One hour electrical stimulation accelerates functional recovery after femoral nerve repair

Electrical stimulation promotes functional recovery after spinal cord injury by activating endogenous spinal cord-derived neural stem/progenitor cell: an in vitro and in vivo study

BACKGROUND CONTEXT

Electrical stimulation is a noninvasive treatment method that has gained popularity in the treatment of spinal cord injury (SCI). Activation of spinal cord-derived neural stem/progenitor cell (SC-NSPC) proliferation and differentiation in the injured spinal cord may elicit considerable neural regenerative effects.

PURPOSE

This study aimed to explore the effect of electrical stimulation on the neurogenesis of SC-NSPCs.

STUDY DESIGN

This study analyzed the effects of electrical stimulation on neurogenesis in rodent SC-NSPCs in vitro and in vivo and evaluated functional recovery and neural circuitry improvements with electrical stimulation using a rodent SCI model.

METHODS

Rats (20 rats/group) were assigned to sham (Group 1), SCI only (Group 2), SCI + electrode implant without stimulation (Group 3), and SCI + electrode with stimulation (Group 4) groups to count total SC-NSPCs and differentiated neurons and to evaluate morphological changes in differentiated neurons. Furthermore, the Basso, Beattie, and Bresnahan scores were analyzed, and the motor- and somatosensory-evoked potentials in all rats were monitored.

RESULTS

Biphasic electrical currents enhanced SC-NSPC proliferation differentiation and caused qualitative morphological changes in differentiated neurons in vitro. Electrical stimulation promoted SC-NSPC proliferation and neuronal differentiation and improved functional outcomes and neural circuitry in SCI models. Increased Wnt3, Wnt7, and β-catenin protein levels were also observed after electrical stimulation.

CONCLUSIONS

Our study proved the beneficial effects of electrical stimulation on SCI. The Wnt/β-catenin pathway activation may be associated with this relationship between electrical stimulation and neuronal regeneration after SCI.

CLINICAL SIGNIFICANCE

The study confirmed the benefits of electrical stimulation on SCI based on cellular, functional, electrophysiological, and histological evidence. Based on these findings, we expect electrical stimulation to make a positive and significant difference in SCI treatment strategies.

Brief Electrical Stimulation Promotes Recovery after Surgical Repair of Injured Peripheral Nerves

Injured peripheral nerves regenerate their axons in contrast to those in the central nervous system. Yet, functional recovery after surgical repair is often disappointing. The basis for poor recovery is progressive deterioration with time and distance of the growth capacity of the neurons that lose their contact with targets (chronic axotomy) and the growth support of the chronically denervated Schwann cells (SC) in the distal nerve stumps. Nonetheless, chronically denervated atrophic muscle retains the capacity for reinnervation. Declining electrical activity of motoneurons accompanies the progressive fall in axotomized neuronal and denervated SC expression of regeneration-associated-genes and declining regenerative success. Reduced motoneuronal activity is due to the withdrawal of synaptic contacts from the soma. Exogenous neurotrophic factors that promote nerve regeneration can replace the endogenous factors whose expression declines with time. But the profuse axonal outgrowth they provoke and the difficulties in their delivery hinder their efficacy. Brief (1 h) low-frequency (20 Hz) electrical stimulation (ES) proximal to the injury site promotes the expression of endogenous growth factors and, in turn, dramatically accelerates axon outgrowth and target reinnervation. The latter ES effect has been demonstrated in both rats and humans. A conditioning ES of intact nerve days prior to nerve injury increases axonal outgrowth and regeneration rate. Thereby, this form of ES is amenable for nerve transfer surgeries and end-to-side neurorrhaphies. However, additional surgery for applying the required electrodes may be a hurdle. ES is applicable in all surgeries with excellent outcomes.

State of the Art and Current Challenges on Electroactive Biomaterials and Strategies for Neural Tissue Regeneration

The loss or failure of an organ/tissue stands as one of the healthcare system's most prevalent, devastating, and costly challenges. Strategies for neural tissue repair and regeneration have received significant attention due to their particularly strong impact on patients' well-being. Many research efforts are dedicated not only to control the disease symptoms but also to find solutions to repair the damaged tissues. Neural tissue engineering (TE) plays a key role in addressing this problem and significant efforts are being carried out to develop strategies for neural repair treatment. In the last years, active materials allowing to tune cell-materials interaction are being increasingly used, representing a recent paradigm in TE applications. Among the most important stimuli influencing cell behavior are the electrical and mechanical ones. In this way, materials with the ability to provide this kind of stimuli to the neural cells seem to be appropriate to support neural TE. In this scope, this review summarizes the different biomaterials types used for neural TE, highlighting the relevance of using active biomaterials and electrical stimulation. Furthermore, this review provides not only a compilation of the most relevant studies and results but also strategies for novel and more biomimetic approaches for neural TE.

Clinical applications of electrical stimulation for peripheral nerve injury: a systematic review

Introduction

Peripheral nerve injuries are common neurologic injuries that are challenging to treat with current therapies. Electrical stimulation has been shown to accelerate reinnervation and enhance functional recovery. This study aims to review the literature on clinical application of electrical stimulation for peripheral nerve injury.

Methods

PubMed and Embase were sourced from 1995 to August 2022. Selection was based on predetermined inclusion/exclusion criteria. Eight hundred and thirty-five articles were screened with seven being included in this review.

Results

Two hundred and twenty-nine patients with peripheral nerve injuries were represented. Six of the studies were randomized controlled trials. A variety of nerve injuries were represented with all being in the upper extremity and supraclavicular region. Electrical stimulation protocols and evaluation varied. Electrodes were implanted in four studies with one also implanting the stimulator. Length of stimulation per session was either 20 mins or 1 h. Median stimulation frequency was 20 Hz. Stimulation intensity varied from 3 to 30V; pulse width ranged from 0.1 to 1.007 ms. Three protocols were conducted immediately after surgery. Patients were followed for an average of 13.5 months and were evaluated using electrophysiology and combinations of motor, sensory, and functional criteria.

Discussion

Patients who received electrical stimulation consistently demonstrated better recovery compared to their respective controls. Electrical stimulation for peripheral nerve injury is a novel treatment that has not been well-studied in humans. Our review illustrates the potential benefit in implementing this approach into everyday practice. Future research should aim to optimize protocol for clinical use.

