MAGNETIC STIMULATION EXCITES SKELETAL-MUSCLE VIA MOTOR-NERVE AXONS INTHE CAT

Surface magnetic stimulation has been applied directly over skeletal m uscle (triceps surae) in decerebrated cats. Recordings were made of th e twitch contraction and electromyographic responses in triceps surae, and of the centripetal nerve volley in the sciatic nerve or spinal ro ots. Stimulus/response curves were established up to the maximum outpu t of the magnetic stimulator. Neuromuscular blockade abolished the twi tch contraction and muscle action potential leaving the nerve volley u naffected. Raising the stimulator output to its maximum increased the size of the nerve volley but failed to produce any muscle response. We conclude that magnetic stimulation applied directly to skeletal muscl e excites via stimulation of motor nerve axons. The maximum output of the stimulator was incapable of exciting muscle fibers by direct depol arization. Use of magnetic stimulation in the clinical appraisal of mu scle function should be interpreted in the knowledge that the stimulat or elicits contraction only indirectly through nerve stimulation. (C) 1997 John Wiley & Sons, Inc.