Post-activation depression of Soleus stretch reflexes in healthy and spastic humans

Optimizing assessment of low frequency H-reflex depression in persons with spinal cord injury

Considering the growing interest in clinical applications of neuromodulation, assessing effects of various modulatory approaches is increasingly important. Monosynaptic spinal reflexes undergo depression following repeated activation, offering a means to quantify neuromodulatory influences. Following spinal cord injury (SCI), changes in reflex modulation are associated with spasticity and impaired motor control. To assess disrupted reflex modulation, low-frequency depression (LFD) of Hoffman (H)-reflex excitability is examined, wherein the amplitudes of conditioned reflexes are compared to an unconditioned control reflex. Alternatively, some studies utilize paired-pulse depression (PPD) in place of the extended LFD train. While both protocols induce similar amounts of H-reflex depression in neurologically intact individuals, this may not be the case for persons with neuropathology. We compared the H-reflex depression elicited by PPD and by trains of 3-10 pulses to an 11-pulse LFD protocol in persons with incomplete SCI. The amount of depression produced by PPD was less than an 11-pulse train (mean difference = 0.137). When compared to the 11-pulse train, the 5-pulse train had a Pearson's correlation coefficient (R) of 0.905 and a coefficient of determination (R2) of 0.818. Therefore, a 5-pulse train for assessing LFD elicits modulation similar to the 11-pulse train and thus we recommend its use in lieu of longer trains.

Modulations in neural pathways excitability post transcutaneous spinal cord stimulation among individuals with spinal cord injury: a systematic review

Introduction

Transcutaneous spinal cord stimulation (TSCS), a non-invasive form of spinal cord stimulation, has been shown to improve motor function in individuals living with spinal cord injury (SCI). However, the effects of different types of TSCS currents including direct current (DC-TSCS), alternating current (AC-TSCS), and spinal paired stimulation on the excitability of neural pathways have not been systematically investigated. The objective of this systematic review was to determine the effects of TSCS on the excitability of neural pathways in adults with non-progressive SCI at any level.

Methods

The following databases were searched from their inception until June 2022: MEDLINE ALL, Embase, Web of Science, Cochrane Library, and clinical trials. A total of 4,431 abstracts were screened, and 23 articles were included.

Results

Nineteen studies used TSCS at the thoracolumbar enlargement for lower limb rehabilitation (gait & balance) and four studies used cervical TSCS for upper limb rehabilitation. Sixteen studies measured spinal excitability by reporting different outcomes including Hoffmann reflex (H-reflex), flexion reflex excitability, spinal motor evoked potentials (SMEPs), cervicomedullay evoked potentials (CMEPs), and cutaneous-input-evoked muscle response. Seven studies measured corticospinal excitability using motor evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS), and one study measured somatosensory evoked potentials (SSEPs) following TSCS. Our findings indicated a decrease in the amplitude of H-reflex and long latency flexion reflex following AC-TSCS, alongside an increase in the amplitudes of SMEPs and CMEPs. Moreover, the application of the TSCS-TMS paired associative technique resulted in spinal reflex inhibition, manifested by reduced amplitudes in both the H-reflex and flexion reflex arc. In terms of corticospinal excitability, findings from 5 studies demonstrated an increase in the amplitude of MEPs linked to lower limb muscles following DC-TSCS, in addition to paired associative stimulation involving repetitive TMS on the brain and DC-TSCS on the spine. There was an observed improvement in the latency of SSEPs in a single study. Notably, the overall quality of evidence, assessed by the modified Downs and Black Quality assessment, was deemed poor.

Discussion

This review unveils the systematic evidence supporting the potential of TSCS in reshaping both spinal and supraspinal neuronal circuitries post-SCI. Yet, it underscores the critical necessity for more rigorous, high-quality investigations.

Sensory Stimulation of the Triceps Surae Muscle Complex Modulates Spinal Reflex Responses-A Comparison between Tapotement Massage and Repetitive Peripheral Magnetic Stimulation (rPMS)

BACKGROUND

The reduction of muscular hypertonia is important in the treatment of various diseases or rehabilitation. This study aims to test the efficacy of a 5 Hz mechanical muscle stimulation (tapotement massage) in comparison to a 5 Hz repetitive peripheral magnetic stimulation (rPMS) on the neuromuscular reflex response.

METHODS

In a randomized control trial, 15 healthy volunteers were administered with either 5 Hz rPMS, tapotement massage, or rPMS sham stimulation. The posterior tibial nerve was stimulated with rPMS and sham stimulation. The Achilles tendon was exposed to a mechanically applied high-amplitude 5 Hz repetitive tendon tapotement massage (rTTM). The tendon reflex (TR) was measured for the spinal response of the soleus muscle.

RESULTS

After rPMS, there was a reduction of the TR response (-9.8%, p ≤ 0.034) with no significant changes after sham stimulation. Likewise, TR decreased significantly (-17.4%, p ≤ 0.002) after Achilles tendon tapotement intervention.

CONCLUSIONS

These findings support the hypothesis that both afferent 5 Hz sensory stimulations contributed to a modulation within the spinal and/or supraspinal circuits, which resulted in a reduction of the spinal reflex excitability. The effects could be beneficial for patients with muscle hypertonia and could improve the functional results of rehabilitation programs.

