of a transcortical loop for the cutaneous reflex producing inhibition of the H-reflex recovery.