Spinal motor neurons serve as the link between the nervous system and muscles. As the final common pathway of the neuromuscular system, they receive inputs from both higher-level controllers and afferent pathways. It is often assumed that spinal motor neurons are primarily driven by continuous common inputs (cCI) within different frequency bands. Within this framework, the motor neuron pool behaves as a linear amplifier of the cCI. This implies that the frequency content of descending and spinal oscillatory signals is preserved and faithfully transmitted to the muscles; thus, the spectral content at the output of the motor neuron pool corresponds to that of the cCI. However, this framework overlooks the possibility that motor neurons could also be driven by impulsive common inputs (iCI), which can induce synchronization among them and disrupt the linear transmission of other synaptic inputs at the pool level. To test this hypothesis, computational simulations and experimental data from two different human muscles were used to characterize different aspects related to motor neuron spiking synchronization at the pool level. Our findings suggest that, indeed, iCI can account for relevant features observed in experimental data such as the presence of synchronization events at the pool level. We also observed that such impulsive inputs can affect the linearity in the transmission of cCI by the motor neuron pool. This study represents pioneering indirect evidence of the existence of iCI as inputs to motor neurons.Key points The current understanding of the motor control of voluntary movements assumes a continuous control, driven by oscillatory common signals. Some aspects of motor unit pool behaviour (particularly in terms of spiking synchronization and spectral content) typically observed in experimental recordings cannot be reproduced in simulations that only use continuous common inputs (cCI) to motor neurons. This study provides evidence indicating that spinal motor neurons receive a portion of their synaptic input in the form of impulsive common inputs (iCI) that synchronize their activity. The study also shows how such iCI can affect the linear transmission of other cCI by the motor neuron pool. These findings constitute a fundamental paradigm shift in the understanding of motor control and impact the development of interfaces that extract information from the activity of spinal motor neurons.

Background Post-COVID fatigue (pCF) affects 2.3% of the UK population and causes physical and mental fatigue, impacting daily life. Management is largely adapted from chronic fatigue syndrome, with limited evidence in post-COVID populations. Fatigue in pCF is linked to dysfunction in central, peripheral, and autonomic nervous systems, suggesting a role for vagus nerve dysfunction. Transcutaneous auricular vagus nerve stimulation (taVNS) is a non-invasive, home-based intervention, but its effectiveness in pCF remains unclear. Methods PAuSing-pCF was a single-site, single-blind, randomised, sham-controlled trial in adults with pCF. Participants were assigned to active non-invasive vagus nerve stimulation (taVNS), sham tragus, or active pinna stimulation for 8 weeks, then all crossed over to taVNS for another 8 weeks. The primary outcome was change in Fatigue Visual Analogue Scale (F-VAS) at 8 weeks. Analyses used an intention-to-treat basis, applying regression and mixed-effects models, with additional complier average causal effect (CACE) analyses. Secondary outcomes included patient-reported questionnaires and neurophysiological measures from wearable devices. Results Of 114 participants (taVNS n = 39; sham n = 36; placebo n = 39), 90 completed the trial. Fatigue decreased over time across all groups. At 8 weeks, F-VAS change did not differ between taVNS and controls (sham vs taVNS: 2.76, 95% CI −5 to 11, p = 0.50; placebo vs taVNS: 2.05, 95% CI −6 to 10, p = 0.62), with similar results in CACE analyses. In the taVNS group, regression analyses showed associations between fatigue and baseline neurophysiological measures at 16 weeks, but not 8 weeks. Extending taVNS to 16 weeks yielded no further improvement. Among secondary outcomes, only the fatigue impact scale score showed a significant effect at 8 weeks, with higher fatigue in placebo than taVNS. Conclusions Non-invasive vagus nerve stimulation did not significantly improve fatigue at 8 weeks compared to sham or placebo. Further research is needed to clarify mechanisms and identify subgroups who may benefit from neuromodulation in pCF.