Electroceuticals for Regeneration of Long Nerve Gap Using Biodegradable Conductive Conduits and Implantable Wireless Stimulator

Regeneration of over 10 mm long peripheral nerve defects remains a challenge due to the failure of regeneration by prolonged axotomy and denervation occurring in long-term recovery. Recent studies reveal that conductive conduits and electrical stimulation accelerate the regeneration of long nerve defects. In this study, an electroceutical platform combining a fully biodegradable conductive nerve conduit and a wireless electrical stimulator is proposed to maximize the therapeutic effect on nerve regeneration. Fully biodegradable nerve conduit fabricated using molybdenum (Mo) microparticles and polycaprolactone (PCL) can eliminate the unwanted effects of non-degradable implants, which occupy nerve paths and need to be removed through surgery increasing the risk of complications. The electrical and mechanical properties of Mo/PCL conduits are optimized by controlling the amounts of Mo and tetraglycol lubricant. The dissolution behavior and electrical conductivity of biodegradable nerve conduits in the biomimetic solutions are also evaluated. In in vivo experiments, the integrated strategy of a conductive Mo/PCL conduit with controlled therapeutic electrical stimulation shows accelerated axon regeneration for long sciatic nerve defects in rats compared to the use of the Mo/PCL conduit without stimulation and has a significant therapeutic effect based on the results obtained from the functional recovery test.

Perspectives of Electroacupuncture as a New Option for the Treatment of Denervated Muscles

Introduction

Conservative treatment of peripheral nerve injuries is based on physical therapy approaches, including electrostimulation of denervated muscle. Electrostimulation retards denervation atrophy and prolongs the time window for axon reinnervation.

Aim

This article focuses on the potential of electroacupuncture, which combines electrostimulation with acupuncture, in the context of the latest knowledge on the mechanisms of axonal regeneration.

Results and conclusions

The possibilities of influencing the growth rate of the axon itself through neurotrophic factors have primarily been previously proven in rodent models. Electroacupuncture as mini-invasive electrostimulation using acupuncture needles appears to be a promising option for the treatment of peripheral nerve paresis. However, this therapy needs to be evaluated in the context of human medicine.

Effects of transcutaneous neuromuscular electrical stimulation on post-stroke dysphagia: a systematic review and meta-analysis

Background

Dysphagia is one of the common complications after stroke. It is closely related to lung infection and malnutrition. Neuromuscular electrical stimulation (NMES) is widely used in the treatment of post-stroke dysphagia, but the evidence-based medical evidence of NMES is limited. Therefore, this study aimed to evaluate the clinical efficacy of NMES in patients with post-stroke dysphagia by systematic review and meta-analysis.

Methods

We searched the CNKI, Wanfang, VIP, SinoMed, PubMed, Embase, Cochrane Library, and Web of Science databases for all randomized controlled trials (RCTs) of NMES in the treatment of post-stroke dysphagia from the establishment of the database to 9 June 2022. The risk of bias assessment tool recommended by Cochrane and the GRADE method was used to assess the risk of bias and the quality of evidence. RevMan 5.3 was used for statistical analysis. Sensitivity and subgroup analyses were performed to evaluate the intervention effect more specifically.

Results

A total of 46 RCTs and 3,346 patients with post-stroke dysphagia were included in this study. Our meta-analysis showed that NMES combined with routine swallowing therapy (ST) could effectively improve swallowing function in Penetration-Aspiration Scale (MD = -0.63, 95% CI [-1.15, -0.12], P = 0.01), Functional Oral Intake Scale (MD = 1.32, 95% CI [0.81, 1.83], P < 0.00001), Functional Dysphagia Scale (MD = - 8.81, 95% CI [-16.48, -1.15], P = 0.02), the Standardized Swallowing Assessment (MD = -6.39, 95% CI [-6.56, -6.22], P < 0.00001), the Videofluoroscopic Swallow Study (MD = 1.42, 95% CI [1.28, 1.57], P < 0.00001) and the Water swallow test (MD = -0.78, 95% CI [-0.84, -0.73], P < 0.00001). Furthermore, it could improve the quality of life (MD = 11.90, 95% CI [11.10, 12.70], P < 0.00001), increase the upward movement distance of hyoid bone (MD = 2.84, 95% CI [2.28, 3.40], P < 0.00001) and the forward movement distance of hyoid bone (MD = 4.28, 95% CI [3.93, 4.64], P < 0.00001), reduce the rate of complications (OR = 0.37, 95%CI [0.24, 0.57], P < 0.00001). Subgroup analyses showed that NMES+ST was more effective at 25 Hz, 7 mA or 0-15 mA, and at courses ( ≤ 4 weeks). Moreover, patients with an onset of fewer than 20 days and those older than 60 years appear to have more positive effects after treatment.

Conclusion

NMES combined with ST could effectively increase the forward and upward movement distance of the hyoid bone, improve the quality of life, reduce the rate of complications, and improve the swallowing function of patients with post-stroke dysphagia. However, its safety needs to be further confirmed.

Systematic review registration

https://www.crd.york.ac.uk/PROSPERO, identifier: CRD42022368416.

Intraoperative electrical stimulation promotes the short-term recovery of patients with cubital tunnel syndrome after surgery

BACKGROUND

This study was designed to investigate whether intraoperative electrical nerve stimulation has effects on the short-term recovery of cubital tunnel syndrome patients after ulnar nerve release.

METHODS

Patients diagnosed as cubital tunnel syndrome were selected. At the same time, they received conventional surgery treatment. The patients were divided by a randomized digits table into two groups. The control group underwent conventional surgery, and the electrical stimulation (ES) group underwent intraoperative electrical stimulation. All the patients were tested for sensory and motor functions, grip strength, key pinch strength, motor conductivity velocity (MCV), and maximum compound muscle action potential (CMAP) before operation and 1 month and 6 months after operation.

RESULTS

In patients treated with intraoperative ES, the sensory and motor functions and the strength of muscle were significantly improved after 1-month and 6-month follow-up than the control group. After the follow-up, the patients in the ES group had significantly higher grip strength and key pinch strength than the control group. After the follow-up, the patients in the ES group had significantly higher MCV and CMAP than the control group.

CONCLUSION

Intraoperative electrical stimulation of nerve muscle can significantly promote the short-term recovery of nerve and muscle functions after the surgery in cubital tunnel syndrome patients.

Electrical stimulation accelerates Wallerian degeneration and promotes nerve regeneration after sciatic nerve injury

Following peripheral nerve injury (PNI), Wallerian degeneration (WD) in the distal stump can generate a microenvironment favorable for nerve regeneration. Brief low-frequency electrical stimulation (ES) is an effective treatment for PNI, but the mechanism underlying its effect on WD remains unclear. Therefore, we hypothesized that ES could enhance nerve regeneration by accelerating WD. To verify this hypothesis, we used a rat model of sciatic nerve transection and provided ES at the distal stump of the injured nerve. The injured nerve was then evaluated after 1, 4, 7, 14 and 21 days post injury (dpi). The results showed that ES significantly promoted the degeneration and clearance of axons and myelin, and the dedifferentiation of Schwann cells. It upregulated the expression of BDNF and NGF and increased the number of monocytes and macrophages. Through transcriptome sequencing, we systematically investigated the effect of ES on the molecular processes involved in WD at 4 dpi. Evaluation of nerves bridged using silicone tubing after transection showed that ES accelerated early axonal and vascular regeneration while delaying gastrocnemius atrophy. These results demonstrate that ES promotes nerve regeneration by accelerating WD and upregulating the expression of neurotrophic factors.

