Evidence for Renshaw cell-motoneuron decoupling during tonic vestibular stimulation in man.

The influence of static head-body tilts in the sagittal plane on the activity of Renshaw cells coupled to the soleus extensor alpha-motoneurons was studied in eight human subjects. Head-body rotation was carried out using a tilting seat and its effect was evaluated at 80 degrees (normal sitting position) and at 40 degrees of backward inclination (nose-up). Renshaw cell activity was assessed through a specially designed method of paired H-reflexes first described by Bussel and Pierrot-Deseilligny. alpha-Motoneuron excitability was also independently studied by mapping a reference H-reflex amplitude as a function of static head-body displacements. In almost all subjects Renshaw cell activity was increased at 40 degrees backward inclination with respect to control values at 80 degrees. These changes were attributed to the tonic labyrinthine reflexes capable of decoupling Renshaw cell activity from their motoneurons when the body was tilted backward from the upright position. We discuss the hypothetical functional modalities of the recurrent inhibitory circuit during postural adjustments elicited by labyrinthine input.