Dissociation of the pathways mediating ipsilateral and contralateral motor‐evoked potentials in human hand and arm muscles

Abstract

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. 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. 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. The corticofugal pathways of ipsilateral and contralateral MEPs could be dissociated through differences in cortical map location and preferred stimulating current direction. 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. Large ipsilateral MEPs were obtained in a patient with complete agenesis of the corpus callosum. 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.

Mapping the cortical representation of the lumbar paravertebral muscles

“…Ipsilateral responses had a longer latency than contralateral responses of around 4 msecs. These findings are in agreement with those of Ziemann et al (1999) who demonstrated slower ipsilateral responses in upper limb muscles obtained more prominently from sites lateral to the optimal stimulation site for contralateral responses.…”

“…As with the findings of Ferbert et al (1992) in the erector spinae and Ziemann et al (1999) in hand and arm muscles, ipsilateral responses were evoked when stimulating at lateral sites. The midline ipsilateral responses seen using the original mapping protocol may be the result of unintentional stimulation of the opposite cortex since they disappear when mapping using a sagittal coil orientation.…”

“…Stimulation was applied with the coil handle oriented 45 degrees from the sagittal plane so that the induced current flowed in an anteromedial direction as suggested by Eguchi et al (1995) and Ziemann et al (1999). Ferbert et al(1992) used 100% of maximum stimulator output to elicit responses from these muscles but reduced the stimulus intensity in subjects with readily indentifiable responses.…”

Stabilization of bimanual coordination due to active interhemispheric inhibition: a dynamical account

“…The inhibitory influences of M1 on the activation of the ipsilateral hand, however, may to a significant extent be relayed (i.e., timed) below the cortex. This relay probably includes multiple (active) levels along 107 the neuroaxis-like ipsilateral oligosynaptic pathways and corticoreticulo-or corticopropriospinal projections (Ziemann et al 1999;Leocani et al 2000). Irrespective of the exact location of the postulated inhibitory mechanisms, the proposed model indicates that an eventual loss of inhibition may result in instabilities and phase transitions or, more generally, unintended motor output (Armatas et al 1994;Leinsinger et al 1997;Daffertshofer et al 1999).…”

Ipsilateral versus contralateral cortical motor projections to a shoulder adductor in chronic hemiparetic stroke: implications for the expression of arm synergies

“…Ipsilateral MEPs can be elicited in selected arm muscles in non-impaired adults when TMS is applied at high intensity and there is background activation of the target muscle (Carr et al 1994;Colebatch et al 1990;MacKinnon et al 2004;Ziemann et al 1999). Ipsilateral MEPs are more easily elicited in proximal than distal muscles, however, even when present, the contralateral projections are much stronger (Bawa et al 2004).…”