A Velocity Stretch Reflex Threshold Based on Muscle-Tendon Unit Peak Acceleration to Detect Possible Occurrences of Spasticity during Gait in Children with Cerebral Palsy

Spasticity might affect gait in children with cerebral palsy. Quantifying its occurrence during locomotion is challenging. One approach is to determine kinematic stretch reflex thresholds, usually on the velocity, during passive assessment and to search for their exceedance during gait. These thresholds are determined through EMG-Onset detection algorithms, which are variable in performance and sensitive to noisy data, and can therefore lack consistency. This study aimed to evaluate the feasibility of determining the velocity stretch reflex threshold from maximal musculotendon acceleration. Eighteen children with CP were recruited and underwent clinical gait analysis and a full instrumented assessment of their soleus, gastrocnemius lateralis, semitendinosus, and rectus femoris spasticity, with EMG, kinematics, and applied forces being measured simultaneously. Using a subject-scaled musculoskeletal model, the acceleration-based stretch reflex velocity thresholds were determined and compared to those based on EMG-Onset determination. Their consistencies according to physiological criteria, i.e., if the timing of the threshold was between the beginning of the stretch and the spastic catch, were evaluated. Finally, two parameters designed to evaluate the occurrence of spasticity during gait, i.e., the proportion of the gait trial time with a gait velocity above the velocity threshold and the number of times the threshold was exceeded, were compared. The proposed method produces velocity stretch reflex thresholds close to the EMG-based ones. For all muscles, no statistical difference was found between the two parameters designed to evaluate the occurrence of spasticity during gait. Contrarily to the EMG-based methods, the proposed method always provides physiologically consistent values, with median electromechanical delays of between 50 and 130 ms. For all subjects, the semitendinosus velocity during gait usually exceeded its stretch reflex threshold, while it was less frequent for the three other muscles. We conclude that a velocity stretch reflex threshold, based on musculotendon acceleration, is a reliable substitute for EMG-based ones.

Testing spasticity mechanisms in chronic stroke before and after intervention with contralesional motor cortex 1 Hz rTMS and physiotherapy

BACKGROUND

Previous studies showed that repetitive transcranial magnetic stimulation (rTMS) reduces spasticity after stroke. However, clinical assessments like the modified Ashworth scale, cannot discriminate stretch reflex-mediated stiffness (spasticity) from passive stiffness components of resistance to muscle stretch. The mechanisms through which rTMS might influence spasticity are also not understood.

METHODS

We measured the effects of contralesional motor cortex 1 Hz rTMS (1200 pulses + 50 min physiotherapy: 3×/week, for 4-6 weeks) on spasticity of the wrist flexor muscles in 54 chronic stroke patients using a hand-held dynamometer for objective quantification of the stretch reflex response. In addition, we measured the excitability of three spinal mechanisms thought to be related to post-stroke spasticity: post-activation depression, presynaptic inhibition and reciprocal inhibition before and after the intervention. Effects on motor impairment and function were also assessed using standardized stroke-specific clinical scales.

RESULTS

The stretch reflex-mediated torque in the wrist flexors was significantly reduced after the intervention, while no change was detected in the passive stiffness. Additionally, there was a significant improvement in the clinical tests of motor impairment and function. There were no significant changes in the excitability of any of the measured spinal mechanisms.

CONCLUSIONS

We demonstrated that contralesional motor cortex 1 Hz rTMS and physiotherapy can reduce the stretch reflex-mediated component of resistance to muscle stretch without affecting passive stiffness in chronic stroke. The specific physiological mechanisms driving this spasticity reduction remain unresolved, as no changes were observed in the excitability of the investigated spinal mechanisms.

Effect of transcutaneous spinal direct current stimulation on spasticity in upper motor neuron conditions: a systematic review and meta-analysis

STUDY DESIGN

A systematic review and meta-analysis of clinical trials.

OBJECTIVES

To determine the effect of non-invasive transcutaneous spinal direct current stimulation (tsDCS) on spasticity, activity limitations and participation restrictions in various upper motor neuron diseases.

METHODS

Six databases including CINAHL plus, Cochrane CENTRAL, Embase, MEDLINE, SCOPUS and Web of Science were searched for the relevant records from January 2008 to December 2022. Two reviewers independently selected and extracted data on spasticity, activity limitations and participation restrictions. The risk of bias was evaluated using the PEDro scale while the GRADE approach established the certainty of the evidence.

RESULTS

Eleven studies were identified of which 5 (45.5%) were rated as having a low risk of bias and 8 (72.7%) were meta-analyzed. The meta-analyses did not show any significant differences between cathodal (SMD = -0.67, 95% CI = -1.50 to 0.15, P = 0.11, I2 = 75%, 6 RCTs) or anodal (SMD = 0.11, 95% CI = -0.43 to -0.64, p = 0.69, I2 = 0%, 2 RCTs) and sham tsDCS for spasticity. There was also no significant difference between active and sham tsDCS for activity limitations (SMD = -0.42, 95% CI = -0.04 to 0.21, p = 0.2, I2 = 0%, 2 RCTs) and participation restrictions (MD = -8.10, 95% CI = -18.02 to 1.82, p = 0.11, 1 RCT).

CONCLUSIONS

The meta-analysis of the available evidence provides an uncertain estimate of the effect of cathodal tsDCS on spasticity, activity limitation and participation restriction. It might be very helpful, or it may make no difference at all. However, considering the level of the evidence and the limitation in the quality of the majority of the included studies, further well-designed research may likely change the estimate of effect.

TRIAL REGISTRATION

PROSPERO CRD42021245601.

Effects of Dry Needling on Spasticity in Multiple Sclerosis Evaluated Through the Rate-Dependent Depression of the H Reflex: A Case Report

Background

Spasticity is a common symptom of multiple sclerosis (MS) which affects mobility. Dry Needling (DN) has shown a reduction in spasticity in neuromuscular conditions such as stroke and spinal cord injury although the mechanism of action is still unclear. In spastic individuals, the Rate-Dependent Depression (RDD) of the H reflex is decreased as compared to controls and analyzing the effects of DN in the RDD may help to understand its mechanism of action.