Upper limb postural tremor is common in both Spinocerebellar Ataxia type 12 (SCA12) and Essential Tremor (ET). In the early stages, their clinical presentation can be similar, leading to potential misdiagnosis. This study aimed to develop and evaluate a low-cost device and methods to quantify 2D reaching kinematics objectively, with the goal of improving differentiation between ET and SCA12 using machine learning (ML) models. We recruited 48 participants, including 16 SCA12 patients, 16 ET patients, and 16 healthy controls (HC). Clinical severity was scored using TETRAS and SARA scales. A Raspberry Pi-based device was used to record upper limb reaching during a button-press task. The videos were processed with DeepLabCut and six 2D kinematic parameters were extracted via MATLAB scripts. We expanded the dataset using Generative Adversarial Networks (GAN). Classification was performed using multiple ML algorithms, with and without GAN-based data augmentation. Both patient cohorts showed significant alterations in kinematic features compared to HC, with SCA12 demonstrating more pronounced abnormality. Kinematic parameters including maximum reaching speed, endpoint precision, and time fraction to peak speed emerged as potential markers for differentiating ET and SCA12. The ML models demonstrated high classification performance in distinguishing HC from patients (CA 88.9%) and SCA12 from ET (CA 83.3%) using original data. The device-guided reaching kinematics provide objective markers of motor impairment. Combined with ML, this approach could enhance diagnostic accuracy; it also has potential for remote monitoring and as an outcome measure in clinical trials.

Obligate flexor synergies are a defining feature of the hemiparetic phenotype following stroke in humans. Although these intrusive synergies can diminish over time, recovery may plateau, leaving some individuals with movements permanently constrained to synergies. Despite their clinical significance, the neural mechanisms underlying the emergence and persistence of abnormal synergies remain poorly understood. To investigate this mechanistically, three macaque monkeys were trained on a reach and grasp task prior to receiving one of three unilateral lesion types: 1) a focal sensorimotor cortical lesion, 2) a combined sensorimotor cortical and magnocellular red nucleus (RNm) lesion, or 3) a lesion of the internal capsule. Upper limb three-dimensional kinematics and EMG cross correlation were used to measure the intrusion of synergies during in synergy vs out of synergy reaching. A combined RNm and cortical lesion produced weakness but no flexor synergy. A similar-sized cortical lesion generated mild synergies which substantially recovered. By contrast, a large internal capsule lesion produced severe, persistent flexor synergy. Collectively, these findings suggest that the emergence of abnormal synergies is determined by the extent of corticofugal disruption, and their persistence depends on the ability of surviving supraspinal motor pathways to regain selective control over muscle contractions.

Stroke survivors frequently develop the flexor synergy, an obligate cocontraction of shoulder abductors and elbow flexors. The neural substrate has to date proven elusive. Here we trained two healthy female monkeys to generate isometric elbow and shoulder torques to move an on-screen cursor and recorded neuron firing from motor cortical areas and the reticular formation. All regions contained some cells coding for independent contractions about elbow or shoulder. For neurons coding cocontractions, there was a surprising bias: More cells were related to combinations orthogonal to the flexor synergy, e.g., shoulder abduction with elbow extension. We then used threshold microstimulation to examine patterns of muscle activation elicited from the primary motor cortex, reticular formation, and spinal cord in five female monkeys. Only in the spinal cord did microstimulation generate coactivation aligned to the flexor synergy. Our results suggest that primitive spinal circuits are limited to synergistic coactivation, a pattern which perhaps evolved for locomotion. Prehensile reaching movements aligned to these synergies require only limited descending control from the cortex and brainstem. In contrast, reaching orthogonal to the flexor synergy relies heavily on descending drive both to suppress spinal circuits and to sculpt motoneuron activity. The findings suggest that apparently similar reaches in different directions have different neural substrates. After a stroke, loss of descending drive leaves movements limited by spinal motor primitives.

