Date Published: April 7, 2017
Publisher: Public Library of Science
Author(s): Shinya Suzuki, Tsuyoshi Nakajima, Shun Irie, Ryohei Ariyasu, Tomoyoshi Komiyama, Yukari Ohki, Maurice J. Chacron.
It is unclear how descending inputs from the vestibular system affect the excitability of cervical interneurons in humans. To elucidate this, we investigated the effects of galvanic vestibular stimulation (GVS) on the spatial facilitation of motor-evoked potentials (MEPs) induced by combined pyramidal tract and peripheral nerve stimulation. To assess the spatial facilitation, electromyograms were recorded from the biceps brachii muscles (BB) of healthy subjects. Transcranial magnetic stimulation (TMS) over the contralateral primary motor cortex and electrical stimulation of the ipsilateral ulnar nerve at the wrist were delivered either separately or together, with interstimulus intervals of 10 ms (TMS behind). Anodal/cathodal GVS was randomly delivered with TMS and/or ulnar nerve stimulation. The combination of TMS and ulnar nerve stimulation facilitated BB MEPs significantly more than the algebraic summation of responses induced separately by TMS and ulnar nerve stimulation (i.e., spatial facilitation). MEP facilitation significantly increased when combined stimulation was delivered with GVS (p < 0.01). No significant differences were found between anodal and cathodal GVS. Furthermore, single motor unit recordings showed that the short-latency excitatory peak in peri-stimulus time histograms during combined stimulation increased significantly with GVS. The spatial facilitatory effects of combined stimulation with short interstimulus intervals (i.e., 10 ms) indicate that facilitation occurred at the premotoneuronal level in the cervical cord. The present findings therefore suggest that GVS facilitates the cervical interneuron system that integrates inputs from the pyramidal tract and peripheral nerves and excites motoneurons innervating the arm muscles.
Anatomical and electrophysiological studies in animals have suggested that the interneurons (INs) located in the cervical cord integrate the vestibular signals related to altered head position in space and the motor signals related to forelimb movement [1–3]. A variety of INs in the cat cervical cord receive synaptic inputs from vestibular afferents [2, 4]. Based on the anatomical locations of their cell bodies in the grey matter and their caudally projecting axons, some INs are considered propriospinal neurons (PNs) that regulate the vestibular reflexes of the fore- and hind limbs . Furthermore, PNs receive pyramidal tract inputs [6, 7]. Therefore, the vestibular system might communicate with the cervical IN system that conveys outputs from the pyramidal tract to the motoneurons innervating the arm muscles.
The present study demonstrated that GVS significantly increased the ulnar-induced facilitation of MEPs in the BB. Moreover, we substantiated that GVS explicitly increased the effects of the combined stimulation on the MU firing probability with recordings of single MUs. The increase in the firing probability was consistently seen ~ 1 ms after the onset of TMS-induced excitation in most MUs.