Inhibitory and facilitatory effects from the peroneal nerve onto the soleus H-reflex in normal and spinal man

The effects of conditioning stimulation of a mixed nerve in the leg, the common peroneal nerve (CPN), on the ipsilateral soleus H-reflex were compared with the effects of stimulating its cutaneous branch, the superficial peroneal nerve (SPN), in two groups of subjects--normals and patients with spinal spasticity subsequent to a clinically complete transection of the spinal cord. Condition-test delays of 20 msec to 2 sec, measured from the end of the 20 msec train (3 pulses at 100 Hz), were investigated. In normal subjects, CPN stimulation at 1.4 X MT profoundly depressed the soleus H-reflex. There was an initial depression (peak 40-90 msec) followed by a slow recovery which was incomplete at condition-test delays of 2 sec. One-half of the subjects showed a late facilitation, or disinhibition, peaking at 170-190 msec. The inhibitory effects were attributed to activation of low threshold, groups I and II, muscle afferents because stimulation of the SPN, at 1.5 X threshold for a compound action potential recorded from the CPN, had only facilitatory effects on the soleus H-reflex. Facilitation occurred at condition-test delays of 30-190 msec. The cutaneous stimulation was presumed to activate the largest, A beta, cutaneous afferents as it elicited a weak paraesthesia on the dorsum of the foot. The results suggested that cutaneous afferents may have contributed to the late facilitation seen with CPN conditioning stimulation. In spinal cord-lesioned subjects, CPN stimulation depressed the soleus H-reflex but the decrease was less and the recovery was faster and more complete than in normals. The magnitude of the initial depression at 20-100 msec varied with the severity of the spasticity, subjects with mild spasticity showing less of a depression. Weak cutaneous conditioning stimulation either had no effect or produced a slight depression of the soleus H-reflex, providing clear evidence that transmission in the pathways mediating the facilitatory effects of cutaneous afferents onto extensor motoneuronal pools is depressed in spinal spasticity. This may shift the balance of activity toward the flexor motoneurones, thus favouring the development of, for example, flexor spasms and flexor hypertonia. Since inhibitory effects from cutaneous stimulation are associated with activation of higher threshold afferents in normal man, the present results may reflect a decrease in the threshold for flexor withdrawal reflexes commonly associated with spasticity of spinal origin.