Recruitment of upper-limb motoneurons with epidural electrical stimulation of the cervical spinal cord

Cervical spinal cord stimulation for treatment of upper limb paralysis: a narrative review

Recent advances in cervical spinal cord stimulation (SCS) have demonstrated improved efficacy as a therapeutic intervention for restoring hand functions in individuals with spinal cord injuries or stroke. Accumulating evidence consistently shows that cervical SCS yields significant improvements in grip force, proximal arm strength and muscle activation, with both immediate and sustained effects. This review synthesizes the evidence that electrical stimulations modulate the spinal and supraspinal organization of uninjured descending motor tracts, primarily the residual corticospinal tract, reticulospinal tract and propriospinal network of neurons, as well as increasing the sensitivity of spinal interneurons at the stimulated segments to these inputs. Additionally, we examine contemporary strategies aimed at achieving more precise patterned stimulations, including intraspinal microstimulation, ventral cord stimulation and closed-loop neuromodulation, and discuss the potential benefits of incorporating cervical SCS into a multimodal treatment paradigm.Level of evidence: V.

Spinal cord ultrasound stimulation modulates corticospinal excitability in humans

BACKGROUND

Low-intensity focused ultrasound offers deep tissue penetration and holds promise as a noninvasive tool for neuromodulating brain circuits. However, its effects on neural activity within the human spinal cord remain largely unexplored.

OBJECTIVE

To investigate the effects of spinal cord ultrasound stimulation (SCUS) on corticospinal excitability by varying acoustic parameters: spatial-peak pulse-average intensity (ISPPA), duty cycle (DC), and pulse repetition frequency (PRF).

METHODS

Two experiments were conducted involving 62 healthy adult participants. In Experiment 1 (N = 36), participants were randomly assigned to either a low-intensity group (2.5 W/cm2, N = 18) or a relatively high-intensity group (10 W/cm2, N = 18) to examine parameter-dependent SCUS effects. SCUS was delivered as a 500 ms pulse train to the C8 spinal cord segment using two DCs (10 % and 30 %) and two PRFs (500 and 1000 Hz). Stimulation was applied concurrently with posterior-anterior (PA) oriented transcranial magnetic stimulation (TMS) over the left primary motor cortex (M1), and motor-evoked potentials (MEPs) were recorded from the right first dorsal interosseous (FDI) muscle. Active and sham SCUS trials were interleaved and compared. In Experiment 2 (N = 26), the specificity of SCUS effects was assessed in relation to TMS coil orientation and target muscle using the sonication parameter set of 10 W/cm2 ISPPA, 1000 Hz PRF, and 30 % DC. MEPs from the FDI were recorded using anterior-posterior (AP) and latero-medial (LM) coil orientations, while MEPs from the abductor digiti minimi (ADM) muscle were assessed using PA, AP, and LM orientations. MEPs were also recorded from the biceps brachii with PA orientation. Active and sham SCUS conditions were interleaved and compared.

RESULTS

SCUS significantly suppressed MEP amplitudes compared to sham stimulation under relatively high-intensity (10 W/cm2), high-PRF (1000 Hz) conditions, regardless of DC. No significant effects were observed at lower intensity (2.5 W/cm2) or lower PRF (500 Hz). Follow-up experiments confirmed consistent inhibitory effects across multiple TMS coil orientations (PA, AP, LM) and in C8-innervated hand muscles (FDI and ADM), with no significant modulation observed in the more rostrally innervated biceps brachii muscle, supporting the specificity of C8-targeted SCUS.

CONCLUSIONS

These findings demonstrate that SCUS can modulate corticospinal excitability in a parameter-specific manner, with suppression observed primarily under relatively high-intensity and high-PRF conditions. SCUS represents a promising noninvasive technique for targeted neuromodulation of spinal neural circuits in humans.

Efficient, Robust, and Accurate CNN Predictor for Neuronal Activation in Directional Deep Brain Stimulation

The programming of clinical deep brain stimulation (DBS) systems involves numerous combinations of stimulation parameters, such as stimulus amplitude, pulse width, and frequency. As more complex electrode designs, such as directional electrodes, are introduced, the traditional trial-and-error approach to manual DBS programming becomes increasingly impractical. Visualization of the volume of tissue activated (VTA) can assist in selecting stimulation parameters by showing the direct effects of DBS on neural tissue. However, the standard method for VTA calculation, which involves modeling biological nerve fibers, is highly time-consuming and limits clinical applicability. In this study, we used finite element models (FEM) of implanted DBS systems to compute electric fields and obtained a large dataset of axonal responses under electrical stimulation using multicompartment cable models. We then trained a convolutional neural network (CNN) to replace the cable models. The CNN model's performance in calculating VTA was evaluated across various electrode configurations and stimulation parameters, and compared with existing activation function (AF) methods. The CNN model achieved a mean absolute error (MAE) of 0.032V in predicting nerve fiber activation thresholds, demonstrating greater stability and accuracy in VTA prediction compared to the AF method. Additionally, the CNN reduced computation time by five orders of magnitude compared to standard axonal modeling methods. We demonstrate that the CNN-based neural fiber predictor can quickly, accurately, and robustly predict neural activation responses to DBS, thereby improving the efficiency of DBS programming.

Timing-dependent synergies between noninvasive motor cortex and spinal cord stimulation in chronic cervical spinal cord injury

Precise movement requires integrating descending motor control with sensory feedback. Sensory networks interact strongly with descending motor circuits within the spinal cord. We targeted this interaction by pairing stimulation of the motor cortex with coordinated stimulation of the cervical spinal cord. We used separate non-invasive and epidural experiments to test the hypothesis that the strongest muscle response would occur when paired brain and spinal cord stimuli simultaneously converge within the spinal cord. For non-invasive experiments, we measured arm and hand muscle motor evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) and transcutaneous spinal cord stimulation (TSCS) in 16 individuals with chronic spinal cord injury (SCI) and 15 uninjured individuals. We compared this noninvasive approach to intraoperative paired stimulation experiments using dorsal epidural electrodes in 38 individuals undergoing surgery for cervical myelopathy. We observed augmented muscle responses to suprathreshold TMS when subthreshold TSCS stimuli were timed to converge synchronously in the spinal cord. At convergent timing, target muscle MEPs increased by 11.0% overall (13.3% in people with SCI, 6.2% in uninjured individuals) compared to non-convergent time intervals. Facilitation correlated with TSCS intensity, with intensity close to movement threshold being most effective. Facilitation did not correlate with SCI level or severity, indicating spared circuits were sufficient for this effect. Noninvasive pairing produced less facilitation compared to intraoperative (epidural) pairing. Thus, sensorimotor interactions in the cervical spinal spinal cord can be targeted with paired stimulation in health and after SCI.

Harnessing spinal circuit reorganization for targeted functional recovery after spinal cord injury

Spinal cord injury (SCI) disrupts the communication between the brain and spinal cord, resulting in the loss of motor function below the injury site. However, spontaneous structural and functional plasticity occurs in neural circuits after SCI, with unaffected synaptic inputs forming new connections and detour pathways to support recovery. The review discusses various mechanisms of circuit reorganization post-SCI, including supraspinal pathways, spinal interneurons, and spinal central pattern generators. Functional recovery may rely on maintaining a balance between excitatory and inhibitory neural activity, as well as enhancing proprioceptive input, which plays a key role in limb stability. The review emphasizes the importance of endogenous neuronal regeneration, neuromodulation therapies (such as electrical stimulation) and proprioception in SCI treatment. Future research should integrate advanced technologies such as gene targeting, imaging, and single-cell mapping to better understand the mechanisms underpinning SCI recovery, aiming to identify key neuronal subpopulations for targeted reconstruction and enhanced functional recovery. By harnessing spinal circuit reorganization, these efforts hold the potential to pave the way for more precise and effective strategies for functional recovery after SCI.

First-in-human study of epidural spinal cord stimulation in individuals with spinal muscular atrophy

Spinal muscular atrophy (SMA) is an inherited neurodegenerative disease causing motoneuron dysfunction, muscle weakness, fatigue and early mortality. Three new therapies can slow disease progression, enabling people to survive albeit with lingering motor impairments. Indeed, weakness and fatigue are still among patients' main concerns. Here we show that epidural spinal cord stimulation (SCS) improved motoneuron function, thereby increasing strength, endurance and gait quality, in three adults with type 3 SMA. Preclinical works demonstrated that SMA motoneurons show low firing rates because of a loss of excitatory input from primary sensory afferents. In the present study, we hypothesized that correcting this loss with electrical stimulation of the sensory afferents could improve motoneuron function. To test this hypothesis, we implanted three adults with SMA with epidural electrodes over the lumbosacral spinal cord, targeting sensory axons of the legs. We delivered SCS for 4 weeks, 2 h per day during motor tasks. Our intervention led to improvements in strength (up to +180%), gait quality (mean step length: +40%) and endurance (mean change in 6-minute walk test: +26 m), paralleled by increased motoneuron firing rates. These changes persisted even when SCS was turned OFF. Notably, no adverse events related to the stimulation were reported. ClinicalTrials.gov identifier: NCT05430113 .