Objective

To evaluate the effect of Dry Needling on spasticity measured by the Rate-dependent Depression (RDD) of the H reflex in an MS patient.

Methods

Three time points were evaluated: Pre-intervention (T1), Post-intervention assessments were carried out in the seventh week at two-time points: Before DN (T2) and After DN (T3). Main outcomes included the RDD and latency of the H reflex in the lower limbs at stimulation frequencies of 0.1, 1, 2, and 5 Hz in a five consecutive pulses protocol.

Results

An impairment of the RDD of the H reflex at frequencies ≥1 Hz was found. Statistically significant differences were found when comparing the mean RDD of the H reflex in Pre-intervention compared to Post-intervention at 1, 2, and 5 Hz stimulation frequencies. Mean latencies were statistically lower when comparing Pre- vs Post-intervention.

Conclusion

Results suggest a partial reduction in spasticity represented by decrease of the excitability of the neural elements involved in the RDD of the H reflex following DN. The RDD of the H reflex could be implemented as an objective tool to monitor changes in spasticity in larger DN trials.

Spinal cord H-reflex post-activation depression is linked with hand motor control in adults with cerebral palsy

OBJECTIVE

Cerebral palsy (CP) is associated with upper extremity motor impairments that are largely assumed to arise from alterations in the supraspinal networks. The objective of this study was to determine if post-activation depression of the spinal H-reflexes is altered in adults with CP and connected with altered upper extremity function.

METHODS

The post-activation depression of the flexor carpi radialis (FCR) H-reflex of adults with CP and healthy adults (HA) controls were assessed by 1) a 1 Hz continuous single-pulse stimulus train and 2) 0.11 Hz / 1 Hz paired-pulse stimuli. Secondarily, we measured the maximum key grip force and the box and blocks assessment of manual dexterity.

RESULTS

Our results revealed that adults with CP had reduced post-activation depression of the FCR H-reflex during the stimulus train and the paired pulse protocol. A greater reduction in H-reflex post-activation depression was connected to lower manual dexterity and weaker grip forces.

CONCLUSIONS

Our results indicate that the post-activation depression of the upper extremity spinal H-reflex pathways is altered in adults with CP and possibly linked with their uncharacteristic upper extremity motor performance. Alterations in the spinal networks may also play a significant role in the altered motor control of adults with CP.

SIGNIFICANCE

Our results identify spinal H-reflex modulation as a possible locus for hand motor control in CP.

Quantitative measurement of resistance force and subsequent attenuation during passive isokinetic extension of the wrist in patients with mild to moderate spasticity after stroke

Background

Spasticity is evaluated by measuring the increased resistance to passive movement, primarily by manual methods. Few options are available to measure spasticity in the wrist more objectively. Furthermore, no studies have investigated the force attenuation following increased resistance. The aim of this study was to conduct a safe quantitative evaluation of wrist passive extension stiffness in stroke survivors with mild to moderate spastic paresis using a custom motor-controlled device. Furthermore, we wanted to clarify whether the changes in the measured values could quantitatively reflect the spastic state of the flexor muscles involved in the wrist stiffness of the patients.

Materials and methods

Resistance forces were measured in 17 patients during repetitive passive extension of the wrist at velocities of 30, 60, and 90 deg/s. The Modified Ashworth Scale (MAS) in the wrist and finger flexors was also assessed by two skilled therapists and their scores were averaged (i.e., average MAS) for analysis. Of the fluctuation of resistance, we focused on the damping just after the peak forces and used these for our analysis. A repeated measures analysis of variance was conducted to assess velocity-dependence. Correlations between MAS and damping parameters were analyzed using Spearman’s rank correlation.

Results

The damping force and normalized value calculated from damping part showed significant velocity-dependent increases.

There were significant correlations (ρ = 0.53–0.56) between average MAS for wrist and the normalized value of the damping part at 90 deg/s. The correlations became stronger at 60 deg/s and 90 deg/s when the MAS for finger flexors was added to that for wrist flexors (ρ = 0.65–0.68).

Conclusions

This custom-made isokinetic device could quantitatively evaluate spastic changes in the wrist and finger flexors simultaneously by focusing on the damping part, which may reflect the decrease in resistance we perceive when manually assessing wrist spasticity using MAS.

Trial registration UMIN Clinical Trial Registry, as UMIN000030672, on July 4, 2018

