U. Ziemann et al., Dissociation of the pathways mediating ipsilateral and contralateral motor-evoked potentials in human hand and arm muscles, J PHYSL LON, 518(3), 1999, pp. 895-906
1. Growing evidence points toward involvement of the human motor cortex in
the control of the ipsilateral hand. We used focal transcranial magnetic st
imulation (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 fing
er and wrist extensors and the biceps, but no response or inhibitory respon
ses were observed in the opponens pollicis, finger and wrist flexors and th
e triceps.
3. The production of ipsilateral MEPs required contraction of the target mu
scle. The threshold TMS intensity for ipsilateral MEPs was on average 1.8 t
imes 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 sti
mulating 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, t
he 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 cro
ssed corticomotoneuronal system can be activated by TMS. Our data suggest a
n ipsilateral oligosynaptic pathway, such as a corticoreticulospinal or a c
orticopropriospinal projection as the route for the ipsilateral MEP. Other
pathways, such as branching of corticomotoneuronal axons, a transcallosal p
rojection or a slow-conducting monosynaptic ipsilateral pathway are very un
likely or can be excluded.