Magnetic stimulation of the descending and ascending tracts at the foramen magnum level.
To test the possibility that stimulation over the foramen magnum activates ascending tracts as well as descending tracts, we studied 4 patients with myoclonic epilepsy all of whom had enhanced cortical long loop reflexes (LLRs) and 10 normal subjects, using our previously reported method (Ugawa et al., Ann. Neurol., 1994, 36: 618-624). For latency comparisons, peripheral nerve stimulation at the elbow and spinal motor root were also performed. In all patients, magnetic stimulation at the foramen magnum consistently elicited long loop reflexes as well as direct responses caused by stimulation of the descending tracts. In contrast, 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 fixed position. The threshold for LLRs was lower than that for activation of the descending tracts. This threshold difference is compatible with the hypothesis that large diameter fibers from muscle afferent conduct the central afferent volley for LLRs (Marsden et al., Brain, 1977, 100: 185-200).
Ugawa
Y
Y
Department of Neurology, School of Medicine, University of Tokyo, Japan.
Uesaka
Y
Y
Terao
Y
Y
Hanajima
R
R
Kanazawa
I
I
D000344:Afferent Pathways / Q000502:physiology / Q000503:physiopathology; D004056:Differential Threshold; D004525:Efferent Pathways / Q000502:physiology / Q000503:physiopathology; D004550:Elbow / Q000294:innervation; D004831:Epilepsies, Myoclonic / Q000503:physiopathology; D005539:Foramen Magnum / Q000502:physiology* / Q000503:physiopathology; D006801:Humans; D008280:Magnetics; D009333:Neck; D010525:Peripheral Nerves / Q000502:physiology; D010812:Physical Stimulation; D011930:Reaction Time; D012016:Reference Values; D012018:Reflex / Q000502:physiology; D013126:Spinal Nerve Roots / Q000502:physiology