Comparison metrics and power trade-offs for BCI motor decoding circuit design

Brain signal decoders are increasingly being used in early clinical trials for rehabilitation and assistive applications such as motor control and speech decoding. As many Brain-Computer Interfaces (BCIs) need to be deployed in battery-powered or implantable devices, signal decoding must be performed using low-power circuits. This paper reviews existing hardware systems for BCIs, with a focus on motor decoding, to better understand the factors influencing the power and algorithmic performance of such systems. We propose metrics to compare the energy efficiency of a broad range of on-chip decoding systems covering Electroencephalography (EEG), Electrocorticography (ECoG), and Microelectrode Array (MEA) signals. Our analysis shows that achieving a given classification rate requires an Input Data Rate (IDR) that can be empirically estimated, a finding that is helpful for sizing new BCI systems. Counter-intuitively, our findings show a negative correlation between the power consumption per channel (PpC) and the Information Transfer Rate (ITR). This suggests that increasing the number of channels can simultaneously reduce the PpC through hardware sharing and increase the ITR by providing new input data. In fact, for EEG and ECoG decoding circuits, the power consumption is dominated by the complexity of signal processing. To better understand how to minimize this power consumption, we review the optimizations used in state-of-the-art decoding circuits.

Epidural electrical stimulation combined with photobiomodulation restores hindlimb motor function in rats with thoracic spinal cord injury

Epidural electrical stimulation (EES) could restore motor function of paralyzed limbs of patients with spinal cord injury (SCI). However, its invasiveness limits its application in early stage of injury. Photobiomodulation (PBM) utilizes infrared light for percutaneous irradiation of the spinal cord to protect nerve tissue, delay muscle atrophy, and can be applied in early stage of SCI due to its non-invasiveness. This study tested the effect of the combination of EES and PBM on promoting motor function recovery in SCI rats. Severe contusion was induced at the T9 spinal segment in female rats, EES (applied to the L2 and S1 spinal cord segments) with treadmill training was conducted one week after the injury, and PBM percutaneous irradiation started at the injured segment on the day of surgery. In the third week post-injury, electromyographic and gait performance during training were recorded. Besides, the muscles of the hind limbs and the spinal cord on the caudal side of the injured segment were extracted. The results demonstrate that compared to the EES- or PBM-only group, this combined therapy led to several indicators returning to intact levels, including behavioral and electrophysiological, the gait patterns was also closer to intact rats. Additionally, the combined treatment group showed minimal muscle atrophy and maximal preservation of the injured spinal cord on the caudal side, with this histological improvement correlated with motor function recovery. Taken together, our results showed that this combined therapy was a more effective treatment for improving motor dysfunction after SCI which could protect the damaged spinal cord and promote the recovery of motor function in rats with SCI.

Anisotropic hydrogel microelectrodes for intraspinal neural recordings in vivo

Creating durable, motion-compliant neural interfaces is crucial for accessing dynamic tissues under in vivo conditions and linking neural activity with behaviors. Utilizing the self-alignment of nano-fillers in a polymeric matrix under repetitive tension, here, we introduce conductive carbon nanotubes with high aspect ratios into semi-crystalline polyvinyl alcohol hydrogels, and create electrically anisotropic percolation pathways through cyclic stretching. The resulting anisotropic hydrogel fibers (diameter of 187 ± 13 µm) exhibit fatigue resistance (up to 20,000 cycles at 20% strain) with a stretchability of 64.5 ± 7.9% and low electrochemical impedance (33.20 ± 9.27 kΩ @ 1 kHz in 1 cm length). We observe the reconstructed nanofillers' axial alignment and a corresponding anisotropic impedance decrease along the direction of cyclic stretching. We fabricate fiber-shaped hydrogels into bioelectronic devices and implant them into wild-type and transgenic Thy1::ChR2-EYFP mice to record electromyographic signals from muscles in anesthetized and freely moving conditions. These hydrogel fibers effectively enable the simultaneous recording of electrical signals from ventral spinal cord neurons and the tibialis anterior muscles during optogenetic stimulation. Importantly, the devices maintain functionality in intraspinal electrophysiology recordings over eight months after implantation, demonstrating their durability and potential for long-term monitoring in neurophysiological studies.

Pharmacological blocking of spinal GABAA receptors in monkeys reduces sensory transmission to the spinal cord, thalamus, and cortex

A century of research established that GABA inhibits proprioceptive inputs presynaptically to sculpt spinal neural inputs into skilled motor output. Recent results in mice challenged this theory by showing that GABA can also facilitate action potential conduction in proprioceptive afferents. Here, we tackle this controversy in monkeys, the most human-relevant animal model, and show that GABAA receptors (GABAARs) indeed facilitate sensory inputs to spinal motoneurons and interneurons and that this mechanism also influences sensory transmission to supraspinal centers. We performed causal manipulations of GABAARs with intrathecal pharmacology in anesthetized monkeys while recording electrical signals in the muscles, spinal cord, thalamus, and cortex. We show that blocking GABAARs suppresses spinal reflexes to hand muscles, sensory-evoked single-unit firing in the spinal cord, and sensory-evoked potentials in the thalamus and somatosensory cortex. Our results portray a sophisticated picture of presynaptic modulation of sensory inputs by GABA in the spinal cord.

Neurostimulation on lumbosacral nerves as a new treatment for spinal cord injury impairments and its impact on cortical activity: a narrative review

Spinal cord injury (SCI) can cause significant motor, sensory, and autonomic dysfunction by disrupting neural connections. As a result, it is a global health challenge that requires innovative interventions to improve outcomes. This review assesses the wide-ranging impacts of SCI and focuses on the laparoscopic implantation of neuroprosthesis (LION) as an emerging and promising rehabilitation technique. The LION technique involves the surgical implantation of electrodes on lumbosacral nerves to stimulate paralyzed muscles. Recent findings have demonstrated significant improvements in mobility, sexual function, and bladder/bowel control in chronic SCI patients following LION therapy. This manuscript revisits the potential physiological mechanisms underlying these results, including neuroplasticity and modulation of autonomic activity. Additionally, we discuss potential future applications and amendments of LION therapy. This study emphasizes the potential of neuromodulation as a complementary approach to traditional rehabilitation, that can provide a beacon of hope for improving functionality and quality of life for individuals with SCI.

Repeated epidural stimulation modulates cervical spinal cord excitability in healthy adult rats

Spinal evoked motor responses (SEMR) are utilized in longitudinal pre-clinical and human studies to reflect the in-vivo physiological changes in neural networks secondary to a spinal cord injury (SCI) or neuro-rehabilitative treatments utilizing epidural stimulation (ES). However, it remains unknown whether the repeated ES exposure during SEMR testing itself modulates spinal cord physiology and accompanying SEMR characteristics. To answer this, ES was delivered to the cervical spinal cord using standard stimulation paradigms during multiple SEMR data acquisition sessions (~ 17 h spanning across 100 days) in ten healthy adult rats. Cervical SEMR at rest and forelimb muscle activity during reaching and grasping task were collected before and after 100 days. We noted a persistent increase in SEMR activity relative to baseline, with prominent changes in the mono and poly-synaptic components of SEMR. The findings indicate increased spinal cord excitability. Increased spinal cord excitability translated into increased forelimb muscle activation during the reaching and grasping task. For the majority of SEMR and muscle activity increase, effect size was large or very large. Cervical SEMR are amenable to modulation by routine ES testing protocols, with prominent changes in the mono and poly-synaptic components of SEMR. Since repeated stimulation during multiple testing alone increases cord excitability, we recommend (1) SEMR may be used with caution to infer the physiological status of the spinal circuitry (2) utilizing appropriate control groups and motor behavioral correlates for meaningful functional interpretation in longitudinal neuromodulation studies involving multiple SEMR testing sessions following a SCI.

Unaltered Responses of Distal Motor Neurons to Non-Targeted Thoracic Spinal Cord Stimulation in Chronic Pain Patients

INTRODUCTION

Spinal cord stimulation (SCS) represents an established interventional pain therapeutic; however, the SCS effects of SCS waveforms on motor neuron recruitment of the lower limbs of chronic pain patients remain largely unknown.