Breathing rhythms influence brain activity, but whether they modulate the excitability of the reticulospinal tract (RST; a key pathway for motor control and recovery after stroke) remains unknown. In this study, we used the StartReact paradigm to examine how respiratory rhythms modulate RST excitability during motor tasks, measuring reaction times across visual, visual-auditory and visual-auditory startling conditions in three arm muscles (first dorsal interosseous, flexor digitorum superficialis and biceps) of healthy adults (n = 13). Reaction times decreased significantly from visual to visual-auditory to visual-auditory startling conditions. Crucially, respiratory-phase transitions, particularly from inspiration to expiration, significantly enhanced RST excitability specifically during startle-evoked responses, with StartReact effects being significantly stronger during respiratory transitions compared with mid-phases (P ≤ 0.011). These findings suggest that respiratory rhythms modulate RST excitability dynamically in a phase- and condition-specific manner. The identification of respiratory transition phases as optimal periods for RST activation could inform new neurorehabilitation strategies, such as respiratory-phase-aligned stimulation, to enhance motor recovery following corticospinal lesions.

INTRODUCTION: Functional gait disorders (FGDs) are characterized by disabling gait and balance impairments, which lead to a significant decline in quality of life. Although recent studies have revealed some neural correlates in functional movement disorders, the findings remain inconsistent and are poorly understood. AIMS: This study aimed to explore the role of cortical inhibitory and excitatory mechanisms in the pathophysiology of FGDs using standard transcranial magnetic stimulation (TMS) paradigms. METHODS: Five patients diagnosed with FGDs based on the Fahn and Williams criteria and 15 age- and sex-matched healthy controls were recruited from neurology and psychiatry outpatient services. Cortical excitability was assessed by recording motor evoked potentials via surface electromyography from the first dorsal interosseous muscle of the dominant hand following TMS to the contralateral motor cortex. Short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) were evaluated using a paired-pulse protocol, with conditioning stimuli at 80% of the resting motor threshold (RMT) and test stimuli at 120% RMT, using interstimulus intervals of 3 ms for SICI and 10 ms for ICF. RESULTS: Patients with FGDs exhibited significantly reduced cortical inhibition compared to healthy controls, as indicated by attenuated SICI responses (116 ± 66% vs. 44 ± 27%, p = 0.045). No significant differences were observed in RMT or ICF between the two groups (p = 1.000 and 0.735, respectively). CONCLUSION: These findings suggest a disruption in intracortical inhibitory mechanisms in patients with FGDs, potentially contributing to their motor symptoms. Such alterations may reflect underlying neurophysiological abnormalities associated with FGDs.

OBJECTIVES:Epidural stimulation of the spinal cord evokes distinct electrophysiologic responses that can be recorded epidurally. Here, we characterized evoked compound action potentials (ECAPs), doublets (secondary or tertiary ECAPs, probably of different physiologic origin from primary ECAPs), evoked synaptic activity potentials (ESAPs), and electromyographic (EMG) signals in preclinical models. Our objective was to clarify the features and distinct physiologic origins of these signals, to advance mechanistic studies and support clinical applications of spinal cord stimulation (SCS) therapy. MATERIALS AND METHODS:Adult male Sprague-Dawley rats (300-440 g) were implanted with two epidural leads (caudal and rostral; each with eight electrodes) and received monopolar, biphasic stimulation (200-μs pulse width) at 2 and 50 Hz, with current increased stepwise to motor threshold. Rhesus macaques (11.5 and 10.2 kg) were implanted with a single 12-electrode epidural lead and stimulated using either tripolar, triphasic pulses at 10 Hz (100 μs) or tripolar, biphasic pulses at 3 Hz (80 μs) up to 3 × ECAP threshold. Recordings were taken from nonstimulating electrodes. RESULTS:ECAPs and EMG signals were recorded across multiple spinal segments in both rats (L1-T7) and macaques (L2-T11). Doublets presented as complex waveforms with multiple negative peaks, two in rats and three in macaques, likely representing distinct ECAPs at T11-T6 in a rat and L1-T11 in macaques. ESAPs, detectable in rats, showed anatomical specificity over the L1/T13 vertebrae, with peak responses at L1. Signal analysis included activation thresholds, amplitudes, latencies, and conduction velocities. CONCLUSIONS:This study outlines electrophysiologic signals evoked by SCS in terms of their waveform, recruitment thresholds, and putative physiologic origins. We propose that to some extent, these signals reflect different aspects of spinal processing and may serve as biomarkers of dysregulated nociceptive pathways, and as indicators of SCS efficacy or potential side effects.

