Influence of different static head-body positions on spinal lumbar interneurons in man: the role of the vestibular system.

The present experiments were made in man with the aim of studying the possible influences of different head-body tilts on the activity of the interneurons Ia, Ib and the Renshaw cells functionally coupled to the soleus alpha-motoneurons. Subjects were seated on a chair, rotable with respect to the vertical axis, and were studied at 80 degrees and 40 degrees to the horizontal. The excitability of the soleus alpha-motoneurons slightly decreased when the body was placed at 40 degrees of backward inclination whereas the Renshaw cell activity showed a reinforcement of inhibition on the same motoneurons. The reciprocal inhibition from the anterior tibial to the soleus muscle increased at 40 degrees of backward inclination with respect to the control values at 80 degrees. Finally, short-latency homonymous facilitation and inhibition showed no significant change in relation to body position. The results indicate that different head-body positions are able to modify the bias of spinal interneurons in man. We discuss the hypothetical role of the vestibular system in producing the effects seen.