METHODS

We investigated these effects by performing isometric ankle-dorsal flexions at varying force levels under four SCS conditions: SCS Off (1 week), burst SCS (40 Hz), SCS Off (acute), and tonic SCS (130 Hz). Muscle activity was recorded via high-density surface electromyography (64-electrode grid) on the tibialis anterior muscle. Motor unit action (MUs) potentials were analyzed for recruitment and de-recruitment thresholds, discharge rate, inter-spike interval, and common synaptic input.

RESULTS

In this prospective study, we included nine patients (five females; four males; mean age 59 years) with chronic pain treated with thoracic (Th7-Th8) epidural spinal stimulation. A total of 97 MUs were found for 15% maximal voluntary torque (MVT) and 83 for 30%MVT, an average of 10.8 ± 3.7 for 15%MVT and 10.4 ± 3.5 for 30%MVT. While a few subject-specific variations were observed, our study suggests that the different SCS frequencies applied do not significantly influence motor unit discharge characteristics in the TA muscle among the participants (p values at 15%MVT were 0.586 (Chi2 = 1.933), 0.737 (Chi2 = 1.267), 0.706 (Chi2 = 1.4) and 0.586 (Chi2 = 1.933), respectively. The p values of the Friedman test at 30%MVT were 0.896 (Chi2 = 0.6), 0.583 (Chi2 = 1.95), 0.896 (Chi2 = 0.6) and 0.256 (Chi2 = 4.05). No significant difference was found for the different stimulation types for the delta (0-5 Hz), alpha (5-12 Hz), and beta (15-30 Hz) bands at both force levels.

CONCLUSIONS

In summary, we did not observe any changes in motor unit oscillatory activity at any low and high bandwidths, indicating that SCS using different waveforms (tonic/burst) does not significantly influence motor neuron recruitment for non-motor individuals with chronic pain.

Advanced neuroprosthetic electrode design optimized by electromagnetic finite element simulation: innovations and applications

Based on electrophysiological activity, neuroprostheses can effectively monitor and control neural activity. Currently, electrophysiological neuroprostheses are widely utilized in treating neurological disorders, particularly in restoring motor, visual, auditory, and somatosensory functions after nervous system injuries. They also help alleviate inflammation, regulate blood pressure, provide analgesia, and treat conditions such as epilepsy and Alzheimer's disease, offering significant research, economic, and social value. Enhancing the targeting capabilities of neuroprostheses remains a key objective for researchers. Modeling and simulation techniques facilitate the theoretical analysis of interactions between neuroprostheses and the nervous system, allowing for quantitative assessments of targeting efficiency. Throughout the development of neuroprostheses, these modeling and simulation methods can save time, materials, and labor costs, thereby accelerating the rapid development of highly targeted neuroprostheses. This article introduces the fundamental principles of neuroprosthesis simulation technology and reviews how various simulation techniques assist in the design and performance enhancement of neuroprostheses. Finally, it discusses the limitations of modeling and simulation and outlines future directions for utilizing these approaches to guide neuroprosthesis design.

Potentiation of cortico-spinal output via targeted electrical stimulation of the motor thalamus

Cerebral white matter lesions prevent cortico-spinal descending inputs from effectively activating spinal motoneurons, leading to loss of motor control. However, in most cases, the damage to cortico-spinal axons is incomplete offering a potential target for therapies aimed at improving volitional muscle activation. Here we hypothesize that, by engaging direct excitatory connections to cortico-spinal motoneurons, stimulation of the motor thalamus could facilitate activation of surviving cortico-spinal fibers thereby immediately potentiating motor output. To test this hypothesis, we identify optimal thalamic targets and stimulation parameters that enhance upper-limb motor-evoked potentials and grip forces in anesthetized monkeys. This potentiation persists after white matter lesions. We replicate these results in humans during intra-operative testing. We then design a stimulation protocol that immediately improves strength and force control in a patient with a chronic white matter lesion. Our results show that electrical stimulation targeting surviving neural pathways can improve motor control after white matter lesions.

Targeted deep brain stimulation of the motor thalamus improves speech and swallowing motor functions after cerebral lesions

Speech and swallowing are complex motor acts that depend upon the integrity of input neural signals from motor cortical areas to control muscles of the head and neck. Lesions damaging these neural pathways result in weakness of key muscles causing dysarthria and dysphagia, leading to profound social isolation and risk of aspiration and suffocation. Here we show that Deep Brain Stimulation (DBS) of the motor thalamus improved speech and swallowing functions in two participants with dysarthria and dysphagia. First, we proved that DBS increased excitation of the face motor cortex, augmenting motor evoked potentials, and range and speed of motion of orofacial articulators in n = 10 volunteers with intact neural pathways. Then, we demonstrated that this potentiation led to immediate improvement in swallowing functions in a patient with moderate dysphagia and profound dysarthria as a consequence of a traumatic brain lesion. In this subject and in another with mild dysarthria, we showed that DBS immediately ameliorated impairments of respiratory, phonatory, resonatory, and articulatory control thus resulting in a clinically significant improvement in speech intelligibility. Our data provide first-in-human evidence that DBS can be used to treat dysphagia and dysarthria in people with cerebral lesions.

Scalable networks of wireless bioelectronics using magnetoelectrics

Networks of miniature bioelectronic implants would enable precise measurement and manipulation of the complex and distributed physiological systems in the body. For example, sensing and stimulation nodes throughout the heart, brain, or peripheral nervous system would more accurately track and treat disease or support prosthetic technologies with many degrees of freedom. A main challenge to creating this type of in-body bioelectronic network is the fact that wireless power and data transfer are often inefficient when communicating through biological tissues. This challenge is typically compounded as one increases the number of implants within the network. Here, we show that magnetoelectric wireless data and power transfer enable a network of millimeter-sized bioelectronic implants where the power transfer efficiency of the system improves as the number of implanted devices increases. Using this property, we demonstrate networks of wireless battery-free bioelectronics ranging from 1 to 6 implants where the wireless power transfer efficiency for the system increases from 0.2% to 1.3%, with each node in the network receiving 2.2 mW at a distance of 1 cm. We use this system for efficient and robust wireless data and power transfer to demonstrate proof-of-concept networks of miniature spinal cord stimulators and cardiac pacing devices in large animals. The scalability of this network architecture enabled by magnetoelectric wireless power transfer provides a platform for building wireless closed-loop networks of bioelectronic implants for next-generation electronic medicine.

Ventral epidural stimulation for motor recovery after spinal cord injury: illustrative case

BACKGROUND

Spinal cord stimulation (SCS) has demonstrated potential as a therapy to enhance motor functional recovery after spinal cord injury (SCI). Epidural SCS for motor recovery is traditionally performed via the dorsal electrode. While ventral epidural stimulation may provide more direct and specific stimulation of the ventral motor neurons involved in motor control, it is largely unstudied, and its role in motor recovery after SCI is unclear. In order to profile the safety and feasibility of ventral epidural spinal stimulation (VSS), the authors present a patient who underwent VSS following a corpectomy to treat SCI related to metastatic epidural cord compression.

OBSERVATIONS

A patient underwent transpedicular corpectomy for spinal cord decompression, as well as the placement of 2 ventral epidural electrodes, followed by concurrent physical therapy and ventral epidural stimulation. He was nonambulatory preoperatively but was able to walk over 300 feet with the assistance of a rolling walker at the conclusion of the 3-week study period. VSS was noted to produce improvements in muscle contraction when stimulation was on.

LESSONS

VSS appears to be safe, feasible, and well tolerated. VSS, as compared to standard-of-care therapy for SCI, can be used in conjunction with physical therapy and may lead to improvements in motor function. https://thejns.org/doi/10.3171/CASE24155.

Anisotropic Hydrogel Microelectrodes for Intraspinal Neural Recordings in vivo

Creating durable, motion-compliant neural interfaces is crucial for accessing dynamic tissues under in vivo conditions and linking neural activity with behaviors. Utilizing the self-alignment of nano-fillers in a polymeric matrix under repetitive tension, here, we introduce conductive carbon nanotubes with high aspect ratios into semi-crystalline polyvinyl alcohol hydrogels and create electrically anisotropic percolation pathways through cyclic stretching. The resulting anisotropic hydrogel fibers (diameter of 187 ± 13 µm) exhibit fatigue resistance (20,000 cycles at 20% strain) with a stretchability of 64.5 ± 7.9%, and low electrochemical impedance (900 ± 149 kΩ @ 1kHz). We observe the re-constructed nanofillers' axial alignment and a corresponding anisotropic impedance decrease along the direction of cyclic stretching. We fabricate fiber-shaped hydrogels into bioelectronic devices and implant them into wild-type and transgenic Thy1-ChR2-EYFP mice to record electromyographic signals from muscles in anesthetized and freely moving conditions. These hydrogel fibers effectively enable the simultaneous recording of electrical signals from ventral spinal cord neurons and the tibialis anterior muscles during optogenetic stimulation. Importantly, the devices maintain functionality with repeatable recording results over eight months after implantation, demonstrating their durability and potential for long-term monitoring in neurophysiological studies.