Approximately 97% of patients with amyotrophic lateral sclerosis (ALS) have cytoplasmic mislocalization and aggregation of the ubiquitous nuclear protein, TDP-43. Current rodent models of this disease fail to replicate the progressive motor weakness and characteristic histopathology, possibly because of fundamental neuroanatomical and genetic differences between rodents and humans. In this study, the TDP-43 protein was overexpressed in the motor neuron pool of the brachioradialis muscle unilaterally in two six-year-old female rhesus macaques, using an intersectional genetics approach involving infection with genetically modified adeno-associated virus. Magnetic resonance images demonstrated delayed signal hyperintensities limited to the injected brachioradialis that persisted for 6-7 weeks, consistent with motor neuron degeneration and denervation of the targeted muscle. At post-mortem , the virus-mediated focal protein overexpression event was found to induce widespread deposits of pathological phosphorylated TDP-43 throughout the cervical spinal cord and motor cortex bilaterally, indicating an ALS-like spread of proteinopathy from the transfection site.

Cortical and subcortical lesions to the motor system, as often occur with stroke, typically lead to transitions through a stereotyped upper limb recovery sequence. After initial weakness and loss of dexterity, spasticity and fixed muscle coactivation patterns (synergies) appear. Early work suggested that different features arise from distinct primary motor cortex (M1) subdivisions. Here we investigated this with modern methods, using ischemic lesions of various cortical areas and electrocoagulation lesions of magnocellular red nucleus (RNm) in rhesus monkeys. Nine animals were trained on a reach and grasp task; hand kinematics were assessed with markerless tracking. The proportion of damaged cortical layer V cells in each cortical area was quantified, and corresponding kinematic effects evaluated. Reaching speed showed greater and more persistent reductions with larger lesions to the posterior part of M1 on the gyrus (Posterior Old M1 in Strick’s terminology). Initial increases in trajectory variability were more consistent with greater damage within the central sulcus (New M1); these partially recovered. Lesions involving Anterior Old M1 (Area 4s in Hines’ terminology) had no additive negative effects. An extensive cortical lesion, which combined New and Old M1 with pre-motor and somatosensory cortex damage did not produce a worse or more persistent deficit than lesions limited to M1, suggesting that loss of arm control arose mainly from damage to descending pathways rather than cortico-cortical interactions. Lesions of RNm led to long-lasting slowing of reach, but no increase in variability. Subsequent cortical lesions to Old M1 led to more severe effects, and worse recovery, than without the preceding RNm lesion. This suggests an important neural compensatory role for the rubrospinal tract following cortical damage in monkey, which is not available in humans where the rubrospinal tract is vestigial. None of the lesions investigated led to overt abnormal synergies. The results are consistent with known differences in descending connections from each area: New M1 has fast cortico-motoneuronal output, known to be important for fine motor control (here assessed by trajectory variability); Old M1 has cortico-reticular connections able to activate the reticulospinal tract, important for generating the high forces needed for fast movements.

The latest hypothesis regarding the source of enhanced neural activation from resistance training is the reticulospinal rather than the corticospinal tract, based on invasive animal and emerging human data. The present study employed a six-week isometric resistance training intervention in a randomized controlled design to address this knowledge gap. Thirty-nine healthy, untrained males (age 23 y, sustained contraction group n = 13, explosive contraction group n = 9, control group n = 17) underwent neuromuscular and electrophysiological testing and completed all study requirements. Maximal isometric torque (MVC) and rate of torque development (RTD) were measured during a familiarization session as well as before and after the six-week period. Transcranial magnetic stimulation was used to assess motor-evoked potential (MEP) area and silent period duration while subjects contracted to 10

