[Changes in magnetic coil orientation affect the stimulation effects of human peripheral nerve]

Transcranial magnetic stimulation is a non-invasive method for assessing the motor function in humans. It is well established that orientation of the magnetic coil plays an important role on the stimulation effects of the motor cortex. However, there has been a few study regarding the effect of magnetic coil orientation on the efficacy of peripheral nerve stimulation. Therefore, in the present study, I carried out two experiments to clarify the relationship between the magnetic coil orientation and the stimulation effects of human peripheral nerve. First, median nerve was stimulated at the elbow and compound muscle action potentials (CMAPs) of abductor pollicis brevis muscle were recorded in 8 subjects at 4 different directions of the induced current. Two eight-shaped coils, 10 cm and 3.5 cm in outer diameter, were used. Stimulus intensities of big and small coils were 850 V and 90%, respectively. Motor threshold was measured at 4 directions with the big coil by increasing stimulus intensity up to 850 V from 150 V at a step of 50 V. It was found that the amplitude of the CMAP was the greatest in outer direction (toward the radial side) using either the big coil or the small coil. There was no significant difference between the amplitude in outer direction and that of supramaximal electrical stimulation. Motor threshold was the lowest in outer direction. Then, I measured the induced current of the big and small coils in a tank filled with saline that mimicked the forearm. The induced currents by both big and small coils were the largest and the first spatial derivatives of the induced electric field were the greatest in outer direction. These results suggest that the orientation of the eight-shaped coil is important for peripheral nerve magnetic stimulation. The fact that the forearm is a restrictive volume conductor may result in the different effects of coil orientation on the excitement of the peripheral nerve.