Human Cervical Epidural Spinal Electrogram Topographically Maps Distinct Volitional Movements

Little is known about the electrophysiologic activity of the intact human spinal cord during volitional movement. We analyzed epidural spinal recordings from a total of five human subjects of both sexes during a variety of upper extremity movements and found that these spinal epidural electrograms contain spectral information distinguishing periods of movement, rest, and sensation. Cervical epidural electrograms also contained spectral changes time-locked with movement. We found that these changes were primarily associated with increased power in the theta (4-8 Hz) band and feature increased theta phase to gamma amplitude coupling, and this increase in theta power can be used to topographically map distinct upper extremity movements onto the cervical spinal cord in accordance with established myotome maps of the upper extremity. Our findings have implications for the development of neurostimulation protocols and devices focused on motor rehabilitation for the upper extremity, and the approach presented here may facilitate spatiotemporal mapping of naturalistic movements.

FUNDAMENTAL LIMITATIONS OF KILOHERTZ-FREQUENCY CARRIERS IN AFFERENT FIBER RECRUITMENT WITH TRANSCUTANEOUS SPINAL CORD STIMULATION

The use of kilohertz-frequency (KHF) waveforms has rapidly gained momentum in transcutaneous spinal cord stimulation (tSCS) to restore motor function after paralysis. However, the mechanisms by which these fast-alternating currents depolarize efferent and afferent fibers remain unknown. Our study fills this research gap by providing a hypothesis-and evidence-based investigation using peripheral nerve stimulation, lumbar tSCS, and cervical tSCS in 25 unimpaired participants together with computational modeling. Peripheral nerve stimulation experiments and computational modeling showed that KHF waveforms negatively impact the processes required to elicit action potentials, thereby increasing response thresholds and biasing the recruitment towards efferent fibers. While these results translate to tSCS, we also demonstrate that lumbar tSCS results in the preferential recruitment of afferent fibers, while cervical tSCS favors recruitment of efferent fibers. Given the assumed importance of proprioceptive afferents in motor recovery, our work suggests that the use of KHF waveforms should be reconsidered to maximize neurorehabilitation outcomes, particularly for cervical tSCS. We posit that careful analysis of the mechanisms that mediate responses elicited by novel approaches in tSCS is crucial to understanding their potential to restore motor function after paralysis.

The role of spinal cord neuroanatomy and the variances of epidurally evoked spinal responses

BACKGROUND

Spinal cord stimulation (SCS) has demonstrated multiple benefits in treating chronic pain and other clinical disorders related to sensorimotor dysfunctions. However, the underlying mechanisms are still not fully understood, including how electrode placement in relation to the spinal cord neuroanatomy influences epidural spinal recordings (ESRs). To characterize this relationship, this study utilized stimulation applied at various anatomical sections of the spinal column, including at levels of the intervertebral disc and regions correlating to the dorsal root entry zone.

METHOD

Two electrode arrays were surgically implanted into the dorsal epidural space of the swine. The stimulation leads were positioned such that the caudal-most electrode contact was at the level of a thoracic intervertebral segment. Intraoperative cone beam computed tomography (CBCT) images were utilized to precisely determine the location of the epidural leads relative to the spinal column. High-resolution microCT imaging and 3D-model reconstructions of the explanted spinal cord illustrated precise positioning and dimensions of the epidural leads in relation to the surrounding neuroanatomy, including the spinal rootlets of the dorsal and ventral columns of the spinal cord. In a separate swine cohort, implanted epidural leads were used for SCS and recording evoked ESRs.

RESULTS

Reconstructed 3D-models of the swine spinal cord with epidural lead implants demonstrated considerable distinctions in the dimensions of a single electrode contact on a standard industry epidural stimulation lead compared to dorsal rootlets at the dorsal root entry zone (DREZ). At the intervertebral segment, it was observed that a single electrode contact may cover 20-25% of the DREZ if positioned laterally. Electrode contacts were estimated to be ~0.75 mm from the margins of the DREZ when placed at the midline. Furthermore, ventral rootlets were observed to travel in proximity and parallel to dorsal rootlets at this level prior to separation into their respective sides of the spinal cord. Cathodic stimulation at the level of the intervertebral disc, compared to an 'off-disc' stimulation (7 mm rostral), demonstrated considerable variations in the features of recorded ESRs, such as amplitude and shape, and evoked unintended motor activation at lower stimulation thresholds. This substantial change may be due to the influence of nearby ventral roots. To further illustrate the influence of rootlet activation vs. dorsal column activation, the stimulation lead was displaced laterally at ~2.88 mm from the midline, resulting in variances in both evoked compound action potential (ECAP) components and electromyography (EMG) components in ESRs at lower stimulation thresholds.

CONCLUSION

The results of this study suggest that the ECAP and EMG components of recorded ESRs can vary depending on small differences in the location of the stimulating electrodes within the spinal anatomy, such as at the level of the intervertebral segment. Furthermore, the effects of sub-centimeter lateral displacement of the stimulation lead from the midline, leading to significant changes in electrophysiological metrics. The results of this pilot study reveal the importance of the small displacement of the electrodes that can cause significant changes to evoked responses SCS. These results may provide further valuable insights into the underlying mechanisms and assist in optimizing future SCS-related applications.

Restoration of coherent reach-grasp-pull movement via sequential intraneural peripheral nerve stimulation in rats

Objective.Peripheral nerve stimulation (PNS) has been demonstrated as an effective way to selectively activate muscles and to produce fine hand movements. However, sequential multi-joint upper limb movements, which are critical for paralysis rehabilitation, has not been tested with PNS. Here, we aimed to restore multiple upper limb joint movements through an intraneural interface with a single electrode, achieving coherent reach-grasp-pull movement tasks through sequential stimulation.Approach.A transverse intrafascicular multichannel electrode was implanted under the axilla of the rat's upper limb, traversing the musculocutaneous, radial, median, and ulnar nerves. Intramuscular electrodes were implanted into the biceps brachii (BB), triceps brachii (TB), flexor carpi radialis (FCR), and extensor carpi radialis (ECR) muscles to record electromyographic (EMG) activity and video recordings were used to capture the kinematics of elbow, wrist, and digit joints. Charge-balanced biphasic pulses were applied to different channels to recruit distinct upper limb muscles, with concurrent recording of EMG signals and joint kinematics to assess the efficacy of the stimulation. Finally, a sequential stimulation protocol was employed by generating coordinated pulses in different channels.Main results.BB, TB, FCR and ECR muscles were selectively activated and various upper limb movements, including elbow flexion, elbow extension, wrist flexion, wrist extension, digit flexion, and digit extension, were reliably generated. The modulation effects of stimulation parameters, including pulse width, amplitude, and frequency, on induced joint movements were investigated and reach-grasp-pull movement was elicited by sequential stimulation.Significance.Our results demonstrated the feasibility of sequential intraneural stimulation for functional multi-joint movement restoration, providing a new approach for clinical rehabilitation in paralyzed patients.

Selective Activation of the Spinal Cord with Epidural Electrical Stimulation

Spinal cord epidural electrical stimulation (EES) has been successfully employed to treat chronic pain and to restore lost functions after spinal cord injury. Yet, the efficacy of this approach is largely challenged by the suboptimal spatial distribution of the electrode contacts across anatomical targets, limiting the spatial selectivity of stimulation. In this study, we exploited different ESS paradigms, designed as either Spatial-Selective Stimulation (SSES) or Orientation-Selective Epidural Stimulation (OSES), and compared them to Conventional Monopolar Epidural Stimulation (CMES). SSES, OSES, and CMES were delivered with a 3- or 4-contact electrode array. Amplitudes and latencies of the Spinally Evoked Motor Potentials (SEMPs) were evaluated with different EES modalities. The results demonstrate that the amplitudes of SEMPs in hindlimb muscles depend on the orientation of the electrical field and vary between stimulation modalities. These findings show that the electric field applied with SSES or OSES provides more selective control of amplitudes of the SEMPs as compared to CMES. We demonstrate that spinal cord epidural stimulation applied with SSES or OSES paradigms in the rodent model could be tailored to the functional spinal cord neuroanatomy and can be tuned to specific target fibers and their orientation, optimizing the effect of neuromodulation.

