Magnetic stimulation of corticospinal pathways at the foramen magnum level in humans

Abstract

Magnetic stimulation done with a double cone coil placed over the back of the head activated descending motor pathways and produced electromyographic responses in muscles of the arms and legs. The latencies of these responses were the same as those of responses to electrical brainstem stimulation. The threshold was lowest when the coil was placed over 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 occurs at the foramen magnum level (just below the pyramidal decussation). Collision experiments that used cortical and magnetic brainstem stimulation indicated that the major part of the responses to the latter stimulation were conducted via the large diameter component of the corticospinal tract. Collision experiments done with the peripheral nerve and magnetic brainstem stimulation showed that this stimulation produced 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 discomfort.

Laterality Differences in Cerebellar–Motor Cortex Connectivity

“…To avoid potential artifacts caused by antidromic stimulation of the pyramidal tract itself (Fisher et al 2009), we first assessed the brainstem threshold. To this end, we searched for MEPs by stimulating directly at the midline (over the inion) with a double cone coil (diameter 110 mm) with the stimulator current directed downward (Ugawa, Uesaka et al 1994). We asked subjects to preactivate both their left and right FDI by lifting the index finger, and searched for a stimulation intensity that evoked MEPs in either hand in 5 out of 10 pulses, with a maximal intensity cutoff of 80% of stimulator output (MSO).…”

Effects of sex, height and age on motor evoked potentials with magnetic stimulation

“…The eight-shaped coil consists of two wings with an inner diameter of 5 cm and an outer diameter of 9 cm. The double cone coil consists of two loops with a diameter of 11 cm, and the planes of the two loops are set at an angle of about 110° [24]. The stimulus intensity was set at 90% of the maximal output.…”

“…Two different types of magnetic coils were used: an eight-shaped coil (YM-111B, Nihon Kohden) in which the direction of the maximal induced current occurs beneath the central contiguous segment of the coil, and a double cone coil (Magstim Co. Ltd., type 9902) that can induce an eddy current in a deep part of the brain [24]. The eight-shaped coil consists of two wings with an inner diameter of 5 cm and an outer diameter of 9 cm.…”

“…To stimulate the leg motor area, the double cone coil was positioned over the vertex [24]. Magnetic stimulation to lumbar roots was performed with the subject in a prone position, and the centre of the eight-shaped coil was placed over the 4th lumbar spinous process.…”

Changes in muscle afferents, motoneurons and motor drive during muscle fatigue

“…Their size is in¯uenced by excitability of cortical and corticospinal neurons, as well as by the state of the motoneurons and ®nally by the muscle ®bre action potential. The axons of the corticospinal tract (and probably other descending tracts) can be stimulated directly by an electrical pulse passed between the mastoid bones or a magnetic shock over the back of the head (Ugawa et al 1991(Ugawa et al , 1994Gandevia et al 1999). Given the relatively strong monosynaptic corticospinal connections to many motoneuron pools in human subjects (Palmer and Ashby 1992) and the lack of presynaptic inhibition at these synapses (Nielsen and Petersen 1994), this stimulus gives a very direct access to the motoneurons.…”