Rizzardi, A. | bioRxiv Science

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↗

bioRxiv2025

Adaptations in common synaptic inputs to spinal motor neurons during grasping versus a less functional hand task

Previous evidence suggests that shared synaptic inputs across spinal motor neurons play a key role in coordinating multiple muscles during hand movements, reducing control complexity. In this study, we investigated how the nervous system modulates these common synaptic inputs during a functionally relevant grip (grasping) compared to less functionally relevant hand tasks. Seventeen participants performed three different tasks: simultaneous four-finger flexion without thumb involvement (four-finger flexion), thumb flexion, and simultaneous flexion of both fingers and thumb (grasping). For each task, subjects sustained isometric contractions at 5% and 15% of maximal voluntary contraction, while high-density surface electromyograms (HDsEMG) were recorded from the superficial extrinsic flexor muscles of the hand. Motor unit spike trains were decomposed from HDsEMG and tracked across tasks, and their mean discharge rate was calculated. Coherence between motor units was quantified within the delta, alpha, and beta bands to estimate common synaptic oscillations. At both force levels, the mean discharge rate decreased during grasping compared to four-finger flexion but increased during grasping compared to thumb flexion. Additionally, the area under the curve of coherence within the alpha band decreased by [~]20% during grasping compared to the four-finger flexion task, with no significant delta or beta bands changes. These reductions in alpha band coherence were reflected in force oscillations, showing decreased force-neural drive coupling within the alpha band and increased force steadiness during grasping compared to four-finger flexion. Our findings suggest that a functionally relevant and frequently used grip involves distinct neural control mechanisms that ultimately enhance force control.

neuroscience↗