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Abstract

Stimulation over the base of the skull can activate descending motor pathways to produce electromyograpnic (EMG) responses in muscles of the arm and leg. The evoked EMG responses were larger when the muscles were preactivated by a small voluntary contraction compared to when they were completely relaxed. The latency of these responses in preactivated muscles was approximately midway between that produced by electrical stimulation over the motor cortex, and by electrical stimulation over the cervical enlargements. With horizontally spaced electrodes, the latency difference between cortical and brainstem stimulation was 1.8 milliseconds in all muscles tested. The latency difference between cervical and brainstem stimulation was 3.9 milliseconds for the first dorsal interosseous and 2.6 milliseconds for tibialis anterior muscles. These values suggest that brainstem stimulation occurs at the level of the cervicomedullary junction. With vertically spaced electrodes in the midline, stimulation often occurs at a higher level. The EMG responses from brainstem stimulation differed from those following cortical stimulation in two ways: (1) They were simpler in form, and (2) their onset latency was the same in active as it was in relaxed muscles. This suggests that brainstem stimulation evoked a large descending motor volley in comparison with the multiple volleys that cortical stimulation can produce. Collision experiments between cortical and brainstem volleys indicated that the major part of the responses evoked by brainstem stimulation were conducted via the large-diameter component of the corticospinal tract.