Accurate and robust recording and decoding from the central nervous system (CNS) is essential for advances in human–machine interfacing. Technologies for direct measurements of CNS activity are limited by their resolution, sensitivity to interference and invasiveness. Motor neurons (MNs) represent the motor output layer of the CNS, receiving and sampling signals from different regions in the nervous system and generating the neural commands that control muscles. Muscle recordings and deep learning decode the spiking activity of spinal MNs in real time and with high accuracy. The input signals to MNs can be estimated from MN outputs. Here we argue that peripheral neural interfaces using muscle sensors represent a promising, non-invasive approach to estimate some of the neural activity from the CNS that reaches the MNs but does not directly modulate force production. We discuss the evidence supporting this concept and the advances needed to consolidate and test MN-based CNS interfaces in controlled and real-world settings. This Perspective argues that neural activity from the central nervous system that is not directly correlated to force production can be sensed via peripheral neural interfaces.

A major consequence of the COVID-19 pandemic has been the emergence of post-COVID syndrome (PCS), and more specifically, post-COVID fatigue (pCF), with an estimated prevalence of ~2%. We previously showed that, compared to healthy controls, people with pCF exhibit changes in muscle physiology, cortical circuitry, and autonomic function. Here we present results from a cohort of people with pCF (N=145), between 12 weeks and 45 months post-infection. We report self-perception of fatigue; objective measures of cortical circuits via transcranial magnetic stimulation and reaction time tasks; peripheral muscle fatigue; and autonomic function such as heart rate variability. Those with pCF persisting >200 days had significantly more fatigue and showed increased cortical excitability, slower reaction times and increased peripheral muscle fatigue compared to those with <200 days of pCF . In pCF, if there is no spontaneous recovery, fatigue worsens, and patients continue to accumulate significant neurophysiologic abnormalities. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This work is independent research jointly funded by the National Institute for Health and Care Research (NIHR) and UK Research and Innovation (UKRI) [COV-LT2- 0022, MR/W004798/1]. The views expressed in this publication are those of the author(s) and not necessarily those of NIHR, The Department of Health and Social Care or UKRI. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Ethics Committee of Newcastle University Faculty of Medical Sciences gave ethical approval for this work I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors

The reticulospinal tract (RST) plays a pivotal role in motor control, especially during recovery after neurological injuries such as stroke and spinal cord injury (SCI). Understanding how RST activity is modulated offers valuable insights into improving motor function recovery. Recent studies have demonstrated that breathing rhythms influence brain activity. This study explores how respiratory rhythms modulate RST excitability during motor tasks, using the StartReact paradigm to examine reaction times (RTs) across visual (VRT), visual-auditory (VART), and visual-auditory startling (VSRT) conditions. We measured RTs in three muscles (first dorsal interosseous, flexor digitorum superficialis, and biceps) in healthy adult participants (n=13, both sexes) performing multi-joint movements. RTs were longest in the VRT condition and significantly decreased when auditory stimuli were added (VART), with further reductions observed in the VSRT condition. Additionally, respiratory phase transitions, particularly from inspiration to expiration (IE), significantly influenced RTs, with the shortest RTs observed during these transitions in the VSRT condition. These findings suggest that RST excitability is dynamically modulated by respiratory rhythms. This modulation of the RST by respiratory phase transitions could inform future neurorehabilitation strategies, such as respiratory-phase-aligned stimulation, to enhance motor recovery following corticospinal lesions. Ultimately, this approach may optimize the timing of interventions, improving outcomes in conditions such as stroke and SCI. Brainstem pathways play a crucial role in motor recovery after stroke, and understanding how these pathways change during recovery is key to optimizing their participation in rehabilitation. This study demonstrates how respiratory rhythms influence these brainstem pathways. Using the StartReact paradigm, we show that muscle response times are faster when transitioning from inspiration to expiration. These findings suggest that the body’s natural breathing rhythms can enhance motor output by activating these pathways. This could inform innovative rehabilitation strategies, such as aligning interventions with specific respiratory phases, to improve motor recovery in stroke and spinal cord injury. Our research highlights the potential for personalized therapies that harness the body’s intrinsic rhythms to optimize recovery.

