Abstract
Modulation of cortical excitability by sensory inputs is a critical component of sensorimotor integration. Sensory afferents, including muscle and joint afferents, to somatosensory cortex (S1) modulate primary motor cortex (M1) excitability, but the effects of muscle and joint afferents specifically activated by muscle contraction are unknown. We compared motor evoked potentials (MEPs) following median nerve stimulation (MNS) above and below the contraction threshold based on the persistence of M-waves. Peripheral nerve electrical stimulation (PES) conditions, including right MNS at the wrist at 110% motor threshold (MT; 110% MNS condition), right MNS at the index finger (sensory digit nerve stimulation [DNS]) with stimulus intensity approximately 110% MNS (DNS condition), and right MNS at the wrist at 90% MT (90% MNS condition) were applied. PES was administered in a 4 s ON and 6 s OFF cycle for 20 min at 30 Hz. In Experiment 1 (n = 15), MEPs were recorded from the right abductor pollicis brevis (APB) before (baseline) and after PES. In Experiment 2 (n = 15), M- and F-waves were recorded from the right APB. Stimulation at 110% MNS at the wrist evoking muscle contraction increased MEP amplitudes after PES compared with those at baseline, whereas DNS at the index finger and 90% MNS at the wrist not evoking muscle contraction decreased MEP amplitudes after PES. M- and F-waves, which reflect spinal cord or muscular and neuromuscular junctions, did not change following PES. These results suggest that muscle contraction and concomitant muscle/joint afferent inputs specifically enhance M1 excitability.
Highlights
Converging evidence suggests that afferent somatosensory inputs such as peripheral nerve electrical stimulation (PES), muscle tendon vibration and active and passive movements can induce changes in primary motor cortex (M1) excitability (Naito et al, 1999, 2002; Ridding et al, 2000; KaelinLang et al, 2002; Macé et al, 2008; Miyaguchi et al, 2013; Onishi et al, 2013; Kotan et al, 2015)
There were no significant differences in the Peripheral nerve electrical stimulation (PES) intensity between experiments in each of the PES conditions (110% median nerve stimulation (MNS) conditions, P = 0.080; DNS conditions, P = 0.080; 90% MNS conditions, P = 0.890; unpaired t-test)
Sample motor evoked potentials (MEPs) waveforms recorded from a representative subject in each PES condition (Figure 2) demonstrate our basic findings; application of 110% MNS increased the MEP amplitude in response to constant transcranial magnetic stimulation (TMS) output, indicating enhanced M1 excitability, while both DNS and 90% MNS reduced MEP amplitude
Summary
Converging evidence suggests that afferent somatosensory inputs such as peripheral nerve electrical stimulation (PES), muscle tendon vibration and active and passive movements can induce changes in primary motor cortex (M1) excitability (Naito et al, 1999, 2002; Ridding et al, 2000; KaelinLang et al, 2002; Macé et al, 2008; Miyaguchi et al, 2013; Onishi et al, 2013; Kotan et al, 2015). Previous studies have reported changes in motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) over the M1 after a prolonged period of PES (Ridding et al, 2000; Fraser et al, 2002; Tinazzi et al, 2005; Chipchase et al, 2011b) These MEP alterations have been proposed depending on the intensity (Chipchase et al, 2011a; Schabrun et al, 2012), frequency (Mang et al, 2010; Golaszewski et al, 2012), and duration (Andrews et al, 2013) of PES. The mechanism underlying the alteration in excitability in the M1 after PES remains poorly understood
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