Corticospinal modulation of vibration-induced H-reflex depression

The purpose of this study was to examine corticospinal modulation of spinal reflex excitability, by determining the effect of transcranial magnetic stimulation (TMS) on soleus H-reflexes while they were almost completely suppressed by lower extremity vibration. In 15 healthy adults, a novel method of single-limb vibration (0.6 g, 30 Hz, 0.33 mm displacement) was applied to the non-dominant leg. Soleus muscle responses were examined in six stimulation conditions: (1) H-reflex elicited by tibial nerve stimulation, (2) tibial nerve stimulation during vibration, (3) subthreshold TMS, (4) subthreshold TMS during vibration, (5) tibial nerve stimulation 10 ms after a subthreshold TMS pulse, and (6) tibial nerve stimulation 10 ms after a subthreshold TMS pulse, during vibration. With or without vibration, subthreshold TMS produced no motor evoked potentials and had no effect on soleus electromyography (p > 0.05). In the absence of vibration, H-reflex amplitudes were not affected by subthreshold TMS conditioning (median (md) 35, interquartile range (IQ) 18-56 vs. md 46, IQ 22-59% of the maximal M wave (Mmax), p > 0.05). During vibration, however, unconditioned H-reflexes were nearly abolished, and a TMS conditioning pulse increased the H-reflex more than fourfold (md 0.3, IQ 0.1-0.7 vs. md 2, IQ 0.9-5.0% of Mmax, p < 0.008). Limb vibration alone had no significant effect on corticospinal excitability. In the absence of vibration, a subthreshold TMS pulse did not influence the soleus H-reflex. During limb vibration, however, while the H-reflex was almost completely suppressed, a subthreshold TMS pulse partially restored the H-reflex. This disinhibition of the H-reflex by a corticospinal signal may represent a mechanism involved in the control of voluntary movement. Corticospinal signals that carry the descending motor command may also reduce presynaptic inhibition, temporarily increasing the impact of sensory inputs on motoneuron activation.

Targeting Sensory and Motor Integration for Recovery of Movement After CNS Injury

The central nervous system (CNS) integrates sensory and motor information to acquire skilled movements, known as sensory-motor integration (SMI). The reciprocal interaction of the sensory and motor systems is a prerequisite for learning and performing skilled movement. Injury to various nodes of the sensorimotor network causes impairment in movement execution and learning. Stimulation methods have been developed to directly recruit the sensorimotor system and modulate neural networks to restore movement after CNS injury. Part 1 reviews the main processes and anatomical interactions responsible for SMI in health. Part 2 details the effects of injury on sites critical for SMI, including the spinal cord, cerebellum, and cerebral cortex. Finally, Part 3 reviews the application of activity-dependent plasticity in ways that specifically target integration of sensory and motor systems. Understanding of each of these components is needed to advance strategies targeting SMI to improve rehabilitation in humans after injury.

Effect of Long-Term Classical Ballet Dance Training on Postactivation Depression of the Soleus Hoffmann-Reflex

Classical ballet dancing is a good model for studying the long-term activity-dependent plasticity of the central nervous system in humans, as it requires unique ankle movements to maintain ballet postures. The purpose of this study was to investigate whether postactivation depression is changed through long-term specific motor training. Eight ballet dancers and eight sedentary subjects participated in this study. The soleus Hoffmann reflexes were elicited at after the completion of a slow, passive dorsiflexion of the ankle. The results demonstrated that the depression of the soleus Hoffmann reflex (i.e., postactivation depression) was larger in classical ballet dancers than in sedentary subjects at two poststretch intervals. This suggests that the plastic change through long-term specific motor training is also expressed in postactivation depression of the soleus Hoffmann reflex. Increased postactivation depression would strengthen the supraspinal control of the plantarflexors and may contribute to fine ankle movements in classical ballet dancers.

Transcutaneous spinal cord stimulation effects on spasticity in patients with spinal cord injury: A systematic review

CONTEXT

Spasticity is one of the most prevalent impairments following spinal cord injury (SCI). It can lead to a decrease in the patient's functional level. Transcutaneous spinal cord stimulation (tSCS) has demonstrated motor function improvements following SCI. No systematic reviews were published examining the influences of tSCS on spasticity post-SCI.

OBJECTIVES

This review aimed to investigate the effects of tSCS on spasticity in patients with SCI.

METHODS

PubMed, SCOPUS, PEDro, CINAHL, MEDLINE, REHABDATA, AMED, and Web of Science databases were searched until June 2021. The Physiotherapy Evidence Database (PEDro) scale was used to assess the methodological quality of the selected studies.

RESULTS

Six studies met the inclusion criteria. Five studies were pilot studies, and one was a case series. The scores on the PEDro scale ranged from two to four, with a median score of four. The results showed heterogenous evidence for the effects of tSCS on spasticity reduction post-SCI.

CONCLUSIONS

TSCS appears safe and well-tolerated intervention in patients with SCI. The evidence for the effectiveness of tSCS on spasticity in chronic SCI patients is limited. Further randomized controlled studies are strongly needed to study the effects of tSCS on patients with SCI.

Functional assessment of stretch hyperreflexia in children with cerebral palsy using treadmill perturbations

BACKGROUND

As hyperactive muscle stretch reflexes hinder movement in patients with central nervous system disorders, they are a common target of treatment. To improve treatment evaluation, hyperactive reflexes should be assessed during activities as walking rather than passively. This study systematically explores the feasibility, reliability and validity of sudden treadmill perturbations to evoke and quantify calf muscle stretch reflexes during walking in children with neurological disorders.

METHODS

We performed an observational cross-sectional study including 24 children with cerebral palsy (CP; 6-16 years) and 14 typically developing children (TD; 6-15 years). Short belt accelerations were applied at three different intensities while children walked at comfortable speed. Lower leg kinematics, musculo-tendon lengthening and velocity, muscle activity and spatiotemporal parameters were measured to analyze perturbation responses.

RESULTS

We first demonstrated protocol feasibility: the protocol was completed by all but three children who ceased participation due to fatigue. All remaining children were able to maintain their gait pattern during perturbation trials without anticipatory adaptations in ankle kinematics, spatiotemporal parameters and muscle activity. Second, we showed the protocol's reliability: there was no systematic change in muscle response over time (P = 0.21-0.54) and a bootstrapping procedure indicated sufficient number of perturbations, as the last perturbation repetition only reduced variability by ~ 2%. Third, we evaluated construct validity by showing that responses comply with neurophysiological criteria for stretch reflexes: perturbations superimposed calf muscle lengthening (P < 0.001 for both CP and TD) in all but one participant. This elicited increased calf muscle activity (359 ± 190% for CP and 231 ± 68% for TD, both P < 0.001) in the gastrocnemius medialis muscle, which increased with perturbation intensity (P < 0.001), according to the velocity-dependent nature of stretch reflexes. Finally, construct validity was shown from a clinical perspective: stretch reflexes were 1.7 times higher for CP than TD for the gastrocnemius medialis muscle (P = 0.017).

