Handedness-related asymmetry in transmission in a system of human cervicalpremotoneurones

The possibility was investigated that human handedness is associated with a n asymmetrical cortical and/or peripheral control of the cervical premotone urones (PreMNs) that have been shown to mediate part of the descending comm and to motoneurones of forearm muscles. Heteronymous facilitation evoked in the ongoing voluntary extensor carpi radialis (ECR) electromyographic acti vity (EMG) by weak (0.8 times motor threshold) stimulation of the musculo-c utaneous (MC) nerve was assessed during tonic co-contraction of biceps and ECR. Suppression evoked by stimulation of a cutaneous nerve (superficial ra dial, SR) at 4 times perception threshold in both the voluntary EMG and in the motor evoked potential (MEP) elicited in ECR by transcranial magnetic s timulation (TMS) was investigated during isolated ECR contraction. Measurem ents were performed within time windows or at interstimulus intervals where peripheral and cortical inputs may interact at the level of PreMNs. Result s obtained on both sides were compared in consistent right- and left-hander s. MC-induced facilitation of the voluntary ECR EMG was significantly large r on the preferred side, whereas there was no asymmetry in the SR-evoked de pression of the ongoing ECR EMG. In addition, the suppression of the ECR ME P by the same SR stimulation was more pronounced on the dominant side durin g unilateral, but not during bilateral, ECR contraction. It is argued that (1) asymmetry in MC-induced facilitation of the voluntary EMG reflects a gr eater efficiency of the peripheral heteronymous volley in facilitating PreM Ns on the dominant side; (2) asymmetry in SR-induced suppression of the MEP during unilateral ECR contraction, which is not paralleled by a similar as ymmetry of voluntary EMG suppression, reflects a higher excitability of cor tical neurones controlling inhibitory spinal pathways to cervical PreMNs on the preferred side.