MAGNETIC STIMULATION OF CORTICOSPINAL PATHWAYS AT THE FORAMEN MAGNUM LEVEL IN HUMANS

Magnetic stimulation done with a double cone coil placed over the back of the head activated descending motor pathways and produced electrom yographic responses in muscles of the arms and legs. The latencies of these responses were the same as those of responses to electrical brai nstem stimulation. The threshold was lowest when the coil was placed o ver the inion or below it on the median line. Placement of the coil on the side ipsilateral to the muscle was more effective than placement on the contralateral side. These results indicate that activation occu rs at the foramen magnum level (just below the pyramidal decussation). Collision experiments that used cortical and magnetic brainstem stimu lation indicated that the major part of the responses to the latter st imulation were conducted via the large diameter component of the corti cospinal tract. Collision experiments done with the peripheral nerve a nd magnetic brainstem stimulation showed that this stimulation produce d a single descending volley in the descending tract. We conclude that magnetic brainstem stimulation produces a single descending volley in the corticospinal tract at the foramen magnum level with less discomf ort.