CONCLUSIONS

The feasibility and reliability of the protocol, as well as the construct validity-shown by the exaggerated velocity-dependent nature of the measured responses-strongly support the use of treadmill perturbations to quantify stretch hyperreflexia during gait. We therefore provided a framework which can be used to inform clinical decision making and treatment evaluation.

Nimodipine Promotes Functional Recovery After Spinal Cord Injury in Rats

Spinal cord injury (SCI) is a devastating condition that results in severe motor, sensory, and autonomic dysfunction. The L-/T-type calcium channel blocker nimodipine (NMD) exerts a protective effect on neuronal injury; however, the protective effects of long-term administration of NMD in subjects with SCI remain unknown. Thus, the aim of this study was to evaluate the role of long-term treatment with NMD on a clinically relevant SCI model. Female rats with SCI induced by 25 mm contusion were subcutaneously injected with vehicle or 10 mg/kg NMD daily for six consecutive weeks. We monitored the motor score, hind limb grip strength, pain-related behaviors, and bladder function in this study to assess the efficacy of NMD in rats with SCI. Rats treated with NMD showed improvements in locomotion, pain-related behaviors, and spasticity-like symptoms, but not in open-field spontaneous activity, hind limb grip strength or bladder function. SCI lesion areas and perilesional neuronal numbers, gliosis and calcitonin gene-related peptide (CGRP⁺) fiber sprouting in the lumbar spinal cord and the expression of K⁺–Cl⁻ cotransporter 2 (KCC2) on lumbar motor neurons were also observed to further explore the possible protective mechanisms of NMD. NMD-treated rats showed greater tissue preservation with reduced lesion areas and increased perilesional neuronal sparing. NMD-treated rats also showed improvements in gliosis, CGRP⁺ fiber sprouting in the lumbar spinal cord, and KCC2 expression in lumbar motor neurons. Together, these results indicate that long-term treatment with NMD improves functional recovery after SCI, which may provide a potential therapeutic strategy for the treatment of SCI.

Transcutaneous Spinal Cord Stimulation Enhances Walking Performance and Reduces Spasticity in Individuals with Multiple Sclerosis

Gait dysfunction and spasticity are common debilitating consequences of multiple sclerosis (MS). Improvements of these motor impairments by lumbar transcutaneous spinal cord stimulation (tSCS) have been demonstrated in spinal cord injury. Here, we explored for the first time the motor effects of lumbar tSCS applied at 50 Hz for 30 min in 16 individuals with MS and investigated their temporal persistence post-intervention. We used a comprehensive protocol assessing walking ability, different presentations of spasticity, standing ability, manual dexterity, and trunk control. Walking ability, including walking speed and endurance, was significantly improved for two hours beyond the intervention and returned to baseline after 24 h. Muscle spasms, clonus duration, and exaggerated stretch reflexes were reduced for two hours, and clinically assessed lower-extremity muscle hypertonia remained at improved levels for 24 h post-intervention. Further, postural sway during normal standing with eyes open was decreased for two hours. No changes were detected in manual dexterity and trunk control. Our results suggest that transcutaneous lumbar SCS can serve as a clinically accessible method without known side effects that holds the potential for substantial clinical benefit across the disability spectrum of MS.

Enhancing KCC2 activity decreases hyperreflexia and spasticity after chronic spinal cord injury

After spinal cord injury (SCI), the majority of individuals develop spasticity, a debilitating condition involving involuntary movements, co-contraction of antagonistic muscles, and hyperreflexia. By acting on GABAergic and Ca2+-dependent signaling, current anti-spastic medications lead to serious side effects, including a drastic decrease in motoneuronal excitability which impairs motor function and rehabilitation efforts. Exercise, in contrast, decreases spastic symptoms without decreasing motoneuron excitability. These functional improvements coincide with an increase in expression of the chloride co-transporter KCC2 in lumbar motoneurons. Thus, we hypothesized that spastic symptoms can be alleviated directly through restoration of chloride homeostasis and endogenous inhibition by increasing KCC2 activity. Here, we used the recently developed KCC2 enhancer, CLP257, to evaluate the effects of acutely increasing KCC2 extrusion capability on spastic symptoms after chronic SCI. Sprague Dawley rats received a spinal cord transection at T12 and were either bike-trained or remained sedentary for 5 weeks. Increasing KCC2 activity in the lumbar enlargement improved the rate-dependent depression of the H-reflex and reduced both phasic and tonic EMG responses to muscle stretch in sedentary animals after chronic SCI. Furthermore, the improvements due to this pharmacological treatment mirror those of exercise. Together, our results suggest that pharmacologically increasing KCC2 activity is a promising approach to decrease spastic symptoms in individuals with SCI. By acting to directly restore endogenous inhibition, this strategy has potential to avoid severe side effects and improve the quality of life of affected individuals.

