Supraspinal influences on recurrent inhibition in humans. Paralysis of descending control of Renshaw cells in patients with mental retardation.
The recurrent inhibition of alpha motoneurons was studied in 8 mentally retarded subjects (age 16-35 years), six of whom also had non-pyramidal or extrapyramidal motor alterations, manifesting as rigid and inflexible voluntary and/or postural movements. Despite a similar degree of mental retardation (Raven spatial general intelligence test), the other 2 cases showed much more modest changes in motor behavior. At rest, recurrent inhibition on soleus motoneurons was normal in all patients. In the 6 cases exhibiting more severe motor abnormality, the changes in Renshaw cell excitability, which occur during postural or voluntary contractions in normal subjects, were not found. This expressed the lack of supraspinal influences on Renshaw cells in these patients. On the other hand, supraspinal modulating influences on Renshaw cells were virtually normal in the remaining 2 patients. The absence of excitability changes of recurrent inhibition to postural or voluntary movements is discussed in relation to the abnormality of motor behavior observed in these patients. In addition, since paralysis of adaptive changes of recurrent inhibition has so far only been described in spastic subjects, the present study demonstrates that the descending pathways, which control recurrent inhibition gain, are different from those which, when damaged, lead to spasticity. Finally, our results indicate that the changes in motor behavior often associated with mental retardation cannot be regarded merely as the consequence of defective motor learning.