Synaptic input to the motoneuron pool is altered during fatiguing muscle contractions. In humans, the corticospinal tract is often studied, with equivocal findings regarding its role in the reduction of force. To date, the involvement of the reticulospinal tract during states of fatigue has not been explored. Fourteen participants (28 ± 6 years, nine males) visited the laboratory twice, first for a familiarisation, then for an experimental trial. Participants completed a 5‐min sustained elbow flexor contraction at an intensity eliciting 40% of the EMG recorded during a maximal isometric voluntary contraction (MVC). Before, during and after the contraction, transcranial magnetic stimulation and electrical cervicomedullary stimulation were used to elicit motor evoked potentials (MEPs) and cervicomedullary evoked potentials during the silent period (SP‐CMEPs), respectively, with CMEPs also being evoked in combination with a startling acoustic sound (CMEPcon). Electrical stimulation of the brachial plexus was used to evoke maximal compound action potentials of the elbow flexors ( M max ). The 5‐min contraction induced a 53% loss of force ( P < 0.001), with no change in background EMG (∼4% M max , P = 0.293). Neither MEP amplitude ( P = 0.246) nor CMEPcon ratio ( P = 0.489) was altered during the contraction, whereas CMEP and SP‐CMEP amplitudes were reduced by ∼20% and 50%, respectively ( P < 0.001) and remained depressed post‐task. The results suggest that neither corticospinal nor reticulospinal tract excitability was altered during a 5‐min constant‐EMG task at 40% maximal EMG. Instead, the aetiology of the neural contribution to fatigability appeared to be primarily related to the loss of motoneuron excitability.

Cutaneomuscular reflexes (CMRs) can be recorded in the hand muscle of human subjects after stimulation of a digital nerve. We hypothesized that repeated synchronous stimulation of nerves from two digits may lead to long-term plastic changes in CMR, by the mechanisms of spike-timing–dependent plasticity (STDP). To test this idea, we conducted experiments in 27 healthy human volunteers. After baseline measurement of CMR, one of four 30-min-long stimulation conditions were tested; the CMR was then remeasured. The four conditions were simultaneous index finger and thumb stimulation; asynchronous index finger and thumb stimulation; thumb 5 ms before index finger stimulation; and thumb-only stimulation. Neither the early (E1) nor late excitatory (E2) components of the CMR showed consistent changes after any stimulation condition. The inhibitory (I1) component was slightly reduced in all cases. To understand why paired stimulation did not produce long-term changes, we conducted a further experiment. In this, we measured the CMR in response to simultaneous stimulation of index finger and thumb, compared with a prediction expected if the responses summed linearly. This revealed sublinear summation, possibly indicating partial response saturation after stimulation of only one digit. We argue such a pattern prevents paired stimuli from generating especially reliable and well-timed outputs relative to synaptic inputs in downstream neurons, which is required to produce plasticity by STDP.

Objective. The identification of individual neuronal activity from multielectrode arrays poses significant challenges, including handling data from numerous electrodes, resolving overlapping action potentials and tracking activity across long recordings. This study introduces NeuroNella, an automated algorithm developed to address these challenges.Approach.NeuroNella employs blind source separation to leverage the sparsity of action potentials in multichannel recordings. It was validated using three datasets, including two publicly available ones: (1)in vitrorecordings (252 channels) of retinal ganglion cells from mice with simultaneous ground-truth loose patch data to assess accuracy; (2) a Neuropixel recording from an awake mouse, comprising 374 channels spanning different brain areas, to demonstrate scalability with dense multielectrode configurations inin vivorecordings; and (3) data (32 channels) recorded from the medullary reticular formation in a terminally anaesthetised macaque, to showcase decomposition over long periods of time.Main results.The algorithm exhibited an error rate of less than 1% compared to ground-truth data. It reliably identified individual neurons, detected neuronal activity across a wide amplitude range, and tolerated minor probe shifts, maintaining robustness in prolonged experimental sessions.Significance.NeuroNella provides an automated and efficient method for neuronal activity identification. Its adaptability to diverse dataset, species, and recording configurations underscores its potential to advance studies of neuronal dynamics and facilitate real-time neuronal decoding systems.