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Dissociation of the pathways mediating ipsilateral and contralateral motor‐evoked potentials in human hand and arm muscles
Author(s) -
Ziemann Ulf,
Ishii Kenji,
Borgheresi Alessandra,
Yaseen Zaneb,
Battaglia Fortunato,
Hallett Mark,
Cincotta Massimo,
Wassermann Eric M.
Publication year - 1999
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1999.0895p.x
Subject(s) - transcranial magnetic stimulation , anatomy , medicine , motor cortex , wrist , biceps , corpus callosum , evoked potential , neuroscience , reflex , inhibitory postsynaptic potential , pyramidal tracts , hand muscles , corticospinal tract , stimulation , physical medicine and rehabilitation , psychology , magnetic resonance imaging , anesthesia , audiology , radiology , diffusion mri
1 Growing evidence points toward involvement of the human motor cortex in the control of the ipsilateral hand. We used focal transcranial magnetic stimulation (TMS) to examine the pathways of these ipsilateral motor effects. 2 Ipsilateral motor‐evoked potentials (MEPs) were obtained in hand and arm muscles of all 10 healthy adult subjects tested. They occurred in the finger and wrist extensors and the biceps, but no response or inhibitory responses were observed in the opponens pollicis, finger and wrist flexors and the triceps. 3 The production of ipsilateral MEPs required contraction of the target muscle. The threshold TMS intensity for ipsilateral MEPs was on average 1.8 times higher, and the onset was 5.7 ms later (in the wrist extensor muscles) compared with size‐matched contralateral MEPs. 4 The corticofugal pathways of ipsilateral and contralateral MEPs could be dissociated through differences in cortical map location and preferred stimulating current direction. 5 Both ipsi‐ and contralateral MEPs in the wrist extensors increased with lateral head rotation toward, and decreased with head rotation away from, the side of the TMS, suggesting a privileged input of the asymmetrical tonic neck reflex to the pathway of the ipsilateral MEP. 6 Large ipsilateral MEPs were obtained in a patient with complete agenesis of the corpus callosum. 7 The dissociation of the pathways for ipsilateral and contralateral MEPs indicates that corticofugal motor fibres other than the fast‐conducting crossed corticomotoneuronal system can be activated by TMS. Our data suggest an ipsilateral oligosynaptic pathway, such as a corticoreticulospinal or a corticopropriospinal projection as the route for the ipsilateral MEP. Other pathways, such as branching of corticomotoneuronal axons, a transcallosal projection or a slow‐conducting monosynaptic ipsilateral pathway are very unlikely or can be excluded.