Transcutaneous and Direct Electrical Stimulation of Mouse Sciatic Nerve Accelerates Functional Recovery After Nerve Transection and Immediate Repair

BACKGROUND

Electrical stimulation can accelerate peripheral nerve regeneration after injury and repair. Clinically, direct electrical stimulation (DES) may involve longer operating times, increasing risks of perioperative complications. Transcutaneous electrical stimulation (TCES) is a noninvasive alternative. In this study, we investigate how transcutaneous and DES compare for accelerating functional nerve recovery in a mouse sciatic nerve model.

METHODS

Twenty-eight mice were divided into sham (n = 4), axotomy (n = 8), DES (n = 8), and TCES (n = 8) groups. After sciatic nerve transection and repair, the proximal nerve was subjected to DES or TCES at 20 Hz for 1 hour. Sciatic functional index was measured before the injury, and at weeks 1, 2, 4, 6, 8, 10, and 12 by walking-track analysis. Electrophysiological measures were taken at week 12.

RESULTS

Kinematic studies showed significant improvement from the 8th week to the 12th week for both electrical stimulation groups compared with the axotomy group (P < 0.05), with no difference between the electrical stimulation groups. At the 12th week, both DES and TCES groups had significantly faster average conduction velocity than the axotomy group.

CONCLUSIONS

Functional recovery was significantly better from 8 weeks onward in mice receiving either DES or TCES stimulation when compared with axotomy and repair alone. Transcutaneous electrical stimulation is a minimally invasive alternative treatment for accelerating functional recovery after peripheral nerve injury.

Injectable 2D flexible hydrogel sheets for optoelectrical/biochemical dual stimulation of neurons

Major challenges in developing implanted neural stimulation devices are the invasiveness, complexity, and cost of the implantation procedure. Here, we report an injectable, nanofibrous 2D flexible hydrogel sheet-based neural stimulation device that can be non-invasively implanted via syringe injection for optoelectrical and biochemical dual stimulation of neuron. Specifically, methacrylated gelatin (GelMA)/alginate hydrogel nanofibers were mechanically reinforced with a poly(lactide-co-ε-caprolactone) (PLCL) core by coaxial electrospinning. The lubricant hydrogel shell enabled not only injectability, but also facile incorporation of functional nanomaterials and bioactives. The nanofibers loaded with photocatatlytic g-C₃N₄/GO nanoparticles were capable of stimulating neural cells via blue light, with a significant 36.3 % enhancement in neurite extension. Meanwhile, the nerve growth factor (NGF) loaded nanofibers supported a sustained release of NGF with well-maintained function to biochemically stimulate neural differentiation. We have demonstrated the capability of an injectable, hydrogel nanofibrous, neural stimulation system to support neural stimulation both optoelectrically and biochemically, which represents crucial early steps in a larger effort to create a minimally invasive system for neural stimulation.

The Role of Physical Exercise and Rehabilitative Implications in the Process of Nerve Repair in Peripheral Neuropathies: A Systematic Review

BACKGROUND

The various mechanisms involved in peripheral nerve regeneration, induced by exercise and electrical nerve stimulation, are still unclear.

OBJECTIVE

The aim of this review was to summarize the influence of physical exercise and/or electrical stimulation on peripheral nerve repair and regeneration and the variation of impact of intervention depending on timing, as well as kind and dosage of the intervention. A literature survey was conducted on PubMed, Scopus, and Web of Science, between February 2021 to July 2021, with an update in September 2022.

METHODOLOGY

The literature search identified 101,386 articles with the keywords: "peripheral nerve" OR "neuropathy" AND "sprouting" OR "neuroapraxia" OR "axonotmesis" OR "neurotmesis" OR "muscle denervation" OR "denervated muscle" AND "rehabilitation" OR "physical activity" OR "physical exercise" OR "activity" OR "electrical stimulation". A total of 60 publications were included. Eligible studies were focused on evaluating the process of nerve repair (biopsy, electromyographic parameters or biomarker outcomes) after electrical stimulation or physical exercise interventions on humans or animals with peripheral sensory or motor nerve injury.

SYNTHESIS

This study shows that the literature, especially regarding preclinical research, is mainly in agreement that an early physical program with active exercise and/or electrical stimulation promotes axonal regenerative responses and prevents maladaptive response. This was evaluated by means of changes in electrophysiological recordings of CMAPs for latency amplitude, and the sciatic functional index (SFI). Furthermore, this type of activity can cause an increase in weight and in muscle fiber diameter. Nevertheless, some detrimental effects of exercising and electrical stimulation too early after nerve repair were recorded.

CONCLUSION

In most preclinical studies, peripheral neuropathy function was associated with improvements after physical exercise and electrical stimulation. For humans, too little research has been conducted on this topic to reach a complete conclusion. This research supports the need for future studies to test the validity of a possible rehabilitation treatment in humans in cases of peripheral neuropathy to help nerve sprouting.

Electroceuticals for peripheral nerve regeneration

Electroceuticals provide promising opportunities for peripheral nerve regeneration, in terms of modulating the extensive endogenous tissue repair mechanisms between neural cell body, axons and target muscles. However, great challenges remain to deliver effective and controllable electroceuticals via bioelectronic implantable device. In this review, the modern fabrication methods of bioelectronic conduit for bridging critical nerve gaps after nerve injury are summarized, with regard to conductive materials and core manufacturing process. In addition, to deliver versatile electrical stimulation, the integration of implantable bioelectronic device is discussed, including wireless energy harvesters, actuators and sensors. Moreover, a comprehensive insight of beneficial mechanisms is presented, including up-to-date in vitro, in vivo and clinical evidence. By integrating conductive biomaterials, 3D engineering manufacturing process and bioelectronic platform to deliver versatile electroceuticals, the modern biofabrication enables comprehensive biomimetic therapies for neural tissue engineering and regeneration in the new era.

Genetic control of neuronal activity enhances axonal growth only on permissive substrates

Background

Neural tissue has limited regenerative ability. To cope with that, in recent years a diverse set of novel tools has been used to tailor neurostimulation therapies and promote functional regeneration after axonal injuries.

Method

In this report, we explore cell-specific methods to modulate neuronal activity, including opto- and chemogenetics to assess the effect of specific neuronal stimulation in the promotion of axonal regeneration after injury.

