MAGNETIC STIMULATION OF THE DESCENDING AND ASCENDING TRACTS AT THE FORAMEN MAGNUM LEVEL

To test the possibility that stimulation over the foramen magnum activ ates ascending tracts as well as descending tracts, we studied 4 patie nts with myoclonic epilepsy all of whom had enhanced cortical long loo p reflexes (LLRs) and 10 normal subjects, using our previously reporte d method (Ugawa et al., Ann. Neurol., 1994, 36:618-624). For latency c omparisons, peripheral nerve stimulation at the elbow and spinal motor root were also performed. In all patients, magnetic stimulation at th e foramen magnum consistently elicited long loop reflexes as well as d irect responses caused by stimulation of the descending tracts. In con trast, no LLRs were ever seen in any normal subjects. The latencies of both types of response were the same whether stimulation used upward or downward current in the brain, although the former was always more effective. This indicates that stimulation at the level of the foramen magnum activates ascending tracts as well as descending tracts at a f ixed position. The threshold for LLRs was lower than that for activati on of the descending tracts. This threshold difference is compatible w ith the hypothesis that large diameter fibers from muscle afferent con duct the central afferent volley for LLRs (Marsden et al., Brain, 1977 , 100:185-200). (C) 1997 Elsevier Science Ireland Ltd.