Aim: 

Uncrossed projections from primary motor cortex (M1) are known to contribute to hand and finger movements. Moreover, M1 Transcranial Magnetic Stimulation (TMS) evokes motor evoked potentials (MEPs) in ipsilateral (IL) muscles following a stroke in the controlateral (CL) M1.

In normal individuals, instead, M1 TMS fails to induce ipsilateral MEPs. Since this failure might conceal subliminal effects onto spinal neurones, we explored whether M1 TMS activation will facilitate the H-reflex response in ipsilateral Flexor Carpi Radialis (FCR) motoneurones.

Methods: 

In 6 subjects, TMS was applied to the right M1 over the optimal focus to elicit MEPs in the left FCR. A single pulse TMS (120% of the CL motor threshold, MT) was then used to condition the H-reflex in the right FCR. In 2 subjects, this same effect has been tested when excitability of the left motor cortex was reduced by a TMS-induced cortical Silent Period (cSP).

Results: 

No MEPs were obtained in the relaxed right (IL) FCR, independently on the intensity of the TMS stimulus. However, TMS (120%MT) facilitated the right (IL) FCR H-reflex starting at a conditioning-test interval of +3 ms (i.e. when the H-reflex lagged the TMS by 3 ms), reaching a peak at +5 ms (p < 0.05) and fading out within +7 / +10 ms). A significant amplification of this facilitatory effect was obtained when the test was performed during the cortical silent-period in the left M1.

Conclusion: 

These preliminary data suggest that i) M1 exerts a short latency subliminal facilitatory effect on motoneurones of ipsilateral upper limb distal muscles; and ii) in normal subject, this effect is significantly facilitated during the presumed depression (cSP) of the opposite M1. This last finding would agree with the occurrence of ipsilateral MEPs in subjects with a structural damage of opposite M1.