Results

Opto- and chemogenetic stimulations of neuronal activity elicited increased in vitro neurite outgrowth in both sensory and cortical neurons, as well as in vivo regeneration in the sciatic nerve, but not after spinal cord injury. Mechanistically, inhibitory substrates such as chondroitin sulfate proteoglycans block the activity induced increase in axonal growth.

Conclusions

We found that genetic modulations of neuronal activity on both dorsal root ganglia and corticospinal motor neurons increase their axonal growth capacity but only on permissive environments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10020-022-00524-2.

Chronic muscle recordings reveal recovery of forelimb function in spinal injured female rats after cortical epidural stimulation combined with rehabilitation and chondroitinase ABC

Cervical level spinal cord injury (SCI) can severely impact upper limb muscle function, which is typically assessed in the clinic using electromyography (EMG). Here, we established novel preclinical methodology for EMG assessments of muscle function after SCI in awake freely moving animals. Adult female rats were implanted with EMG recording electrodes in bicep muscles and received bilateral cervical (C7) contusion injuries. Forelimb muscle activity was assessed by recording maximum voluntary contractions during a grip strength task and cortical motor evoked potentials in the biceps. We demonstrate that longitudinal recordings of muscle activity in the same animal are feasible over a chronic post-injury time course and provide a sensitive method for revealing post-injury changes in muscle activity. This methodology was utilized to investigate recovery of muscle function after a novel combination therapy. Cervical contused animals received intraspinal injections of a neuroplasticity-promoting agent (lentiviral-chondroitinase ABC) plus 11 weeks of cortical epidural electrical stimulation (3 h daily, 5 days/week) and behavioral rehabilitation (15 min daily, 5 days/week). Longitudinal monitoring of voluntary and evoked muscle activity revealed significantly increased muscle activity and upper limb dexterity with the combination treatment, compared to a single treatment or no treatment. Retrograde mapping of motor neurons innervating the biceps showed a predominant distribution across spinal segments C5-C8, indicating that treatment effects were likely due to neuroplastic changes in a mixture of intact and injured motor neurons. Thus, longitudinal assessments of muscle function after SCI correlate with skilled reach and grasp performance and reveal functional benefits of a novel combination therapy.

Selective Electrical Surface Stimulation to Support Functional Recovery in the Early Phase After Unilateral Acute Facial Nerve or Vocal Fold Paralysis

This article addresses the potential clinical value of surface electrical stimulation in the acute phase of denervation after the onset of facial nerve or recurrent laryngeal nerve paralysis. These two nerve lesions are the most frequent head and neck nerve lesions. In this review, we will work out several similarities concerning the pathophysiology features and the clinical scenario between both nerve lesions, which allow to develop some general rules for surface electrical stimulation applicable for both nerve lesions. The focus is on electrical stimulation in the phase between denervation and reinnervation of the target muscles. The aim of electrostimulation in this phase of denervation is to bridge the time until reinnervation is complete and to maintain facial or laryngeal function. In this phase, electrostimulation has to stimulate directly the denervated muscles, i.e. muscle stimulation and not nerve stimulation. There is preliminary data that early electrostimulation might also improve the functional outcome. Because there are still caveats against the use of electrostimulation, the neurophysiology of denervated facial and laryngeal muscles in comparison to innervated muscles is explained in detail. This is necessary to understand why the negative results published in several studies that used stimulation parameters are not suitable for denervated muscle fibers. Juxtaposed are studies using parameters adapted for the stimulation of denervated facial or laryngeal muscles. These studies used standardized outcome measure and show that an effective and tolerable electrostimulation of facial and laryngeal muscles without side effects in the early phase after onset of the lesions is feasible, does not hinder nerve regeneration and might even be able to improve the functional outcome. This has now to be proven in larger controlled trials. In our view, surface electrical stimulation has an unexploited potential to enrich the early therapy concepts for patients with unilateral facial or vocal fold paralysis.

Role of Electrical Stimulation in Peripheral Nerve Regeneration: A Systematic Review

Functional recovery after peripheral nerve injury is often suboptimal despite the intrinsic permissive growth environment of the peripheral nervous system. The objective of this systematic review is to explore the use of electrical stimulation (ES) for peripheral nerve regeneration.

Gate Mechanism and Parameter Analysis of Anodal-First Waveforms for Improving Selectivity of C-Fiber Nerves

PURPOSE

Few investigations have been conducted on the selective stimulation of small-radius unmyelinated C nerves (C), which are critical to both the recovery of damaged nerves and pain suppression. The purpose of this study is to understand how an anodal pulse in an anodal-first stimulation could improve C-selectivity over myelinated nociceptive Aδ nerves (Aδ) and to further clarify the landscape of the solution space.

MATERIALS AND METHODS

An adapted Hodgkin-Huxley (HH) model and the McIntyre-Richardson-Grill (MRG) model were used for modeling C and Aδ, respectively, to analyze the underlying ion dynamics and the influence of relevant stimulation waveforms, including monopolar, polarity-symmetric, and asymmetric pulses.

RESULTS

The results showed that polarity asymmetric waveforms with preceding anodal stimulations benefit C-selectivity the most, underlain by the decrease in the potassium ion current of C.

CONCLUSION

The optimal parameters for C-selectivity have been identified in the low-frequency band, remarkably benefiting the design of selective stimulation waveforms for the recovery of damaged nerves and pain management.

Piezoelectric and Magnetoelectric Scaffolds for Tissue Regeneration and Biomedicine: A Review

Electric fields are ubiquitous throughout the body, playing important role in a multitude of biological processes including osteo-regeneration, cell signaling, nerve regeneration, cardiac function, and DNA replication. An increased understanding of the role of electric fields in the body has led to the development of devices for biomedical applications that incorporate electromagnetic fields as an intrinsically novel functionality (e.g., bioactuators, biosensors, cardiac/neural electrodes, and tissues scaffolds). However, in the majority of the aforementioned devices, an implanted power supply is necessary for operation, and therefore requires highly invasive procedures. Thus, the ability to apply electric fields in a minimally invasive manner to remote areas of the body remains a critical and unmet need. Here, we report on the potential of magnetoelectric (ME)-based composites to overcome this challenge. ME materials are capable of producing localized electric fields in response to an applied magnetic field, which the body is permeable to. Yet, the use of ME materials for biomedical applications is just beginning to be explored. Here, we present on the potential of ME materials to be utilized in biomedical applications. This will be presented alongside current state-of-the-art for in vitro and in vivo electrical stimulation of cells and tissues. We will discuss key findings in the field, while also identifying challenges, such as the synthesis and characterization of biocompatible ME materials, challenges in experimental design, and opportunities for future research that would lead to the increased development of ME biomaterials and their applications.

