Proceedings of The Physiological Society

Physiology 2016 (Dublin, Ireland) (2016) Proc Physiol Soc 37, PCB134

Poster Communications

Paired corticospinal-motoneuronal stimulation increases maximal voluntary activation of human adductor pollicis muscle

J. L. Taylor1,2, S. C. Dongés1,2, J. M. D'Amico1

1. Neuroscience Research Australia, Randwick, New South Wales, Australia. 2. School of Medical Sciences, University of New South Wales, Sydney, New South Wales, Australia.

Paired corticospinal-motoneuronal stimulation (PCMS) is a technique in which transcranial magnetic stimulation (TMS) of motor cortex is repeatedly paired with stimulation of motor axons in a peripheral nerve to produce spike-timing-dependent plasticity at corticospinal-motoneuronal synapses in the human spinal cord. TMS evokes presynaptic volleys in corticospinal terminals and antidromic activation of motoneurones supplies postsynaptic depolarisation. Studies have shown that, depending on the interstimulus interval of the paired stimuli, delivery of 50-100 pairs can facilitate or depress both evoked and voluntary motor responses (Taylor & Martin 2009; Bunday & Perez 2012; Fitzpatrick et al 2016). However, it is not known if such stimulation can increase maximal voluntary output. Hence, this study aimed to determine whether PCMS increases maximal voluntary activation of the hand muscle, adductor pollicis. On two days, subjects (n=14) performed isometric maximal voluntary contractions (MVCs) of the thumb adductor before and after PCMS or a control protocol. PCMS comprised 100 pairs of TMS (figure 8 coil, 60-95% stimulator output) and supramaximal ulnar nerve stimulation at 0.1 Hz. Interstimulus intervals (3.3-7.8 ms) were calculated so that presynaptic volleys were estimated to arrive at the synapse 1.5 ms prior to postsynaptic depolarisation. The control protocol comprised TMS at 0.1 Hz. Five brief 2-3 s MVCs were performed (2 min apart) prior to and starting at 0, 20 and 40 min after the intervention. Single supramaximal ulnar nerve stimuli were delivered during and 2-4 s after each MVC to evoke superimposed and resting twitches from adductor pollicis. Voluntary activation (%) was calculated as 100 x (1 - superimposed twitch/resting twitch). Adductor pollicis EMG was also recorded. Changes from baseline were calculated for each parameter and were compared for PCMS and control protocols with 2-way repeated measures ANOVAs (protocol X time [0, 20, 40 min post]). The superimposed twitch was reduced and hence, voluntary activation increased by 4.4% [1.4,7.5] (mean [95%CI]) after PCMS compared to TMS alone (F1,26=10.241, p=0.007). Maximal EMG was non-significantly higher (8.1% [-0.6,16.8], F1,26=4.014, p=0.066) and the resting twitch did not differ (0.1% [-5.7,5.9] smaller). Thus, there was a small improvement in the ability of subjects to drive the muscle maximally after PCMS. This suggests that PCMS-induced changes in the transmission of corticospinal input are effective for high threshold motoneurones as well as the low threshold ones recruited by evoked responses and weak contractions. If PCMS can be applied reliably, it may have therapeutic potential to improve strength in patients with muscle weakness caused by impaired corticospinal drive.

Where applicable, experiments conform with Society ethical requirements