A QUANTITATIVE ASSESSMENT OF PRESYNAPTIC INHIBITION OF IA AFFERENTS IN SPASTICS - DIFFERENCES IN HEMIPLEGICS AND PARAPLEGICS

Soleus H-reflex facilitation evoked by a supramaximal conditioning sti mulation to the femoral nerve was investigated in 28 healthy control s ubjects and 35 spastic patients of whom 17 were paraplegics with bilat eral spinal cord lesion and 18 were hemiplegics with unilateral cerebr al lesion. Heteronymous facilitation from quadriceps to soleus was mea sured 0.4 ms after onset, while the monosynaptic la excitation is stil l uncontaminated by any non-monosynaptic effect and can be used to ass ess ongoing presynaptic inhibition on la terminals to soleus motor neu rons. In paraplegics, this heteronymous la facilitation was significan tly larger than in control subjects (all individual results in these p atients being above the mean observed in controls). This must reflect a decrease in presynaptic inhibition of la terminals in the paraplegic s explored here. There was no correlation between this decreased presy naptic inhibition of la terminals and the degree of spasticity measure d by Ashworth's scale. Surprisingly, the amount of heteronymous la fac ilitation in hemiplegics was the same as in normal subjects. This indi cates that presynaptic inhibition of la terminals is unchanged in thes e patients and disagrees with the usual interpretation of reduced vibr atory inhibition of the soleus H-reflex in hemiplegics. It is argued t hat this disagreement is due to the fact that vibratory inhibition of the reflex also depends on post-activation depression following repeti tive synaptic transmission