Miyazaki, D. | Search bioRxiv

bioRxiv2025

Intensity-dependent corticospinal facilitation by repetitive peripheral magnetic stimulation: Evidence for a major contribution of group I afferents

BackgroundRepetitive peripheral magnetic stimulation (PMS) is increasingly used in neurorehabilitation, yet the optimal stimulation intensity for inducing corticospinal facilitation and the underlying afferent mechanisms remain unclear. We investigated the intensity-dependent effects of repetitive PMS on corticospinal excitability and single motor unit responses, and tested group I afferent contribution. MethodsHealthy participants received repetitive PMS (25 Hz; 2-s ON/2-s OFF) over the extensor carpi radialis (ECR) in a crossover design at 0.9x motor threshold (MT), 1.2xMT, and high intensity sufficient to induce maximal wrist dorsiflexion (mean 1.8xMT). Motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation were recorded from the ECR and flexor carpi radialis (FCR) before and during the intervention (total 15 min). The lasting effects were assessed after 9 min of high-intensity PMS for 50 min. To examine group I afferent contribution, the same high-intensity protocol was applied during upper-arm ischemia after reducing the ECR H-reflex to <10% of baseline. Sensory-motor input characteristics across stimulation intensities were compared using post-stimulus time histograms of ECR single motor unit firings during weak voluntary contraction. ResultsHigh-intensity PMS significantly increased ECR MEPs after 9 min of intervention, whereas 1.2xMT of PMS required 15 min to induce a marked effect. PMS at 0.9xMT did not induce significant MEP changes. Across all intensities, the FCR MEPs remained unaltered. ECR MEPs remained markedly elevated for up to 30 min after 9 min of high-intensity PMS. In contrast, PMS delivered during ischemia produced no MEP enhancement. The motor unit analysis revealed that suprathreshold PMS elicited an early peak in firing probability--consistent with monosynaptic Ia excitation--whose amplitude increased with stimulation intensity, whereas PMS at 0.9xMT produced no discernible peak. ConclusionsRepetitive PMS above MT facilitates corticospinal excitability in an intensity-dependent manner. Facilitation was abolished during ischemia. Together with the presence of a short-latency peak in motor unit firing via a monosynaptic pathway, this finding supports a major contribution of large-diameter muscle afferents, with a substantial Ia component, to PMS-induced corticospinal facilitation.

neuroscience↗

bioRxiv2024

Becker muscular dystrophy mice showed site-specific decay of type IIa fibers with capillary change in skeletal muscle

Becker muscular dystrophy (BMD), an X-linked muscular dystrophy, is mostly caused by an in-frame deletion of DMD. BMD severity varies from asymptomatic to severe, associated with the genotype of DMD. However, the underlying mechanisms remain unclear. We established BMD mice carrying three representative exon deletions: ex45-48 del., ex45-47 del., and ex45-49 del. (d45-48, d45-47 and d45-49), with high frequencies and different severities in the human BMD hotspot. All three BMD mice showed muscle weakness, muscle degeneration, and fibrosis, but these changes appeared at different times for each exon deletion, consistent with the severities obtained by the natural history study of BMD. BMD mice showed site-specific muscle changes, unlike mdx mice, which showed diffuse muscle changes, and we demonstrated selective type IIa fiber reduction in BMD mice. Furthermore, BMD mice showed sarcolemmal neuronal nitric oxide synthase (nNOS) reduction and morphological capillary changes around type IIa fibers. These results suggest that capillary changes caused by nNOS reduction may be associated with the mechanism of skeletal muscle degeneration and type IIa fiber reduction in BMD mice. BMD mice may be useful in elucidating the pathomechanisms and developing vascular targeted therapies for human BMD.

genetics↗