Postsurgical Electrical Stimulation Enhances Recovery Following Surgery for Severe Cubital Tunnel Syndrome: A Double-Blind Randomized Controlled Trial

BACKGROUND

Patients with severe cubital tunnel syndrome often have poor functional recovery with conventional surgical treatment. Postsurgical electrical stimulation (PES) has been shown to enhance axonal regeneration in animal and human studies.

OBJECTIVE

To determine if PES following surgery for severe cubital tunnel syndrome would result in better outcomes compared to surgery alone.

METHODS

Patients with severe cubital tunnel syndrome in this randomized, double-blind, placebo-controlled trial were randomized in a 1:2 ratio to the control or stimulation groups. Control patients received cubital tunnel surgery and sham stimulation, whereas patients in the stimulation group received 1-h of 20 Hz PES following surgery. Patients were assessed by a blinded evaluator annually for 3 yr. The primary outcome was motor unit number estimation (MUNE) and secondary outcomes were grip and key pinch strength and McGowan grade and compound muscle action potential.

RESULTS

A total of 31 patients were enrolled: 11 received surgery alone and 20 received surgery and PES. Three years following surgery, MUNE was significantly higher in the PES group (176 ± 23, mean + SE) compared to controls (88 ± 11, P < .05). The mean gain in key pinch strength in the PES group was almost 3 times greater than in the controls (P < .05). Similarly, other functional and physiological outcomes showed significantly greater improvements in the PES group.

CONCLUSION

PES enhanced muscle reinnervation and functional recovery following surgery for severe cubital tunnel syndrome. It may be a clinically useful adjunct to surgery for severe ulnar neuropathy, in which functional recovery with conventional treatment is often suboptimal.

Electrical stimulation of co-woven nerve conduit for peripheral neurite differentiation

Electrically stimulable nerve conduits are implants that could potentially be utilized in patients with nerve injury for restoring function and limb mobility. Such conduits need to be developed from specialized scaffolds that are both electrically conductive and allow neuronal attachment and differentiation. In this study, we investigate neural cell attachment and axonal differentiation on scaffolds co-woven with poly-(L-lactic acid) (PLLA) yarns and conducting threads. Yarns obtained from electrospun PLLA were co-woven with polypyrrole (PPy)-coated PLLA yarns or ultrathin wires of copper or platinum using a custom built low-resistance semi-automated weaving machine. The conducting threads were first electrically characterized and tested for stability in cell growth media. Suitability of the conducting threads was further assessed via cell viability studies using PC12 cells. Neurite growth was then quantified after electrically stimulating rat dorsal root ganglion (DRG) sensory neurons cultured on the woven scaffolds. Electrical conductivity tests and cellular viability studies demonstrated better bio-tolerability of platinum wires over PPy-coated PLLA yarns and copper wires. Electrically stimulated DRG neurons cultured on platinum-PLLA co-woven scaffolds showed enhanced neurite outgrowth and length. We demonstrate that a woven scaffold design could be utilized to incorporate conducting materials into cell-tolerable polymer yarns for developing electrically stimulable nerve conduits.

CNT/Sericin Conductive Nerve Guidance Conduit Promotes Functional Recovery of Transected Peripheral Nerve Injury in a Rat Model

Peripheral nerve injury usually leads to poor outcomes such as painful neuropathies and disabilities. Autogenous nerve grafting is the current gold standard; however, the limited source of a donor nerve remains a problem. Numerous tissue engineering nerve guidance conduits have been developed as substitutes for autografts. However, a few conduits can achieve the reparative effect equivalent to autografts. Here, we report for the development and application of a carbon nanotube (CNT)/sericin nerve conduit with electrical conductivity and suitable mechanical properties for nerve repair. This CNT/sericin conduit possesses favorable properties including biocompatibility, biodegradability, porous microarchitecture, and suitable swelling property. We thus applied this conduit for bridging a 10 mm gap defect of a transected sciatic nerve combined with electrical stimulation (ES) in a rat injury model. By the end of 12 weeks, we observed that the CNT/sericin conduit combined with electrical stimulation could effectively promote both structural repair and functional recovery comparable to those of the autografts, evidenced by the morphological and histological analyses, electrophysiological responses, functional studies, and target muscle reinnervation evaluations. These findings suggest that this electric conductive CNT/sericin conduit combined with electrical stimulation may have the potential to serve as a new alternative for the repair of transected peripheral nerves.

Functional Electrical Stimulation and the Modulation of the Axon Regeneration Program

Neural injury in mammals often leads to persistent functional deficits as spontaneous repair in the peripheral nervous system (PNS) is often incomplete, while endogenous repair mechanisms in the central nervous system (CNS) are negligible. Peripheral axotomy elicits growth-associated gene programs in sensory and motor neurons that can support reinnervation of peripheral targets given sufficient levels of debris clearance and proximity to nerve targets. In contrast, while damaged CNS circuitry can undergo a limited amount of sprouting and reorganization, this innate plasticity does not re-establish the original connectivity. The utility of novel CNS circuitry will depend on effective connectivity and appropriate training to strengthen these circuits. One method of enhancing novel circuit connectivity is through the use of electrical stimulation, which supports axon growth in both central and peripheral neurons. This review will focus on the effects of CNS and PNS electrical stimulation in activating axon growth-associated gene programs and supporting the recovery of motor and sensory circuits. Electrical stimulation-mediated neuroplasticity represents a therapeutically viable approach to support neural repair and recovery. Development of appropriate clinical strategies employing electrical stimulation will depend upon determining the underlying mechanisms of activity-dependent axon regeneration and the heterogeneity of neuronal subtype responses to stimulation.

Electrical stimulation to enhance peripheral nerve regeneration: Update in molecular investigations and clinical translation

Peripheral nerve injuries are common and frequently result in incomplete functional recovery even with optimal surgical treatment. Permanent motor and sensory deficits are associated with significant patient morbidity and socioeconomic burden. Despite substantial research efforts to enhance peripheral nerve regeneration, few effective and clinically feasible treatment options have been found. One promising strategy is the use of low frequency electrical stimulation delivered perioperatively to an injured nerve at the time of surgical repair. Possibly through its effect of increasing intraneuronal cyclic AMP, perioperative electrical stimulation accelerates axon outgrowth, remyelination of regenerating axons, and reinnervation of end organs, even with delayed surgical intervention. Building on decades of experimental evidence in animal models, several recent, prospective, randomized clinical trials have affirmed electrical stimulation as a clinically translatable technique to enhance functional recovery in patients with peripheral nerve injuries requiring surgical treatment. This paper provides an updated review of the cellular physiology of electrical stimulation and its effects on axon regeneration, Level I evidence from recent prospective randomized clinical trials of electrical stimulation, and ongoing and future directions of research into electrical stimulation as a clinically feasible adjunct to surgical intervention in the treatment of patients with peripheral nerve injuries.