Timing-dependent synergies between motor cortex and posterior spinal stimulation in humans

Volitional movement requires descending input from the motor cortex and sensory feedback through the spinal cord. We previously developed a paired brain and spinal electrical stimulation approach in rats that relies on convergence of the descending motor and spinal sensory stimuli in the cervical cord. This approach strengthened sensorimotor circuits and improved volitional movement through associative plasticity. In humans, it is not known whether posterior epidural spinal cord stimulation targeted at the sensorimotor interface or anterior epidural spinal cord stimulation targeted within the motor system is effective at facilitating brain evoked responses. In 59 individuals undergoing elective cervical spine decompression surgery, the motor cortex was stimulated with scalp electrodes and the spinal cord was stimulated with epidural electrodes, with muscle responses being recorded in arm and leg muscles. Spinal electrodes were placed either posteriorly or anteriorly, and the interval between cortex and spinal cord stimulation was varied. Pairing stimulation between the motor cortex and spinal sensory (posterior) but not spinal motor (anterior) stimulation produced motor evoked potentials that were over five times larger than brain stimulation alone. This strong augmentation occurred only when descending motor and spinal afferent stimuli were timed to converge in the spinal cord. Paired stimulation also increased the selectivity of muscle responses relative to unpaired brain or spinal cord stimulation. Finally, clinical signs suggest that facilitation was observed in both injured and uninjured segments of the spinal cord. The large effect size of this paired stimulation makes it a promising candidate for therapeutic neuromodulation. KEY POINTS: Pairs of stimuli designed to alter nervous system function typically target the motor system, or one targets the sensory system and the other targets the motor system for convergence in cortex. In humans undergoing clinically indicated surgery, we tested paired brain and spinal cord stimulation that we developed in rats aiming to target sensorimotor convergence in the cervical cord. Arm and hand muscle responses to paired sensorimotor stimulation were more than five times larger than brain or spinal cord stimulation alone when applied to the posterior but not anterior spinal cord. Arm and hand muscle responses to paired stimulation were more selective for targeted muscles than the brain- or spinal-only conditions, especially at latencies that produced the strongest effects of paired stimulation. Measures of clinical evidence of compression were only weakly related to the paired stimulation effect, suggesting that it could be applied as therapy in people affected by disorders of the central nervous system.

Multifunctional Conductive and Electrogenic Hydrogel Repaired Spinal Cord Injury via Immunoregulation and Enhancement of Neuronal Differentiation

Spinal cord injury (SCI) is a refractory neurological disorder. Due to the complex pathological processes, especially the secondary inflammatory cascade and the lack of intrinsic regenerative capacity, it is difficult to recover neurological function after SCI. Meanwhile, simulating the conductive microenvironment of the spinal cord reconstructs electrical neural signal transmission interrupted by SCI and facilitates neural repair. Therefore, a double-crosslinked conductive hydrogel (BP@Hydrogel) containing black phosphorus nanoplates (BP) is synthesized. When placed in a rotating magnetic field (RMF), the BP@Hydrogel can generate stable electrical signals and exhibit electrogenic characteristic. In vitro, the BP@Hydrogel shows satisfactory biocompatibility and can alleviate the activation of microglia. When placed in the RMF, it enhances the anti-inflammatory effects. Meanwhile, wireless electrical stimulation promotes the differentiation of neural stem cells (NSCs) into neurons, which is associated with the activation of the PI3K/AKT pathway. In vivo, the BP@Hydrogel is injectable and can elicit behavioral and electrophysiological recovery in complete transected SCI mice by alleviating the inflammation and facilitating endogenous NSCs to form functional neurons and synapses under the RMF. The present research develops a multifunctional conductive and electrogenic hydrogel for SCI repair by targeting multiple mechanisms including immunoregulation and enhancement of neuronal differentiation.

Neuromodulation techniques in poststroke motor impairment recovery: Efficacy, challenges, and future directions

Cerebrovascular accidents, also known as strokes, represent a major global public health challenge and contribute to substantial mortality, disability, and socioeconomic burden. Multidisciplinary approaches for poststroke therapies are crucial for recovering lost functions and adapting to new limitations. This review discusses the potential of neuromodulation techniques, repetitive transcranial magnetic stimulation (rTMS), transcranial direct current stimulation, spinal cord stimulation (SCS), vagus nerve stimulation (VNS), and deep brain stimulation (DBS), as innovative strategies for facilitating poststroke recovery. Neuromodulation is an emerging adjunct to conventional therapies that target neural plasticity to restore lost function and compensate for damaged brain areas. The techniques discussed in this review have different efficacies in enhancing neural plasticity, optimizing motor recovery, and mitigating poststroke impairments. Specifically, rTMS has shown significant promise in enhancing motor function, whereas SCS has shown potential in improving limb movement and reducing disability. Similarly, VNS, typically used to treat epilepsy, has shown promise in enhancing poststroke motor recovery, while DBS may be used to improve poststroke motor recovery and symptom mitigation. Further studies with standardized protocols are warranted to elucidate the efficacy of these methods and integrate them into mainstream clinical practice to optimize poststroke care.

Neural population dynamics reveals disruption of spinal circuits' responses to proprioceptive input during electrical stimulation of sensory afferents

While neurostimulation technologies are rapidly approaching clinical applications for sensorimotor disorders, the impact of electrical stimulation on network dynamics is still unknown. Given the high degree of shared processing in neural structures, it is critical to understand if neurostimulation affects functions that are related to, but not targeted by, the intervention. Here, we approach this question by studying the effects of electrical stimulation of cutaneous afferents on unrelated processing of proprioceptive inputs. We recorded intraspinal neural activity in four monkeys while generating proprioceptive inputs from the radial nerve. We then applied continuous stimulation to the radial nerve cutaneous branch and quantified the impact of the stimulation on spinal processing of proprioceptive inputs via neural population dynamics. Proprioceptive pulses consistently produce neural trajectories that are disrupted by concurrent cutaneous stimulation. This disruption propagates to the somatosensory cortex, suggesting that electrical stimulation can perturb natural information processing across the neural axis.

Engineering the Physical Microenvironment into Neural Organoids for Neurogenesis and Neurodevelopment

Understanding the signals from the physical microenvironment is critical for deciphering the processes of neurogenesis and neurodevelopment. The discovery of how surrounding physical signals shape human developing neurons is hindered by the bottleneck of conventional cell culture and animal models. Notwithstanding neural organoids provide a promising platform for recapitulating human neurogenesis and neurodevelopment, building neuronal physical microenvironment that accurately mimics the native neurophysical features is largely ignored in current organoid technologies. Here, it is discussed how the physical microenvironment modulates critical events during the periods of neurogenesis and neurodevelopment, such as neural stem cell fates, neural tube closure, neuronal migration, axonal guidance, optic cup formation, and cortical folding. Although animal models are widely used to investigate the impacts of physical factors on neurodevelopment and neuropathy, the important roles of human stem cell-derived neural organoids in this field are particularly highlighted. Considering the great promise of human organoids, building neural organoid microenvironments with mechanical forces, electrophysiological microsystems, and light manipulation will help to fully understand the physical cues in neurodevelopmental processes. Neural organoids combined with cutting-edge techniques, such as advanced atomic force microscopes, microrobots, and structural color biomaterials might promote the development of neural organoid-based research and neuroscience.

Symptomatic Treatment of Myelopathy

OBJECTIVE

This article discusses the effects of myelopathy on multiple organ systems and reviews the treatment and management of some of these effects.

LATEST DEVELOPMENTS

Recent advances in functional electrical stimulation, epidural spinal cord stimulation, robotics, and surgical techniques such as nerve transfer show promise in improving function in patients with myelopathy. Ongoing research in stem cell therapy and neurotherapeutic drugs may provide further therapeutic avenues in the future.

ESSENTIAL POINTS

Treatment for symptoms of spinal cord injury should be targeted toward patient goals. If nerve transfer for upper extremity function is considered, the patient should be evaluated at around 6 months from injury to assess for lower motor neuron involvement and possible time limitations of surgery. A patient with injury at or above the T6 level is at risk for autonomic dysreflexia, a life-threatening condition that presents with elevated blood pressure and can lead to emergent hypertensive crisis. Baclofen withdrawal due to baclofen pump failure or programming errors may also be life-threatening. Proper management of symptoms may help avoid complications such as autonomic dysreflexia, renal failure, heterotopic ossification, and fractures.

