Somatosensory influence on corticomotor evoked potentials

The clinical utility of corticomotor evoked potentials (CMEPs) as a method of evaluating and monitoring patients with spinal cord disorders is being intensively studied. Relatively few neuronal mechanisms responsible for waveform production are clearly known. Although CMEP components are dependent upon activity carried in descending motor pathways, somatosensory information can influence the basic waveform structure. By stimulating peripheral afferent fibers at varying frequencies, intensities, and trains, two CMEP component groups were identified based on latency. The configuration of the short-latency waveforms was influenced primarily by large-diameter afferents. Long-latency waveforms were altered primarily by small-diameter afferents. The present investigation describes both segmental and suprasegmental modification of CMEP characteristics based on afferent fiber group stimulation. If both motor and sensory systems can be accurately assessed, the clinical applications of CMEPs are considerably enhanced.