Thalamic potentials evoked by motor point stimulation

We studied thalamic potentials elicited by stimulation of the extensor digitorum communis muscle motor point during stereotactic surgery for movement disorders. In 6 patients with Parkinson's disease and 1 with cerebral palsy, muscle afferent-evoked thalamic potentials (METPs) were recorded in the ventral intermediate (Vim) nucleus. METPs consisted of three peaks with an average latency of 11.02 +/- 0.80 ms for initial positive peak (PI), 13.04 +/- 0.58 ms for negative (N), and 14.30 +/- 0.87 ms for later positive peak (PII). These METPs were different from those evoked by median nerve stimulation. No peaks corresponding to the N20 component of the somatosensory evoked potential were recorded from C3 or C4 scalp electrodes. The METP amplitudes in the ventrolateral (VL) nucleus were markedly lower than those in the Vim nucleus. These findings suggest that METPs in the Vim nucleus are responses that occur via the fast conducting group Ia afferent fibers rather than somatosensory afferent fibers.