Post-activation depression of primary afferents reevaluated in humans

Amplitude variation of Hoffmann Reflex (H-reflex) was used as a tool to investigate many neuronal networks. However, H-reflex itself is a subject to intrinsic changes including post-activation depression (P-AD). We aimed to investigate P-AD and its implication on motor control in humans. Upon tibial nerve stimulation in 23 healthy participants, peak-to-peak amplitude change of H-reflex was investigated using surface electromyography (SEMG) of soleus muscle. Variety of stimulus intensities, interstimulus intervals (ISIs), voluntary contraction levels/types and force recording were used to investigate the nature of P-AD. We have shown that P-AD was significantly stronger in the shorter ISIs. The only exception was the ISI of 200 msecs which had a weaker P-AD than some of the longer ISIs. Sudden muscle relaxation, on the other hand, further increased the effectiveness of the ongoing P-AD. Moreover, P-AD displayed its full effect with the first stimulus when there was no muscle contraction and was efficient to reduce the muscle force output by about 30%. These findings provide insight about the variations and mechanism of P-AD and could lead to improvements in diagnostic tools in neurological diseases.

Immobilization leads to reduced stretch reflexes but increased central reflex gain in the rat

Plastic adaptations are known to take place in muscles, tendons, joints, and the nervous system in response to changes in muscle activity. However, few studies have addressed how these plastic adaptations are related. Thus this study focuses on changes in the mechanical properties of the ankle plantarflexor muscle-tendon unit, stretch reflex activity, and spinal neuronal pathways in relation to cast immobilization. The left rat hindlimb from toes to hip was immobilized with a plaster cast for 1, 2, 4, or 8 wk followed by acute electrophysiological recordings to investigate muscle stiffness and stretch reflex torque. Moreover, additional acute experiments were performed after 4 wk of immobilization to investigate changes in the central gain of the stretch reflex. Monosynaptic reflexes (MSR) were recorded from the L4 and L5 ventral roots following stimulation of the corresponding dorsal roots. Rats developed reduced range of movement in the ankle joint 2 wk after immobilization. This was accompanied by significant increases in the stiffness of the muscle-tendon complex as well as an arthrosis at the ankle joint at 4 and 8 wk following immobilization. Stretch reflexes were significantly reduced at 4-8 wk following immobilization. This was associated with increased central gain of the stretch reflex. These data show that numerous interrelated plastic changes occur in muscles, connective tissue, and the central nervous system in response to changes in muscle use. The findings provide an understanding of coordinated adaptations in multiple tissues and have important implications for prevention and treatment of the negative consequences of immobilization following injuries of the nervous and musculoskeletal systems.NEW & NOTEWORTHY Immobilization leads to multiple simultaneous adaptive changes in muscle, connective tissue, and central nervous system.

A stimulus rate that is not influenced by homosynaptic post-activation depression in chronic stroke

PURPOSE

To determine a stimulus rate that is not influenced by homosynaptic post-activation depression for H-reflex studies in patients with chronic spasticity.

MATERIALS AND METHODS

A cohort of 15 chronic stroke patients with soleus spasticity who received inpatient treatment at our rehabilitation centre participated in this study. The effect of stimulus frequency related depression on H-reflex size was tested using four different stimulus rates (0.1, 0.2, 0.3 and 1 Hz). The affected sides stibial nerve was stimulated by a bipolar electrode. The H-reflex was recorded from the affected sideed sidee sidehe affected smine stimulus frequency related depression of H-reflex size, amplitude of the first H-reflex response (H₁) was used as control and amplitude of the second H-reflex response (H₂) as test.

RESULTS

H₂ amplitude for frequency of 1 Hz, 0.3 Hz, 0.2 Hz and 0.1 Hz were 74.3, 84.1, 85.5 and 92.7% of H₁, respectively. Depression of H₂ amplitude was statistically significant for 1 Hz, 0.3 Hz and 0.2 Hz (p < 0.001, p = 0.002, p = 0.024, respectively).

CONCLUSIONS

Higher frequency stimulation of Ia afferents than 0.1 Hz induced a stimulus frequency-related depression of H-reflex size in patients with chronic spasticity. The optimal stimulus rate for H-reflex was found to be 0.1 Hz.

Direct evidence for decreased presynaptic inhibition evoked by PBSt group I muscle afferents after chronic SCI and recovery with step-training in rats

KEY POINTS

Presynaptic inhibition is modulated by supraspinal centres and primary afferents in order to filter sensory information, adjust spinal reflex excitability, and ensure smooth movement. After spinal cord injury (SCI), the supraspinal control of primary afferent depolarization (PAD) interneurons is disengaged, suggesting an increased role for sensory afferents. While increased H-reflex excitability in spastic individuals indicates a possible decrease in presynaptic inhibition, it remains unclear whether a decrease in sensory-evoked PAD contributes to this effect. We investigated whether the PAD evoked by hindlimb afferents contributes to the change in presynaptic inhibition of the H-reflex in a decerebrated rat preparation. We found that chronic SCI decreases presynaptic inhibition of the plantar H-reflex through a reduction in PAD evoked by posterior biceps-semitendinosus (PBSt) muscle group I afferents. We further found that step-training restored presynaptic inhibition of the plantar H-reflex evoked by PBSt, suggesting the presence of activity-dependent plasticity of PAD pathways activated by flexor muscle group I afferents.

