Percutaneous electrical stimulation of corticospinal pathways at the level of the pyramidal decussation in humans

Abstract

Stimulation over the base of the skull can activate descending motor pathways to produce electromyographic (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.

Double-Pulse Magnetic Brain Stem Stimulation: Mimicking Successive Descending Volleys

“…We also showed that the latency of MEP to the second pulse of double BST is the same as that to single BST in each healthy subject. Previous studies (Ugawa et al 1991(Ugawa et al , 1994 reported that the MEP latency to single BST was the same in both the active and the relaxed conditions because BST induces a single descending volley, irrespective of the state of muscle activation. For double BST, the MEP latency from the time of second BST is identical to the MEP latency to single BST.…”

“…For double BST, the MEP latency from the time of second BST is identical to the MEP latency to single BST. The identical latencies are explainable by a single descending volley induced by either type of BST (Ugawa et al 1994). Therefore double BSTs in the relaxed condition are applicable to clinical practice by comparing them with normal values of MEP latency to single BSTs.…”

“…Our present finding-that the 2-ms interval led to maximal MEP enhancement-is consistent with results of these previous studies. The BST produces a single descending volley in the corticospinal tract axons (Ugawa et al 1994). Then no intervening synapse is present until the volley reaches the spinal MNs.…”

“…We previously developed methods to activate the descending motor tracts at the level of the pyramidal decussation (foramen magnum) using electrical (Ugawa et al 1991) and magnetic stimulation (Ugawa et al 1994). The methods have been shown to be clinically useful for localizing corticospinal tract lesions in patients with neurological disorders (Ugawa et al 1992(Ugawa et al , 1995(Ugawa et al , 1996.…”

“…This technique is called brain stem stimulation (BST) and is usually performed in clinical practice using magnetic stimulation instead of electrical stimulation because the former can be done with less discomfort (Ugawa 1999(Ugawa , 2002. However, we often fail to elicit discernible motor-evoked potentials (MEPs) in patients with corticospinal tract lesions by magnetic BST, and even in some normal subjects (Ugawa et al 1994). For that reason, a powerful method is needed to obtain MEPs to BST even in patients in whom no MEP is elicited by single-pulse magnetic BST.…”