Muscle anatomy is reflected in the spatial organization of the spinal motoneuron pools

Neural circuits embed limb dynamics for motor control and sensorimotor integration. The somatotopic organization of motoneuron pools in the spinal cord may support these computations. Here, we tested if the spatial organization of motoneurons is related to the musculoskeletal anatomy. We created a 3D model of motoneuron locations within macaque spinal cord and compared the spatial distribution of motoneurons to the anatomical organization of the muscles they innervate. We demonstrated that the spatial distribution of motoneuron pools innervating the upper limb and the anatomical relationships between the muscles they innervate were similar between macaque and human species. Using comparative analysis, we found that the distances between motoneuron pools innervating synergistic muscles were the shortest, followed by those innervating antagonistic muscles. Such spatial organization can support the co-activation of synergistic muscles and reciprocal inhibition of antagonistic muscles. The spatial distribution of motoneurons may play an important role in embedding musculoskeletal dynamics.

Supraspinal control of motoneurons after paralysis enabled by spinal cord stimulation

Spinal cord stimulation (SCS) restores motor control after spinal cord injury (SCI) and stroke. This evidence led to the hypothesis that SCS facilitates residual supraspinal inputs to spinal motoneurons. Instead, here we show that SCS does not facilitate residual supraspinal inputs but directly triggers motoneurons action potentials. However, supraspinal inputs can shape SCS-mediated activity, mimicking volitional control of motoneuron firing. Specifically, by combining simulations, intraspinal electrophysiology in monkeys and single motor unit recordings in humans with motor paralysis, we found that residual supraspinal inputs transform subthreshold SCS-induced excitatory postsynaptic potentials into suprathreshold events. We then demonstrated that only a restricted set of stimulation parameters enables volitional control of motoneuron firing and that lesion severity further restricts the set of effective parameters. Our results explain the facilitation of voluntary motor control during SCS while predicting the limitations of this neurotechnology in cases of severe loss of supraspinal axons.

Spinal cord injury: Olfactory ensheathing cell-based therapeutic strategies

Spinal cord injury (SCI) is a highly disabling neurological disorder that is difficult to treat due to its complex pathophysiology and nerve regeneration difficulties. Hence, effective SCI treatments are necessary. Olfactory ensheathing cells (OECs), glial cells derived from the olfactory bulb or mucosa, are ideal candidates for SCI treatment because of their neuroprotective and regenerative properties, ample supply, and convenience. In vitro, animal model, and human trial studies have reported discoveries on OEC transplantation; however, shortcomings have also been demonstrated. Recent studies have optimized various OEC transplantation strategies, including drug integration, biomaterials, and gene editing. This review aims to introduce OECs mechanisms in repairing SCI, summarize the research progress of OEC transplantation-optimized strategies, and provide novel research ideas for SCI treatment.

POTENTIATION OF CORTICO-SPINAL OUTPUT VIA TARGETED ELECTRICAL STIMULATION OF THE MOTOR THALAMUS

Cerebral white matter lesions prevent cortico-spinal descending inputs from effectively activating spinal motoneurons, leading to loss of motor control. However, in most cases, the damage to cortico-spinal axons is incomplete offering a potential target for new therapies aimed at improving volitional muscle activation. Here we hypothesized that, by engaging direct excitatory connections to cortico-spinal motoneurons, stimulation of the motor thalamus could facilitate activation of surviving cortico-spinal fibers thereby potentiating motor output. To test this hypothesis, we identified optimal thalamic targets and stimulation parameters that enhanced upper-limb motor evoked potentials and grip forces in anesthetized monkeys. This potentiation persisted after white matter lesions. We replicated these results in humans during intra-operative testing. We then designed a stimulation protocol that immediately improved voluntary grip force control in a patient with a chronic white matter lesion. Our results show that electrical stimulation targeting surviving neural pathways can improve motor control after white matter lesions.

SUPRASPINAL CONTROL OF MOTONEURONS AFTER PARALYSIS ENABLED BY SPINAL CORD STIMULATION

Spinal cord stimulation (SCS) restores motor control after spinal cord injury (SCI) and stroke. This evidence led to the hypothesis that SCS facilitates residual supraspinal inputs to spinal motoneurons. Instead, here we show that SCS does not facilitate residual supraspinal inputs but directly triggers motoneurons action potentials. However, supraspinal inputs can shape SCS-mediated activity, mimicking volitional control of motoneuron firing. Specifically, by combining simulations, intraspinal electrophysiology in monkeys and single motor unit recordings in humans with motor paralysis, we found that residual supraspinal inputs transform subthreshold SCS-induced excitatory postsynaptic potentials into suprathreshold events. We then demonstrated that only a restricted set of stimulation parameters enables volitional control of motoneuron firing and that lesion severity further restricts the set of effective parameters. Our results explain the facilitation of voluntary motor control during SCS while predicting the limitations of this neurotechnology in cases of severe loss of supraspinal axons.

A Hyperflexible Electrode Array for Long-Term Recording and Decoding of Intraspinal Neuronal Activity

Neural interfaces for stable access to the spinal cord (SC) electrical activity can benefit patients with motor dysfunctions. Invasive high-density electrodes can directly extract signals from SC neuronal populations that can be used for the facilitation, adjustment, and reconstruction of motor actions. However, developing neural interfaces that can achieve high channel counts and long-term intraspinal recording remains technically challenging. Here, a biocompatible SC hyperflexible electrode array (SHEA) with an ultrathin structure that minimizes mechanical mismatch between the interface and SC tissue and enables stable single-unit recording for more than 2 months in mice is demonstrated. These results show that SHEA maintains stable impedance, signal-to-noise ratio, single-unit yield, and spike amplitude after implantation into mouse SC. Gait analysis and histology show that SHEA implantation induces negligible behavioral effects and Inflammation. Additionally, multi-unit signals recorded from the SC ventral horn can predict the mouse's movement trajectory with a high decoding coefficient of up to 0.95. Moreover, during step cycles, it is found that the neural trajectory of spikes and low-frequency local field potential (LFP) signal exhibits periodic geometry patterns. Thus, SHEA can offer an efficient and reliable SC neural interface for monitoring and potentially modulating SC neuronal activity associated with motor dysfunctions.

Prediction of isometric forces from combined epidural spinal cord and neuromuscular electrical stimulation in the rat lower limb

Both epidural spinal cord and muscle stimulation have been widely used for restoration of movement after spinal cord injury. However, using both approaches simultaneously could provide more flexible control compared to using either approach alone. We evaluate whether responses evoked by combined spinal and muscle stimulation can be predicted by the linear summation of responses produced by each individually. Should this be true, it would simplify the prediction of co-stimulation responses and the development of control schemes for spinal cord injury rehabilitation. In anesthetized rats, we measured hindlimb isometric forces in response to spinal and muscle stimulation across a range of amplitudes. Force prediction errors were calculated as the difference between predicted co-stimulation vectors and observed co-stimulation vectors whereby small errors signified evidence for linear summation. We found that the errors for spinal and muscle co-stimulation were significantly larger than expected. Using a bootstrapping analysis, we find that these larger errors do not reflect a nonlinear interaction between spinal and muscle responses. Instead, they can be attributed to the variability of spinal stimulation responses. We discuss the implications of these results to the use of combined muscle and spinal stimulation for the restoration of movement following spinal cord injury.

Upper limb modeling and motion extraction based on multi-space-fusion

Modeling and motion extraction of human upper limbs are essential for interpreting the natural behavior of upper limb. Owing to the high degrees of freedom (DOF) and highly dynamic nature, existing upper limb modeling methods have limited applications. This study proposes a generic modeling and motion extraction method, named Primitive-Based triangular body segment method (P-BTBS), which follows the physiology of upper limbs, allows high accuracy of motion angles, and describes upper-limb motions with high accuracy. For utilizing the upper-limb modular motion model, the motion angles and bones can be selected as per the research topics (The generic nature of the study targets). Additionally, P-BTBS is suitable in most scenarios for estimating spatial coordinates (The generic nature of equipment and technology). Experiments in continuous motions with seven DOFs and upper-limb motion description validated the excellent performance and robustness of P-BTBS in extracting motion information and describing upper-limb motions, respectively. P-BTBS provides a new perspective and mathematical tool for human understanding and exploration of upper-limb motions, which theoretically supports upper-limb research.

