Could parkinsonian akinesia be attributable to a disturbance in the motor preparatory process?

To test the hypothesis that parkinsonian akinesia could be due to a disturbance in motor preparatory process, we measured the extent to which the normal pattern of H-reflex excitability prior to a ballistic movement, in a simple reaction time (RT) paradigm, is modifiable by akinesia. Nine age-matched normals and 11 parkinsonians were examined. They were instructed to plantarflex their ankle rapidly in response to a visual signal (RS) following a bleep (WS), while EMGs were recorded from soleus and tibialis anterior (TA) simultaneously with ankle position. Under this 'control' condition: mean soleus EMG and movement RTs were significantly longer in parkinsonians than normals. Given no evidence of fatigue or other effects, this finding indicated that our patients were akinetic. Furthermore, the normal pattern of agonist-antagonist bursts was often delayed, reduced or prolonged in these patients. Next, we measured changes in soleus motoneuronal pool excitability at 4 predetermined intervals relative to the RS but prior to EMG onset, by means of H-reflex testing. Three findings emerged: mean soleus EMG and movement RTs were again significantly delayed in parkinsonians. However, no intersensory facilitation/inhibition of the RTs occurred between H-reflex and visual stimuli, in that these values remained unchanged within the group, despite the addition of H-test stimulation. More importantly, facilitation of H-reflex was similarly time-locked to EMG onset for the two groups, with increases in H-reflex amplitude commencing some 60 ms prior to agonist discharge. Bearing in mind the prolonged RTs in parkinsonians, these findings pointed to a delay in the facilitation of the H-reflex relative to the RS as a cause of akinesia. Our proposition that parkinsonian akinesia could be attributable to an impairment in the motor preparatory process therefore remains a tantalizing possibility.