Restoration of Neurological Function Following Peripheral Nerve Trauma

Following peripheral nerve trauma that damages a length of the nerve, recovery of function is generally limited. This is because no material tested for bridging nerve gaps promotes good axon regeneration across the gap under conditions associated with common nerve traumas. While many materials have been tested, sensory nerve grafts remain the clinical “gold standard” technique. This is despite the significant limitations in the conditions under which they restore function. Thus, they induce reliable and good recovery only for patients < 25 years old, when gaps are <2 cm in length, and when repairs are performed <2–3 months post trauma. Repairs performed when these values are larger result in a precipitous decrease in neurological recovery. Further, when patients have more than one parameter larger than these values, there is normally no functional recovery. Clinically, there has been little progress in developing new techniques that increase the level of functional recovery following peripheral nerve injury. This paper examines the efficacies and limitations of sensory nerve grafts and various other techniques used to induce functional neurological recovery, and how these might be improved to induce more extensive functional recovery. It also discusses preliminary data from the clinical application of a novel technique that restores neurological function across long nerve gaps, when repairs are performed at long times post-trauma, and in older patients, even under all three of these conditions. Thus, it appears that function can be restored under conditions where sensory nerve grafts are not effective.

Advances in electrical and magnetic stimulation on nerve regeneration

Central and peripheral nerve injuries pose a great threat to people. Complications such as inflammation, muscle atrophy, traumatic neuromas and delayed reinnervation can bring huge challenges to clinical practices and barriers to complete nerve regrowth. Physical interventions such as electrical and magnetic stimulation show satisfactory results with varying parameters for acute and chronic nerve damages. The biological basis of electrical and magnetic stimulation mainly relies on protein synthesis, ion channel regulation and growth factor secretion. This review focuses on the various paradigms used in different models of electrical and magnetic stimulation and their regenerative potentials and underlying mechanisms in nerve injuries. The combination of physical stimulation and conductive biomaterial scaffolds displays an infinite potentiality in translational application in nerve regeneration.

Spatiotemporal Differences in Gene Expression Between Motor and Sensory Autografts and Their Effect on Femoral Nerve Regeneration in the Rat

To improve the outcome after autologous nerve grafting in the clinic, it is important to understand the limiting variables such as distinct phenotypes of motor and sensory Schwann cells. This study investigated the properties of phenotypically different autografts in a 6 mm femoral nerve defect model in the rat, where the respective femoral branches distally of the inguinal bifurcation served as homotopic, or heterotopic autografts. Axonal regeneration and target reinnervation was analyzed by gait analysis, electrophysiology, and wet muscle mass analysis. We evaluated regeneration-associated gene expression between 5 days and 10 weeks after repair, in the autografts as well as the proximal, and distal segments of the femoral nerve using qRT-PCR. Furthermore we investigated expression patterns of phenotypically pure ventral and dorsal roots. We identified highly significant differences in gene expression of a variety of regeneration-associated genes along the central – peripheral axis in healthy femoral nerves. Phenotypically mismatched grafting resulted in altered spatiotemporal expression of neurotrophic factor BDNF, GDNF receptor GFRα1, cell adhesion molecules Cadm3, Cadm4, L1CAM, and proliferation associated Ki67. Although significantly higher quadriceps muscle mass following homotopic nerve grafting was measured, we did not observe differences in gait analysis, and electrophysiological parameters between treatment paradigms. Our study provides evidence for phenotypic commitment of autologous nerve grafts after injury and gives a conclusive overview of temporal expression of several important regeneration-associated genes after repair with sensory or motor graft.

Effects of Scalp Acupuncture on Functional Deficits Induced by Early Sensorimotor Restriction

The aim of this study was to investigate the effects of scalp acupuncture and electrostimulation, combined or not, in a disuse model consisted of early sensorimotor restriction in rats. Male Wistar pups received sensorimotor restriction from the second postnatal day (P2) until P28. Animals were divided into five different groups (n = 6): control (CT), sensorimotor restricted (SR), acupuncture (AC), electrostimulation (EL), and electroacupuncture (AC+EL). Experimental animals received sham, acupuncture, or electrical stimulation, combined or not, of two scalp regions for 7 days (P29-P35). Before treatment period (P29) and after treatment (P36), animals were evaluated with the narrow suspended bar, horizontal ladder, and stride length tests. SR animals had worse performance in the narrow suspended and horizontal ladder tasks compared with SR animals at P29 (p ≤ 0.005). Significant improvements were observed in both tasks in AC, EL, and EL+AC groups comparing P29 and P36 (p < 0.001). Also, at P35, all treated animals performed significantly better motor tasks compared with SR group (p < 0.05). There was no difference between treated groups. Finally, acupuncture and electrical stimulation, combined or not, have beneficial effect on motor performance following early developmental disuse.

Pharmacological Attenuation of Electrical Effects in a Model of Compression Neuropathy

BACKGROUND

Peripheral nerve compression and entrapment can be debilitating. Using a validated animal model of peripheral nerve compression, we examined the utility of 2 drugs approved for other uses in humans, 4-aminopyridine (4-AP) and erythropoietin (EPO), as treatments for surgically induced ischemia and as adjuvants to surgical decompression.

METHODS

Peripheral nerve compression was induced in wild-type mice by placing an inert silicone sleeve around the sciatic nerve. Decompression surgery was performed at 6 weeks with mice receiving 4-AP, EPO, or saline solution either during and after compression or only after decompression. A nerve conduction study and morphometric analyses were performed to compare the extent of the injury and the efficacy of the therapies, and the findings were subjected to statistical analysis.

RESULTS

During peripheral nerve compression, there was a progressive decline in nerve conduction velocity compared with that in sham-treatment animals, in which nerve conduction velocity remained normal (∼55 m/s). Mice treated with 4-AP or EPO during the compression phase had significantly smaller declines in nerve conduction velocity and increased plateau nerve conduction velocities compared with untreated controls (animals that received saline solution). Histomorphometric analyses of newly decompressed nerves (i.e., nerves that underwent decompression on the day that the mouse was sacrificed) revealed that both treated groups had significantly greater proportions of large (>5-µm) axons than the untreated controls. Following surgical decompression, all animals recovered to a normal baseline nerve conduction velocity by day 15; however, treatment significantly accelerated improvement (in both the 4-AP and the EPO group), even when it was only started after decompression. Histomorphometric analyses at 7 and 15 days following surgical decompression revealed significantly increased myelin thickness and significantly greater proportions of large axons among the treated animals.

