MAGNETIC STIMULATION OF MUSCLE EVOKES CEREBRAL POTENTIALS BY DIRECT ACTIVATION OF NERVE AFFERENTS - A STUDY DURING MUSCLE PARALYSIS

We tested the hypothesis that magnetic stimulation of muscle evokes ce rebral potentials by causing a muscle contraction that then activates muscle receptors, We measured cerebral evoked potentials accompanying magnetic stimulation of muscle in 3 patients during surgery both befor e and after muscle paralysis with succinylcholine, a depolarizing agen t, The magnetic stimulation was at low intensity (30%) and at a 2/s ra te, The administration of succinylcholine sufficient to produce muscle paralysis did not alter cerebral potentials evoked by either low-inte nsity magnetic stimulation of muscle (gastrocnemius/soleus) or electri cal stimulation of peripheral nerve (tibial nerve). In 1 normal subjec t, the S1 nerve root action potentials conducting at rapid velocity (> 60 m/s) were detected at the S1 foramen with a needle electrode using electrical stimulation of the tibial nerve, However, no S1 nerve root potentials could be identified to magnetic stimulation of muscle that evoked a cerebral potential. We conclude that magnetic stimulation of muscle activates terminal afferents in the muscle to provide the affe rent drive for the cerebral potentials independent of muscle contracti on. The failure to detect the afferent volley in S1 nerve root to magn etic stimulation suggests that only a few afferents are activated or t hat the activation of afferents is temporally dispersed. (C) 1996 John Wiley & Sons, Inc.