Entrainment of Motoneuron Firing by Deep Brain Stimulation

Deep brain stimulation (DBS) is an established therapy for reducing the symptoms of Parkison's disease. However, how it affects neural inputs responsible for motor control and, more specifically motor unit firing, is not well understood. In this study, we use decomposition of high-density surface electromyographic (HD-EMG) signals to investigate the effect of subthalamic nucleus DBS on motor unit firing in the first dorsal interosseous muscle. Six participants with bilateral DBS of the subthalamic nucleus for Parkison's disease performed isometric index finger abduction at 10%-30% of maximum voluntary force. In 50% of participants, DBS led to partial entrainment of motor unit firing times. Entrainment of motoneuron firing was evident at the stimulation frequency and higher harmonics, and at half the stimulation frequency. Peaks at the stimulation frequency and its harmonics were evident in the power spectra of individual motor unit firing trains and in the cumulative trains at each force level. Using synthetic data, it was verified that the observed peaks in the power spectra were not due to stimulation artifacts. Partial entrainment of motoneuron firing is consistent with stimulation of descending pyramidal tract neurons adjacent to the electrode during DBS. In addition to providing evidence of pyramidal tract activation by DBS at clinical stimulation settings, the results illustrate the potential of HD-EMG as a marker of pyramidal tract activation during DBS to assist with stimulation parameter setting or in closed-loop stimulation systems.Clinical Relevance-The results provide evidence of stimulation of pyramidal tract neurons by DBS at clinical parameter settings.