CONCLUSIONS

Both the 4-AP and the EPO-treated group demonstrated improvements in tissue architectural and electrodiagnostic measurements, both during and after peripheral nerve compression, compared with untreated mice.

CLINICAL RELEVANCE

Peripheral nerve decompression is one of the most commonly performed procedures in orthopaedic surgery. We believe that there is reason for some optimism about the translation of our findings to the clinical setting. Our findings in this murine model suggest that 4-AP and EPO may lessen the effects of nerve entrapment and that the use of these agents after decompression may speed and perhaps otherwise optimize recovery after surgery.

The Role of BDNF in Peripheral Nerve Regeneration: Activity-Dependent Treatments and Val66Met

Despite the ability of peripheral nerves to spontaneously regenerate after injury, recovery is generally very poor. The neurotrophins have emerged as an important modulator of axon regeneration, particularly brain derived neurotrophic factor (BDNF). BDNF regulation and signaling, as well as its role in activity-dependent treatments including electrical stimulation, exercise, and optogenetic stimulation are discussed here. The importance of a single nucleotide polymorphism in the BDNF gene, Val66Met, which is present in 30% of the human population and may hinder the efficacy of these treatments in enhancing regeneration after injury is considered. Preliminary data are presented on the effectiveness of one such activity-dependent treatment, electrical stimulation, in enhancing axon regeneration in mice expressing the met allele of the Val66Met polymorphism.

Electrical stimulation accelerates neurite regeneration in axotomized dorsal root ganglion neurons by increasing MMP-2 expression

Electrical stimulation (ES) can be useful for promoting the regeneration of injured axons, but the mechanism underlying its positive effects is largely unknown. The current study aimed to investigate whether ES could enhance the regeneration of injured neurites in dorsal root ganglion explants and regulate the MMP-2 expression level, which is correlated with regeneration. Significantly increased neurite regeneration and MMP-2 expression was observed in the ES group compared with the sham group. However, an MMP inhibitor significantly decreased this ES-induced neurite regeneration. Our data suggest that the positive effect of ES on neurite regeneration could likely be mediated by an increase in MMP-2 expression, thereby promoting the regeneration of injured neurites.

Bridging larger gaps in peripheral nerves using neural prosthetics and physical therapeutic agents

Peripheral nerve injuries are relatively common and can be caused by a variety of traumatic events such as motor vehicle accidents. They can lead to long-term disability, pain, and financial burden, and contribute to poor quality of life. In this review, we systematically analyze the contemporary literature on peripheral nerve gap management using nerve prostheses in conjunction with physical therapeutic agents. The use of nerve prostheses to assist nerve regeneration across large gaps (> 30 mm) has revolutionized neural surgery. The materials used for nerve prostheses have been greatly refined, making them suitable for repairing large nerve gaps. However, research on peripheral nerve gap management using nerve prostheses reports inconsistent functional outcomes, especially when prostheses are integrated with physical therapeutic agents, and thus warrants careful investigation. This review explores the effectiveness of nerve prostheses for bridging large nerve gaps and then addresses their use in combination with physical therapeutic agents.

Therapeutic electrical stimulation of injured peripheral nerve tissue using implantable thin-film wireless nerve stimulators

OBJECTIVE

Electrical stimulation of peripheral nerve tissue has been shown to accelerate axonal regeneration. Yet existing methods of applying electrical stimulation to injured peripheral nerves have presented significant barriers to clinical translation. In this study, the authors examined the use of a novel implantable wireless nerve stimulator capable of simultaneously delivering therapeutic electrical stimulation of injured peripheral nerve tissue and providing postoperative serial assessment of functional recovery.

METHODS

Flexible wireless stimulators were fabricated and implanted into Lewis rats. Thin-film implants were used to deliver brief electrical stimulation (1 hour, 20 Hz) to sciatic nerves after nerve crush or nerve transection-and-repair injuries.

RESULTS

Electrical stimulation of injured nerves via implanted wireless stimulators significantly improved functional recovery. Brief electrical stimulation was observed to increase the rate of functional recovery after both nerve crush and nerve transection-and-repair injuries. Wireless stimulators successfully facilitated therapeutic stimulation of peripheral nerve tissue and serial assessment of nerve recovery.

CONCLUSIONS

Implantable wireless stimulators can deliver therapeutic electrical stimulation to injured peripheral nerve tissue. Implantable wireless nerve stimulators might represent a novel means of facilitating therapeutic electrical stimulation in both intraoperative and postoperative settings.

4-Aminopyridine promotes functional recovery and remyelination in acute peripheral nerve injury

Traumatic peripheral nerve damage is a major medical problem without effective treatment options. In repurposing studies on 4-aminopyridine (4-AP), a potassium channel blocker that provides symptomatic relief in some chronic neurological afflictions, we discovered this agent offers significant promise as a small molecule regenerative agent for acute traumatic nerve injury. We found, in a mouse model of sciatic crush injury, that sustained early 4-AP administration increased the speed and extent of behavioral recovery too rapidly to be explained by axonal regeneration. Further studies demonstrated that 4-AP also enhanced recovery of nerve conduction velocity, promoted remyelination, and increased axonal area post-injury. We additionally found that 4-AP treatment enables distinction between incomplete and complete lesions more rapidly than existing approaches, thereby potentially addressing the critical challenge of more effectively distinguishing injured individuals who may require mutually exclusive treatment approaches. Thus, 4-AP singularly provides both a new potential therapy to promote durable recovery and remyelination in acute peripheral nerve injury and a means of identifying lesions in which this therapy would be most likely to be of value.

Nerve Regeneration: Understanding Biology and Its Influence on Return of Function After Nerve Transfers

Poor functional outcomes are frequent after peripheral nerve injuries despite the regenerative support of Schwann cells. Motoneurons and, to a lesser extent, sensory neurons survive the injuries but outgrowth of axons across the injury site is slow. The neuronal regenerative capacity and the support of regenerating axons by the chronically denervated Schwann cells progressively declines with time and distance of the injury from the denervated targets. Strategies, including brief low-frequency electrical stimulation that accelerates target reinnervation and functional recovery, and the insertion of cross-bridges between a donor nerve and a recipient denervated nerve stump, are effective in promoting functional outcomes after complete and incomplete injuries.