ABSTRACT

Spinal cord injury (SCI) results in the disruption of supraspinal control of spinal networks and an increase in the relative influence of afferent feedback to sublesional neural networks, both of which contribute to enhancing spinal reflex excitability. Hyperreflexia occurs in ∼75% of individuals with a chronic SCI and critically hinders functional recovery and quality of life. It is suggested that it results from an increase in motoneuronal excitability and a decrease in presynaptic and postsynaptic inhibitory mechanisms. In contrast, locomotor training decreases hyperreflexia by restoring presynaptic inhibition. Primary afferent depolarization (PAD) is a powerful presynaptic inhibitory mechanism that selectively gates primary afferent transmission to spinal neurons to adjust reflex excitability and ensure smooth movement. However, the effect of chronic SCI and step-training on the reorganization of presynaptic inhibition evoked by hindlimb afferents, and the contribution of PAD has never been demonstrated. The objective of this study is to directly measure changes in presynaptic inhibition through dorsal root potentials (DRPs) and its association with plantar H-reflex inhibition. We provide direct evidence that H-reflex hyperexcitability is associated with a decrease in transmission of PAD pathways activated by posterior biceps-semitendinosus (PBSt) afferents after chronic SCI. More precisely, we illustrate that the pattern of inhibition evoked by PBSt group I muscle afferents onto both L4-DRPs and plantar H-reflexes evoked by the distal tibial nerve is impaired after chronic SCI. These changes are not observed in step-trained animals, suggesting a role for activity-dependent plasticity to regulate PAD pathways activated by flexor muscle group I afferents.

Acute whole-body vibration reduces post-activation depression in the triceps surae muscle

PURPOSE

Acute whole-body vibration (WBV) is known to enhance neuromuscular activation. Especially mechanisms which act presynaptically are discussed to be involved in this modulation, but evidence is still limited. Therefore, this study aimed to investigate if 2 min of WBV might impact the premotoneuronal mechanism of post-activation depression (PAD).

METHODS

PAD in m. soleus was assessed by paired-pulse stimulation in 28 healthy participants prior, 2 min, 4 min and 10 min after 2 min of side-alternating WBV (10 Hz, 2 mm). Methodologies involved electromyography (m. soleus, m. tibialis anterior) and goniometric recordings (ankle, knee joint). H-reflexes were elicited with peripheral nerve stimulation and assessed by means of conditioned H-reflexes (ISI 1 s, H_cond) versus control H-reflexes (ISI₁₀, H).

RESULTS

H_cond/H was significantly enhanced by +55% (2 min), +32% (4 min) and +35% (10 min) following WBV (P < 0.05). Baseline muscle activity and joint positions were shown to be reliable (Cronbach's α values >0.990) throughout the testing procedure.

CONCLUSION

Vibratory-induced spinal inhibition is accompanied by diminished PAD at the presynaptic terminals which interconnect the Ia afferents with the α-motoneuron. Functionally, the PAD reduction might explain enhanced motor performance following vibration therapy, but future studies will be needed to verify this assumption.

Treatment Response to Botulinum Neurotoxin-A in Children With Cerebral Palsy Categorized by the Type of Stretch Reflex Muscle Activation

While Botulinum NeuroToxin-A (BoNT-A) injections are frequently used to reduce the effects of hyperactive stretch reflexes in children with cerebral palsy (CP), the effects of this treatment vary strongly. Previous research, combining electromyography (EMG) with motion analysis, defined different patterns of stretch reflex muscle activation in muscles, those that reacted more to a change in velocity (velocity dependent -VD), and those that reacted more to a change in length (length dependent -LD). The aim of this study was to investigate the relation between the types of stretch reflex muscle activation in the semitendinosus with post-BoNT-A outcome as assessed passively and with 3D gait analysis in children with spastic CP. Eighteen children with spastic CP (10 bilaterally involved) between the ages of 12 and 18 years were assessed before and on average, 8 weeks post-treatment. EMG and motion analysis were used to assess the degree and type of muscle activation dependency in the semitendinosus during passive knee extensions performed at different joint angular velocities. Three-dimensional gait analysis was used to assess knee gait kinematics as a measure of functional outcome. Pre-treatment, 9 muscles were classified as VD and 9 as LD, but no differences between the groups were evident in the baseline knee gait kinematics. Post-treatment, stretch reflex muscle activation decreased significantly in both groups but the reduction was more pronounced in those muscles classified pre-treatment as VD (-72% vs. -50%, p = 0.005). In the VD group, these changes were accompanied by greater knee extension at initial contact and during the swing phase of gait. In the LD group, there was significantly increased post-treatment knee hyperextension in late stance. Although results vary between patients, the reduction of stretch reflex muscle activation in the semitendinosus generally translated to an improved functional outcome, as assessed with 3D gait analysis. However, results were less positive for those muscles with pre-treatment length-dependent type of stretch reflex muscle activation. The study demonstrates the relevance of categorizing the type of stretch reflex muscle activation as a possible predictor of treatment response.

Rehabilitation Decreases Spasticity by Restoring Chloride Homeostasis through the Brain-Derived Neurotrophic Factor-KCC2 Pathway after Spinal Cord Injury

Activity-based therapy is routinely integrated in rehabilitation programs to facilitate functional recovery after spinal cord injury (SCI). Among its beneficial effects is a reduction of hyperreflexia and spasticity, which affects ∼75% of the SCI population. Unlike current anti-spastic pharmacological treatments, rehabilitation attenuates spastic symptoms without causing an active depression in spinal excitability, thus avoiding further interference with motor recovery. Understanding how activity-based therapies contribute to decrease spasticity is critical to identifying new pharmacological targets and to optimize rehabilitation programs. It was recently demonstrated that a decrease in the expression of KCC2, a neuronal Cl- extruder, contributes to the development spasticity in SCI rats. Although exercise can decrease spinal hyperexcitability and increase KCC2 expression on lumbar motoneurons after SCI, a causal effect remains to be established. Activity-dependent processes include an increase in brain-derived neurotrophic factor (BDNF) expression. Interestingly, BDNF is a regulator of KCC2 but also a potent modulator of spinal excitability. Therefore, we hypothesized that after SCI, the activity-dependent increase in KCC2 expression: 1) functionally contributes to reduce hyperreflexia, and 2) is regulated by BDNF. SCI rats chronically received VU0240551 (KCC2 blocker) or TrkB-IgG (BDNF scavenger) during the daily rehabilitation sessions and the frequency-dependent depression of the H-reflex, a monitor of hyperreflexia, was recorded 4 weeks post-injury. Our results suggest that the activity-dependent increase in KCC2 functionally contributes to H-reflex recovery and critically depends on BDNF activity. This study provides a new perspective in understanding how exercise impacts hyperreflexia by identifying the biological basis of the recovery of function.

