Electrophysiological considerations regarding electrical stimulation of motor cortex and brain stem in humans

To provide information about activation of descending motor pathways in humans, motor evoked potentials were obtained from 16 patients without any sensorimotor deficit, after both cortical and brain stem stimulation.

Total anesthesia was achieved in all patients through intravenous administration. Short trains of one to five electrical pulses were delivered separately to the motor cortex and the brain stem. Compound muscle action potentials were recorded from the contralateral upper extremity. Threshold intensity, stimulus polarity, latencies, and effect of increased stimulus intensity on latencies were analyzed.

The threshold intensity was significantly lower when stimulating the brain stem than when stimulating the cortex. A monophasic anodal stimulus was better for cortical stimulation than for brain stem stimulation. Conversely, a monophasic cathodal stimulus was more effective for brain stem stimulation. The rate of unsuccessful stimulations was higher with brain stem stimulation and with increased stimulation intensity. The variability of latencies was so high that a calculation of the conduction velocity of the motor pathways was not possible.

The results indicate that cortical surface and brain stem stimulation act on different nervous elements. Because of the condensation of motor pathway fibers at the brain stem level, much less stimulus intensity for eliciting compound muscle action potentials was necessary. On the other hand, the higher rate of unsuccessful brain stem stimulations may be caused by a block of conduction at either the anterior horn cell pool or the neuromuscular junction. Thus, for cortical and for brain stem stimulation, different stimulating parameters seemed to be necessary with the patient under general anesthesia.