Changes in Ia reciprocal inhibition from the peroneal nerve to the soleus alpha-motoneurons with different static body positions in man.

Experiments were conducted in man to evaluate the changes in Ia inhibitory interneurons activated from the anterior tibial muscle and projecting to the soleus alpha-motoneurons in relation to different static body positions. Subjects were fixed to a tilting chair and the effects of body rotation were evaluated at 80 degrees (normal sitting position) and 40 degrees of backward inclination (head supine, nose-up). A test H-reflex was used to assess changes in excitability of the soleus alpha-motoneurons after a conditioning stimulus applied to the deep peroneal nerve. In 5 out of 6 subjects, we observed a significant increase in the reciprocal inhibition after backward inclination of the body (40 degrees) with respect to the control position (80 degrees). Such increase was attributed to facilitation of the Ia inhibitory interneurons projecting to the soleus motoneurons. We consider the possibility that the observed increment in reciprocal inhibition after backward inclination be sustained by variations of tonic vestibular activity.