Aguiar dos Santos, M. | Search bioRxiv

bioRxiv2026

Bandpass corticospinal transmission during repetitive TMS revealed by motor unit recordings

We employed a noninvasive high-density surface electromyography (HDsEMG) framework to track spinal motor neuron responses during repetitive transcranial magnetic stimulation (rTMS) and characterize corticospinal transmission of different stimulation frequencies and intensities to the alpha motor neuron pool. Eleven healthy individuals performed isometric thumb flexion at 10% of maximal voluntary contraction while rTMS was delivered over the motor cortex at five frequencies (5, 10, 20, 30, and 50 Hz) and three subthreshold intensities (50%, 60%, and 70% of resting motor threshold). Motor units were decomposed from HDsEMG signals before stimulation and tracked during rTMS. Input-output coupling was quantified using coherence between the rTMS train and individual motor unit spike trains or the cumulative spike train (CST), with shuffled spike trains used as surrogate controls. rTMS inputs were robustly transmitted to spinal motor neurons for all frequencies except 5 Hz, indicating widespread corticospinal coupling. Transmission behaved linearly, with CST output spectra reproducing input frequencies and scaling proportionally with stimulation intensity. The estimated transfer function revealed a bandpass-like profile, with maximal transmission between 10 and 60 Hz. Transmitted inputs also induced oscillatory components in the common synaptic input to motor neurons at stimulation frequency. Simulations indicated that this frequency selectivity emerges from balanced excitatory and inhibitory inputs to the motor neuron pool, with specific synaptic dynamics. These findings demonstrate that corticospinal transmission during rTMS acts as a frequency-selective linear system and provide a framework for assessing and modulating corticospinal pathways, with potential application as tool for tracking disease progression and neurorehabilitation. Highlights- HDsEMG decomposition tracks spinal motor neuron activity during rTMS. - Corticospinal transmission scales with rTMS stimulation intensity. - Corticospinal pathways act as a frequency-selective system. - rTMS transfer function shows maximal transmission at 10-60 Hz. - EPSP-IPSP interactions explain bandpass corticospinal transmission during rTMS.

neuroscience↗

bioRxiv2026

Neural and mechanical properties of vastus lateralis and vastus medialis at different rectus femoris muscle lengths

This study investigated how altering the length of one muscle influences motor unit discharge behavior of its synergists. Eighteen participants performed isometric knee extensions at 10% and 30% of maximal voluntary contraction (MVC) with the hip joint at 90{degrees} (shortened rectus femoris, RF) and 180{degrees} (lengthened RF). High-density surface electromyograms were recorded from vastus medialis, RF and vastus lateralis, and decomposed into motor unit spike trains. Mean discharge rate and coefficient of variation of interspike interval were analyzed for tracked units in the vasti and non-tracked in the RF. While no changes were observed in RF motor units, lengthening the RF led to increased discharge rates of vasti motor units at 10% MVC, but not 30% MVC. To further explore these force-dependent changes, two further experiments were conducted. The first showed that the discharge rate at recruitment during ramp-up contractions increased with RF lengthening, but only for low-threshold vasti units. In the second, electrically evoked twitch contractions in the vasti revealed significantly reduced twitches at 180{degrees} during low-frequency, but not high-frequency stimulation. These findings collectively suggest that the force-dependent changes in the vasti motor unit discharge rates are likely driven by RF-length dependent changes in the vasti muscles contractile properties.

physiology↗