Lateralized alpha-motoneuron excitabilities during lying and standing of healthy individuals in relation to Parkinsonian rigidity

Abstract Objectives: To elucidate mechanisms of Parkinsonian rigidity by assessing excitability of alpha-motoneurons innervating right and left soleus muscles in healthy controls and Parkinson’s disease (PD) patients with rigidities in the right, left and both legs. Methods: One group of 45 controls was recruited and 60 PD patients in three groups: rigidities, predominantly in the right, left and both legs. H-reflex (H) and muscle response (M) were recorded from right and left soleus muscles during stimulations of the posterior tibial nerve at the popliteal fossa while lying and standing. The H/M ratio was taken as an index for motoneuron excitability. Results: Mean H/M ratios were significantly different on the right and left sides, modified by postural changes in controls and PD patients. Analysis of variance showed that in healthy subjects the H/M ratio was: standing>lying (right), lying>standing (left). In right leg rigidity patients, the H/M ratio was greatest during standing, and smallest during lying. In left leg rigidity patients, the H/M ratios on the right and left sides were equally independent of posture. In controls, left H/M>right while lying, <right while standing. In right leg rigidity patients, right H/M>left, but <right in left leg rigidity patients, independent of posture. There was no side difference in patients with rigidity in both legs. Conclusions: (i) motoneuron excitability may show side and postural differences in healthy individuals and PD patients; (ii) posture may be associated with lateralized motoneuron excitability in these subjects; and (iii) Parkinsonian rigidity may have spinal motor origins.