No abstract

Epidural electrical stimulation (EES) of the spinal cord restores locomotion in animal models of spinal cord injury (SCI) but is less effective in humans. Here, we hypothesized that this inter-species discrepancy is due to interference between EES and proprioceptive information in humans. Computational simulations, preclinical and clinical experiments reveal that EES blocks a significant amount of proprioceptive input in humans, but not in rats. This transient deafferentation prevents the modulation of reciprocal inhibitory networks involved in locomotion and reduces or abolishes the conscious perception of leg position. Consequently, continuous EES can only facilitate locomotion within a narrow range of stimulation parameters and is unable to provide meaningful locomotor improvements in humans without rehabilitation. Simulations showed that burst stimulation and spatiotemporal stimulation profiles mitigate the cancellation of proprioceptive information, enabling robust control over motoneuron activity. This demonstrates the importance of stimulation protocols that preserve proprioceptive information to facilitate walking with EES.

Epidural electrical stimulation (EES) targeting the dorsal roots of lumbosacral segments restored walking in people with spinal cord injury (SCI). However, EES was delivered with multielectrode paddle leads that were originally designed to target the dorsal column of the spinal cord. Here, we hypothesized that an arrangement of electrodes targeting the ensemble of dorsal roots involved in leg and trunk movements would result in superior efficacy, restoring more diverse motor activities after the most severe SCI. To test this hypothesis, we established a computational framework that informed the optimal arrangement of electrodes on a new paddle lead and guided its neurosurgical positioning. We also developed a software supporting the rapid configuration of activity-specific stimulation programs that reproduced the natural activation of motor neurons underlying each activity. We tested these neurotechnologies in three individuals with complete sensorimotor paralysis, as part of an ongoing clinical trial (clinicaltrials.gov, NCT02936453). Within a single day, activity-specific stimulation programs enabled the three individuals to stand, walk, cycle, swim, and control trunk movements. Neurorehabilitation mediated sufficient improvement to restore these activities in community settings, opening a realistic path to support everyday mobility with EES in people with SCI.

We thank Sim4Life by ZMT, www.zurichmedtech.com for their support. We thank Brian K. Kwon for critically reading the manuscript and his insightful suggestions.

A spinal cord injury interrupts pathways from the brain and brainstem that project to the lumbar spinal cord, leading to paralysis. Here we show that spatiotemporal epidural electrical stimulation (EES) of the lumbar spinal cord1–3 applied during neurorehabilitation4,5 (EESREHAB) restored walking in nine individuals with chronic spinal cord injury. This recovery involved a reduction in neuronal activity in the lumbar spinal cord of humans during walking. We hypothesized that this unexpected reduction reflects activity-dependent selection of specific neuronal subpopulations that become essential for a patient to walk after spinal cord injury. To identify these putative neurons, we modelled the technological and therapeutic features underlying EESREHAB in mice. We applied single-nucleus RNA sequencing6–9 and spatial transcriptomics10,11 to the spinal cords of these mice to chart a spatially resolved molecular atlas of recovery from paralysis. We then employed cell type12,13 and spatial prioritization to identify the neurons involved in the recovery of walking. A single population of excitatory interneurons nested within intermediate laminae emerged. Although these neurons are not required for walking before spinal cord injury, we demonstrate that they are essential for the recovery of walking with EES following spinal cord injury. Augmenting the activity of these neurons phenocopied the recovery of walking enabled by EESREHAB, whereas ablating them prevented the recovery of walking that occurs spontaneously after moderate spinal cord injury. We thus identified a recovery-organizing neuronal subpopulation that is necessary and sufficient to regain walking after paralysis. Moreover, our methodology establishes a framework for using molecular cartography to identify the neurons that produce complex behaviours.

The convergence of materials science, electronics, and biology, namely bioelectronic interfaces, leads novel and precise communication with biological tissue, particularly with the nervous system. However, the translation of lab‐based innovation toward clinical use calls for further advances in materials, manufacturing and characterization paradigms, and design rules. Herein, a translational framework engineered to accelerate the deployment of microfabricated interfaces for translational research is proposed and applied to the soft neurotechnology called electronic dura mater, e‐dura. Anatomy, implant function, and surgical procedure guide the system design. A high‐yield, silicone‐on‐silicon wafer process is developed to ensure reproducible characteristics of the electrodes. A biomimetic multimodal platform that replicates surgical insertion in an anatomy‐based model applies physiological movement, emulates therapeutic use of the electrodes, and enables advanced validation and rapid optimization in vitro of the implants. Functionality of scaled e‐dura is confirmed in nonhuman primates, where epidural neuromodulation of the spinal cord activates selective groups of muscles in the upper limbs with unmet precision. Performance stability is controlled over 6 weeks in vivo. The synergistic steps of design, fabrication, and biomimetic in vitro validation and in vivo evaluation in translational animal models are of general applicability and answer needs in multiple bioelectronic designs and medical technologies.

Single-molecule localization microscopy (SMLM) relies on the switching of fluorescent molecules between a fluorescent and a dark state to achieve super resolution. This process is inherently dependent on the intensity distribution of the laser light used for both activation from the dark state and excitation of the bright state. Typically, laser light is coupled directly or via a single-mode fiber into the microscope, which leads to a Gaussian intensity profile in total internal reflection (TIR) or epi illumination. As a result, switching dynamics and brightness of the fluorescent molecules vary strongly across the field of view, impacting their localization precision and impeding quantitative analysis. Here we present a simple illumination scheme based on the use of a multimode fiber and a laser speckle-reducer, which results in a flat, homogeneous and speckle-free illumination across the entire field of view. In addition, we combined homogeneous multimode excitation of the sample with single-mode based TIR activation to simultaneously obtain the advantages of both approaches: uniform brightness of single fluorophores and TIR-like optical sectioning.

Optoelectronic systems can exert precise control over targeted neurons and pathways throughout the brain in untethered animals, but similar technologies for the spinal cord are not well established. Here, we describe a novel system for ultrafast, wireless, closed-loop manipulation of targeted neurons and pathways across the entire dorsoventral spinal cord in untethered mice. We developed a soft stretchable carrier integrating micro-LEDs that conforms to the dura matter of the spinal cord. A coating of silicone-phosphor matrix over the micro-LEDs provides mechanical protection and light conversion for compatibility with the large library of opsins. A lightweight, head-mounted wireless platform powers the micro-LEDs and performs low-latency on-chip processing of sensed physiological signals to control photostimulation in a closed-loop. We use the device to reveal the role of various neuronal subtypes, sensory pathways and supraspinal projections in the control of locomotion in healthy and spinal-cord injured mice.