Recent progress and challenges in the treatment of spinal cord injury

Spinal cord injury (SCI) disrupts the structural and functional connectivity between the higher center and the spinal cord, resulting in severe motor, sensory, and autonomic dysfunction with a variety of complications. The pathophysiology of SCI is complicated and multifaceted, and thus individual treatments acting on a specific aspect or process are inadequate to elicit neuronal regeneration and functional recovery after SCI. Combinatory strategies targeting multiple aspects of SCI pathology have achieved greater beneficial effects than individual therapy alone. Although many problems and challenges remain, the encouraging outcomes that have been achieved in preclinical models offer a promising foothold for the development of novel clinical strategies to treat SCI. In this review, we characterize the mechanisms underlying axon regeneration of adult neurons and summarize recent advances in facilitating functional recovery following SCI at both the acute and chronic stages. In addition, we analyze the current status, remaining problems, and realistic challenges towards clinical translation. Finally, we consider the future of SCI treatment and provide insights into how to narrow the translational gap that currently exists between preclinical studies and clinical practice. Going forward, clinical trials should emphasize multidisciplinary conversation and cooperation to identify optimal combinatorial approaches to maximize therapeutic benefit in humans with SCI.

Optogenetic spinal stimulation promotes new axonal growth and skilled forelimb recovery in rats with sub-chronic cervical spinal cord injury

Objective.Spinal cord injury (SCI) leads to debilitating sensorimotor deficits that greatly limit quality of life. This work aims to develop a mechanistic understanding of how to best promote functional recovery following SCI. Electrical spinal stimulation is one promising approach that is effective in both animal models and humans with SCI. Optogenetic stimulation is an alternative method of stimulating the spinal cord that allows for cell-type-specific stimulation. The present work investigates the effects of preferentially stimulating neurons within the spinal cord and not glial cells, termed 'neuron-specific' optogenetic spinal stimulation. We examined forelimb recovery, axonal growth, and vasculature after optogenetic or sham stimulation in rats with cervical SCI.Approach.Adult female rats received a moderate cervical hemicontusion followed by the injection of a neuron-specific optogenetic viral vector ipsilateral and caudal to the lesion site. Animals then began rehabilitation on the skilled forelimb reaching task. At four weeks post-injury, rats received a micro-light emitting diode (µLED) implant to optogenetically stimulate the caudal spinal cord. Stimulation began at six weeks post-injury and occurred in conjunction with activities to promote use of the forelimbs. Following six weeks of stimulation, rats were perfused, and tissue stained for GAP-43, laminin, Nissl bodies and myelin. Location of viral transduction and transduced cell types were also assessed.Main Results.Our results demonstrate that neuron-specific optogenetic spinal stimulation significantly enhances recovery of skilled forelimb reaching. We also found significantly more GAP-43 and laminin labeling in the optogenetically stimulated groups indicating stimulation promotes axonal growth and angiogenesis.Significance.These findings indicate that optogenetic stimulation is a robust neuromodulator that could enable future therapies and investigations into the role of specific cell types, pathways, and neuronal populations in supporting recovery after SCI.

Single-cell analysis reveals region-heterogeneous responses in rhesus monkey spinal cord with complete injury

Spinal cord injury (SCI) leads to severe sensory and motor dysfunction below the lesion. However, the cellular dynamic responses and heterogeneity across different regions below the lesion remain to be elusive. Here, we used single-cell transcriptomics to investigate the region-related cellular responses in female rhesus monkeys with complete thoracic SCI from acute to chronic phases. We found that distal lumbar tissue cells were severely impacted, leading to degenerative microenvironments characterized by disease-associated microglia and oligodendrocytes activation alongside increased inhibitory interneurons proportion following SCI. By implanting scaffold into the injury sites, we could improve the injury microenvironment through glial cells and fibroblast regulation while remodeling spared lumbar tissues via reduced inhibitory neurons proportion and improved phagocytosis and myelination. Our findings offer crucial pathological insights into the spared distal tissues and proximal tissues after SCI, emphasizing the importance of scaffold-based treatment approaches targeting heterogeneous microenvironments.

A systematic review of computational models for the design of spinal cord stimulation therapies: from neural circuits to patient-specific simulations

Seventy years ago, Hodgkin and Huxley published the first mathematical model to describe action potential generation, laying the foundation for modern computational neuroscience. Since then, the field has evolved enormously, with studies spanning from basic neuroscience to clinical applications for neuromodulation. Computer models of neuromodulation have evolved in complexity and personalization, advancing clinical practice and novel neurostimulation therapies, such as spinal cord stimulation. Spinal cord stimulation is a therapy widely used to treat chronic pain, with rapidly expanding indications, such as restoring motor function. In general, simulations contributed dramatically to improve lead designs, stimulation configurations, waveform parameters and programming procedures and provided insight into potential mechanisms of action of electrical stimulation. Although the implementation of neural models are relentlessly increasing in number and complexity, it is reasonable to ask whether this observed increase in complexity is necessary for improved accuracy and, ultimately, for clinical efficacy. With this aim, we performed a systematic literature review and a qualitative meta-synthesis of the evolution of computational models, with a focus on complexity, personalization and the use of medical imaging to capture realistic anatomy. Our review showed that increased model complexity and personalization improved both mechanistic and translational studies. More specifically, the use of medical imaging enabled the development of patient-specific models that can help to transform clinical practice in spinal cord stimulation. Finally, we combined our results to provide clear guidelines for standardization and expansion of computational models for spinal cord stimulation.

Enhanced selectivity of transcutaneous spinal cord stimulation by multielectrode configuration

Objective.Transcutaneous spinal cord stimulation (tSCS) has been gaining momentum as a non-invasive rehabilitation approach to restore movement to paralyzed muscles after spinal cord injury (SCI). However, its low selectivity limits the types of movements that can be enabled and, thus, its potential applications in rehabilitation.Approach.In this cross-over study design, we investigated whether muscle recruitment selectivity of individual muscles could be enhanced by multielectrode configurations of tSCS in 16 neurologically intact individuals. We hypothesized that due to the segmental innervation of lower limb muscles, we could identify muscle-specific optimal stimulation locations that would enable improved recruitment selectivity over conventional tSCS. We elicited leg muscle responses by delivering biphasic pulses of electrical stimulation to the lumbosacral enlargement using conventional and multielectrode tSCS.Results.Analysis of recruitment curve responses confirmed that multielectrode configurations could improve the rostrocaudal and lateral selectivity of tSCS. To investigate whether motor responses elicited by spatially selective tSCS were mediated by posterior root-muscle reflexes, each stimulation event was a paired pulse with a conditioning-test interval of 33.3 ms. Muscle responses to the second stimulation pulse were significantly suppressed, a characteristic of post-activation depression suggesting that spatially selective tSCS recruits proprioceptive fibers that reflexively activate muscle-specific motor neurons in the spinal cord. Moreover, the combination of leg muscle recruitment probability and segmental innervation maps revealed a stereotypical spinal activation map in congruence with each electrode's position.Significance. Improvements in muscle recruitment selectivity could be essential for the effective translation into stimulation protocols that selectively enhance single-joint movements in neurorehabilitation.

Targeted transcutaneous spinal cord stimulation promotes persistent recovery of upper limb strength and tactile sensation in spinal cord injury: a pilot study

Long-term recovery of limb function is a significant unmet need in people with paralysis. Neuromodulation of the spinal cord through epidural stimulation, when paired with intense activity-based training, has shown promising results toward restoring volitional limb control in people with spinal cord injury. Non-invasive neuromodulation of the cervical spinal cord using transcutaneous spinal cord stimulation (tSCS) has shown similar improvements in upper-limb motor control rehabilitation. However, the motor and sensory rehabilitative effects of activating specific cervical spinal segments using tSCS have largely remained unexplored. We show in two individuals with motor-complete SCI that targeted stimulation of the cervical spinal cord resulted in up to a 1,136% increase in exerted force, with weekly activity-based training. Furthermore, this is the first study to document up to a 2-point improvement in clinical assessment of tactile sensation in SCI after receiving tSCS. Lastly, participant gains persisted after a one-month period void of stimulation, suggesting that targeted tSCS may lead to persistent recovery of motor and sensory function.

Spatiotemporal Distribution of Electrically Evoked Spinal Compound Action Potentials During Spinal Cord Stimulation

OBJECTIVES

Recent studies using epidural spinal cord stimulation (SCS) have demonstrated restoration of motor function in individuals previously diagnosed with chronic spinal cord injury (SCI). In parallel, the spinal evoked compound action potentials (ECAPs) induced by SCS have been used to gain insight into the mechanisms of SCS-based chronic pain therapy and to titrate closed-loop delivery of stimulation. However, the previous characterization of ECAPs recorded during SCS was performed with one-dimensional, cylindrical electrode leads. Herein, we describe the unique spatiotemporal distribution of ECAPs induced by SCS across the medial-lateral and rostral-caudal axes of the spinal cord, and their relationship to polysynaptic lower-extremity motor activation.