Electrical Stimulation to Enhance Axon Regeneration After Peripheral Nerve Injuries in Animal Models and Humans

Injured peripheral nerves regenerate their lost axons but functional recovery in humans is frequently disappointing. This is so particularly when injuries require regeneration over long distances and/or over long time periods. Fat replacement of chronically denervated muscles, a commonly accepted explanation, does not account for poor functional recovery. Rather, the basis for the poor nerve regeneration is the transient expression of growth-associated genes that accounts for declining regenerative capacity of neurons and the regenerative support of Schwann cells over time. Brief low-frequency electrical stimulation accelerates motor and sensory axon outgrowth across injury sites that, even after delayed surgical repair of injured nerves in animal models and patients, enhances nerve regeneration and target reinnervation. The stimulation elevates neuronal cyclic adenosine monophosphate and, in turn, the expression of neurotrophic factors and other growth-associated genes, including cytoskeletal proteins. Electrical stimulation of denervated muscles immediately after nerve transection and surgical repair also accelerates muscle reinnervation but, at this time, how the daily requirement of long-duration electrical pulses can be delivered to muscles remains a practical issue prior to translation to patients. Finally, the technique of inserting autologous nerve grafts that bridge between a donor nerve and an adjacent recipient denervated nerve stump significantly improves nerve regeneration after delayed nerve repair, the donor nerves sustaining the capacity of the denervated Schwann cells to support nerve regeneration. These reviewed methods to promote nerve regeneration and, in turn, to enhance functional recovery after nerve injury and surgical repair are sufficiently promising for early translation to the clinic.

The use of brief post-surgical low frequency electrical stimulation to enhance nerve regeneration in clinical practice

Despite efforts to enhance peripheral nerve regeneration, there has been little progress in improving clinical outcomes. Recently, a method of brief post-surgical low frequency electrical stimulation of surgically repaired nerves has been developed. It was shown to accelerate axon outgrowth across the repair site and it hastened target reinnervation. In this brief review, we describe the mechanistic insights and functional impacts of the post-surgical electrical stimulation that have been gained through animal studies. Brain-derived neurotrophic factor, cyclic AMP and regeneration-associated genes play a vital role in expediting the outgrowth of axons across the injury site. The method of stimulation has also been shown to be effective in patients with severe compressive neuropathy as well as those with digital nerve laceration. Its clinical feasibility and positive impact open the door of further clinical translation in other peripheral nerve injuries.

Electrical Stimulation to Promote Peripheral Nerve Regeneration

Peripheral nerve injury afflicts individuals from all walks of life. Despite the peripheral nervous system’s intrinsic ability to regenerate, many patients experience incomplete functional recovery. Surgical repair aims to expedite this recovery process in the most thorough manner possible. However, full recovery is still rarely seen especially when nerve injury is compounded with polytrauma where surgical repair is delayed. Pharmaceutical strategies supplementary to nerve microsurgery have been investigated but surgery remains the only viable option. Brief low-frequency electrical stimulation of the proximal nerve stump after primary repair has been widely investigated. This article aims to review the currently known biological basis for the regenerative effects of acute brief low-frequency electrical stimulation on axonal regeneration and outline the recent clinical applications of the electrical stimulation protocol to demonstrate the significant translational potential of this modality for repairing peripheral nerve injuries. The review concludes with a discussion of emerging new advancements in this exciting area of research. The current literature indicates the imminent clinical applicability of acute brief low-frequency electrical stimulation after surgical repair to effectively promote axonal regeneration as the stimulation has yielded promising evidence to maximize functional recovery in diverse types of peripheral nerve injuries.

Strategies to promote peripheral nerve regeneration: electrical stimulation and/or exercise

Enhancing the regeneration of axons is often considered to be a therapeutic target for improving functional recovery after peripheral nerve injury. In this review, the evidence for the efficacy of electrical stimulation (ES), daily exercise and their combination in promoting nerve regeneration after peripheral nerve injuries in both animal models and in human patients is explored. The rationale, effectiveness and molecular basis of ES and exercise in accelerating axon outgrowth are reviewed. In comparing the effects of ES and exercise in enhancing axon regeneration, increased neural activity, neurotrophins and androgens are considered to be common requirements. Similarly, there are sex-specific requirements for exercise to enhance axon regeneration in the periphery and for sustaining synaptic inputs onto injured motoneurons. ES promotes nerve regeneration after delayed nerve repair in humans and rats. The effectiveness of exercise is less clear. Although ES, but not exercise, results in a significant misdirection of regenerating motor axons to reinnervate different muscle targets, the loss of neuromuscular specificity encountered has only a very small impact on resulting functional recovery. Both ES and exercise are promising experimental treatments for peripheral nerve injury that seem to be ready to be translated to clinical use.

Optogenetic control of nerve growth

Due to the limited regenerative ability of neural tissue, a diverse set of biochemical and biophysical cues for increasing nerve growth has been investigated, including neurotrophic factors, topography, and electrical stimulation. In this report, we explore optogenetic control of neurite growth as a cell-specific alternative to electrical stimulation. By investigating a broad range of optical stimulation parameters on dorsal root ganglia (DRGs) expressing channelrhodopsin 2 (ChR2), we identified conditions that enhance neurite outgrowth by three-fold as compared to unstimulated or wild-type (WT) controls. Furthermore, optogenetic stimulation of ChR2 expressing DRGs induces directional outgrowth in WT DRGs co-cultured within a 10 mm vicinity of the optically sensitive ganglia. This observed enhancement and polarization of neurite growth was accompanied by an increased expression of neural growth and brain derived neurotrophic factors (NGF, BDNF). This work highlights the potential for implementing optogenetics to drive nerve growth in specific cell populations.

Brief electrical stimulation improves nerve regeneration after delayed repair in Sprague Dawley rats

Functional recovery after peripheral nerve injury and surgical repair declines with time and distance because the injured neurons without target contacts (chronic axotomy) progressively lose their regenerative capacity and chronically denervated Schwann cells (SCs) atrophy and fail to support axon regeneration. Findings that brief low frequency electrical stimulation (ES) accelerates axon outgrowth and muscle reinnervation after immediate nerve surgery in rats and human patients suggest that ES might improve regeneration after delayed nerve repair. To test this hypothesis, common peroneal (CP) neurons were chronically axotomized and/or tibial (TIB) SCs and ankle extensor muscles were chronically denervated by transection and ligation in rats. The CP and TIB nerves were cross-sutured after three months and subjected to either sham or one hour 20Hz ES. Using retrograde tracing, we found that ES significantly increased the numbers of both motor and sensory neurons that regenerated their axons after a three month period of chronic CP axotomy and/or chronic TIB SC denervation. Muscle and motor unit forces recorded to determine the numbers of neurons that reinnervated gastrocnemius muscle demonstrated that ES significantly increased the numbers of motoneurons that reinnervated chronically denervated muscles. We conclude that electrical stimulation of chronically axotomized motor and sensory neurons is effective in accelerating axon outgrowth into chronically denervated nerve stumps and improving target reinnervation after delayed nerve repair. Possible mechanisms for the efficacy of ES in promoting axon regeneration and target reinnervation after delayed nerve repair include the upregulation of neurotrophic factors.