The influence of stimulus type on the magnetic excitation of nerve structures

The voltage measured was that induced in a measuring coil from 3 different commercially available magnetic stimulators. The strongest stimulus was from the Cadwell, followed by Novametrix and then Digitimer. The Digitimer and Novametrix produced a monophasic pulse, whilst the Cadwell stimulator produced a polyphasic pulse, all measured by an induction coil. This is thought to be the reason why reversed coil polarity does not influence the position of peripheral nerve excitation with a Cadwell stimulator; this is, however, the case with the two other magnetic stimulators. Nevertheless, electrical stimulation was found to be the most useful method for exciting peripheral nerves. The lack of influence of Cadwell coil polarity on the excitation of spinal roots and motor cortex is also thought to be due to the bipolar stimulus effect mentioned above. The stimuli induced by Digitimer and Novametrix are monophasic, exciting one hemisphere first, depending on the direction of the current impulse. The stimulus generated by Cadwell excites both hemispheres by reversal of current direction.