MATERIALS AND METHODS

In each of four sheep, two 24-contact epidural SCS arrays were placed on the lumbosacral spinal cord, spanning the L3 to L6 vertebrae. Spinal ECAPs were recorded during SCS from nonstimulating contacts of the epidural arrays, which were synchronized to bilateral electromyography (EMG) recordings from six back and lower-extremity muscles.

RESULTS

We observed a triphasic P1, N1, P2 peak morphology and propagation in the ECAPs during midline and lateral stimulation. Distinct regions of lateral stimulation resulted in simultaneously increased ECAP and EMG responses compared with stimulation at adjacent lateral contacts. Although EMG responses decreased during repetitive stimulation bursts, spinal ECAP amplitude did not significantly change. Both spinal ECAP responses and EMG responses demonstrated preferential ipsilateral recruitment during lateral stimulation compared with midline stimulation. Furthermore, EMG responses were correlated with stimulation that resulted in increased ECAP amplitude on the ipsilateral side of the electrode array.

CONCLUSIONS

These results suggest that ECAPs can be used to investigate the effects of SCS on spinal sensorimotor networks and to inform stimulation strategies that optimize the clinical benefit of SCS in the context of managing chronic pain and the restoration of sensorimotor function after SCI.

Multi-Site Spinal Cord Transcutaneous Stimulation Facilitates Upper Limb Sensory and Motor Recovery in Severe Cervical Spinal Cord Injury: A Case Study

Individuals with cervical spinal cord injury (SCI) rank regaining arm and hand function as their top rehabilitation priority post-injury. Cervical spinal cord transcutaneous stimulation (scTS) combined with activity-based recovery training (ABRT) is known to effectively facilitate upper extremity sensorimotor recovery in individuals with residual arm and hand function post SCI. However, scTS effectiveness in facilitating upper extremity recovery in individuals with severe SCI with minimal to no sensory and motor preservation below injury level remains largely unknown. We herein introduced a multimodal neuro-rehabilitative approach involving scTS targeting systematically identified various spinal segments combined with ABRT. We hypothesized that multi-site scTS combined with ABRT will effectively neuromodulate the spinal networks, resulting in improved integration of ascending and descending neural information required for sensory and motor recovery in individuals with severe cervical SCI. To test the hypothesis, a 53-year-old male (C2, AIS A, 8 years post-injury) received 60 ABRT sessions combined with continuous multi-site scTS. Post-training assessments revealed improved activation of previously paralyzed upper extremity muscles and sensory improvements over the dorsal and volar aspects of the hand. Most likely, altered spinal cord excitability and improved muscle activation and sensations resulted in observed sensorimotor recovery. However, despite promising neurophysiological evidence pertaining to motor re-activation, we did not observe visually appreciable functional recovery on obtained upper extremity motor assessments.

Human-machine interface for two-dimensional steering control with the auricular muscles

Human-machine interfaces (HMIs) can be used to decode a user's motor intention to control an external device. People that suffer from motor disabilities, such as spinal cord injury, can benefit from the uses of these interfaces. While many solutions can be found in this direction, there is still room for improvement both from a decoding, hardware, and subject-motor learning perspective. Here we show, in a series of experiments with non-disabled participants, a novel decoding and training paradigm allowing naïve participants to use their auricular muscles (AM) to control two degrees of freedom with a virtual cursor. AMs are particularly interesting because they are vestigial muscles and are often preserved after neurological diseases. Our method relies on the use of surface electromyographic records and the use of contraction levels of both AMs to modulate the velocity and direction of a cursor in a two-dimensional paradigm. We used a locking mechanism to fix the current position of each axis separately to enable the user to stop the cursor at a certain location. A five-session training procedure (20-30 min per session) with a 2D center-out task was performed by five volunteers. All participants increased their success rate (Initial: 52.78 ± 5.56%; Final: 72.22 ± 6.67%; median ± median absolute deviation) and their trajectory performances throughout the training. We implemented a dual task with visual distractors to assess the mental challenge of controlling while executing another task; our results suggest that the participants could perform the task in cognitively demanding conditions (success rate of 66.67 ± 5.56%). Finally, using the Nasa Task Load Index questionnaire, we found that participants reported lower mental demand and effort in the last two sessions. To summarize, all subjects could learn to control the movement of a cursor with two degrees of freedom using their AM, with a low impact on the cognitive load. Our study is a first step in developing AM-based decoders for HMIs for people with motor disabilities, such as spinal cord injury.

Walking naturally after spinal cord injury using a brain-spine interface

A spinal cord injury interrupts the communication between the brain and the region of the spinal cord that produces walking, leading to paralysis1,2. Here, we restored this communication with a digital bridge between the brain and spinal cord that enabled an individual with chronic tetraplegia to stand and walk naturally in community settings. This brain-spine interface (BSI) consists of fully implanted recording and stimulation systems that establish a direct link between cortical signals3 and the analogue modulation of epidural electrical stimulation targeting the spinal cord regions involved in the production of walking4-6. A highly reliable BSI is calibrated within a few minutes. This reliability has remained stable over one year, including during independent use at home. The participant reports that the BSI enables natural control over the movements of his legs to stand, walk, climb stairs and even traverse complex terrains. Moreover, neurorehabilitation supported by the BSI improved neurological recovery. The participant regained the ability to walk with crutches overground even when the BSI was switched off. This digital bridge establishes a framework to restore natural control of movement after paralysis.

Neuromodulation for recovery of trunk and sitting functions following spinal cord injury: a comprehensive review of the literature

Trunk stability is crucial for people with trunk paralysis resulting from spinal cord injuries (SCI), as it plays a significant role in performing daily life activities and preventing from fall-related accidents. Traditional therapy used assistive methods or seating modifications to provide passive assistance while restricting their daily functionality. The recent emergence of neuromodulation techniques has been reported as an alternative therapy that could improve trunk and sitting functions following SCI. The aim of this review was to provide a broad perspective on the existing studies using neuromodulation techniques and identify their potentials in terms of trunk recovery for people with SCI. Five databases were searched (PubMed, Embase, Science Direct, Medline-Ovid, and Web of Science) from inception to December 31, 2022 to identify relevant studies. A total of 21 studies, involving 117 participants with SCI, were included in this review. According to these studies, neuromodulation significantly improved the reaching ability, restored trunk stability and seated posture, increased sitting balance, as well as elevated activity of trunk and back muscles, which were considered early predictors of trunk recovery after SCI. However, there is limited evidence regarding neuromodulation techniques on the improvement of trunk and sitting functions. Therefore, future large-scale randomized controlled trials are warranted to validate these preliminary findings.

Design of an adaptable intrafascicular electrode (AIR) for selective nerve stimulation by model-based optimization

Peripheral nerve stimulation is being investigated as a therapeutic tool in several clinical scenarios. However, the adopted devices have restricted ability to obtain desired outcomes with tolerable off-target effects. Recent promising solutions are not yet employed in clinical practice due to complex required surgeries, lack of long-term stability, and implant invasiveness. Here, we aimed to design a neural interface to address these issues, specifically dimensioned for pudendal and sacral nerves to potentially target sexual, bladder, or bowel dysfunctions. We designed the adaptable intrafascicular radial electrode (AIR) through realistic computational models. They account for detailed human anatomy, inhomogeneous anisotropic conductance, following the trajectories of axons along curving and branching fascicles, and detailed biophysics of axons. The model was validated against available experimental data. Thanks to computationally efficient geometry-based selectivity estimations we informed the electrode design, optimizing its dimensions to obtain the highest selectivity while maintaining low invasiveness. We then compared the AIR with state-of-the-art electrodes, namely InterStim leads, multipolar cuffs and transversal intrafascicular multichannel electrodes (TIME). AIR, comprising a flexible substrate, surface active sites, and radially inserted intrafascicular needles, is designed to be implanted in a few standard steps, potentially enabling fast implants. It holds potential for repeatable stimulation outcomes thanks to its radial structural symmetry. When compared in-silico, AIR consistently outperformed cuff electrodes and InterStim leads in terms of recruitment threshold and stimulation selectivity. AIR performed similarly or better than a TIME, with quantified less invasiveness. Finally, we showed how AIR can adapt to different nerve sizes and varying shapes while maintaining high selectivity. The AIR electrode shows the potential to fill a clinical need for an effective peripheral nerve interface. Its high predicted performance in all the identified requirements was enabled by a model-based approach, readily applicable for the optimization of electrode parameters in any peripheral nerve stimulation scenario.