Recovery cycles of posterior root-muscle reflexes evoked by transcutaneous spinal cord stimulation and of the H reflex in individuals with intact and injured spinal cord

Posterior root-muscle (PRM) reflexes are short-latency spinal reflexes evoked by epidural or transcutaneous spinal cord stimulation (SCS) in clinical and physiological studies. PRM reflexes share key physiological characteristics with the H reflex elicited by electrical stimulation of large-diameter muscle spindle afferents in the tibial nerve. Here, we compared the H reflex and the PRM reflex of soleus in response to transcutaneous stimulation by studying their recovery cycles in ten neurologically intact volunteers and ten individuals with traumatic, chronic spinal cord injury (SCI). The recovery cycles of the reflexes, i.e., the time course of their excitability changes, were assessed by paired pulses with conditioning-test intervals of 20–5000 ms. Between the subject groups, no statistical difference was found for the recovery cycles of the H reflexes, yet those of the PRM reflexes differed significantly, with a striking suppression in the intact group. When comparing the reflex types, they did not differ in the SCI group, while the PRM reflexes were more strongly depressed in the intact group for durations characteristic for presynaptic inhibition. These differences may arise from the concomitant stimulation of several posterior roots containing afferent fibers of various lower extremity nerves by transcutaneous SCS, producing multi-source heteronymous presynaptic inhibition, and the collective dysfunction of inhibitory mechanisms after SCI contributing to spasticity. PRM-reflex recovery cycles additionally obtained for bilateral rectus femoris, biceps femoris, tibialis anterior, and soleus all demonstrated a stronger suppression in the intact group. Within both subject groups, the thigh muscles showed a stronger recovery than the lower leg muscles, which may reflect a characteristic difference in motor control of diverse muscles. Based on the substantial difference between intact and SCI individuals, PRM-reflex depression tested with paired pulses could become a sensitive measure for spasticity and motor recovery.

Interrater Reliability of the Modified Ashworth Scale with Standardized Movement Speeds: A Pilot Study

Purpose: The purpose of this study was to estimate the interrater reliability of the Modified Ashworth Scale (MAS) using consistent movement speeds guided by a metronome when measuring wrist flexor and ankle plantar flexor spasticity in patients with an upper motor neuron (UMN) lesion. The MAS has been found to have inconsistent interrater reliability for scoring the degree of spasticity. Method: This quantitative cross-sectional pilot study occurred in the outpatient spasticity clinic of a community rehabilitation hospital. Participants were a convenience sample of 20 patients with UMN lesions receiving botulinum toxin type A injections for focal spasticity. For each of the muscle groups tested, different pairs of evaluators scored spasticity on the MAS at a velocity of 130° per second. Evaluators were physical therapy students trained by an experienced physical therapist in MAS assessment. Weighted κ values were calculated to measure interrater reliability for each muscle group. Results: The weighted κ values for the wrist flexors and ankle plantar flexors were, respectively, moderate (κ_w = 0.54; 95% CI: 0.21, 0.87) and slight (κ_w = 0.08; 95% CI: -0.34, 0.50). Conclusion: Using consistent movement velocity, novice evaluators were able to score spasticity with reasonable interrater reliability when assessing the wrist flexors but not the ankle plantar flexors.

Repeated transspinal stimulation decreases soleus H-reflex excitability and restores spinal inhibition in human spinal cord injury

Transcutaneous spinal cord or transspinal stimulation over the thoracolumbar enlargement, the spinal location of motoneurons innervating leg muscles, modulates neural circuits engaged in the control of movement. The extent to which daily sessions (e.g. repeated) of transspinal stimulation affects soleus H-reflex excitability in individuals with chronic spinal cord injury (SCI) remains largely unknown. In this study, we established the effects of repeated cathodal transspinal stimulation on soleus H-reflex excitability and spinal inhibition in individuals with and without chronic SCI. Ten SCI and 10 healthy control subjects received monophasic transspinal stimuli of 1-ms duration at 0.2 Hz at subthreshold and suprathreshold intensities of the right soleus transspinal evoked potential (TEP). SCI subjects received an average of 16 stimulation sessions, while healthy control subjects received an average of 10 stimulation sessions. Before and one or two days post intervention, we used the soleus H reflex to assess changes in motoneuron recruitment, homosynaptic depression following single tibial nerve stimuli delivered at 0.1, 0.125, 0.2, 0.33 and 1.0 Hz, and postactivation depression following paired tibial nerve stimuli at the interstimulus intervals of 60, 100, 300, and 500 ms. Soleus H-reflex excitability was decreased in both legs in motor incomplete and complete SCI but not in healthy control subjects. Soleus H-reflex homosynaptic and postactivation depression was present in motor incomplete and complete SCI but was of lesser strength to that observed in healthy control subjects. Repeated transspinal stimulation increased homosynaptic depression in all SCI subjects and remained unaltered in healthy controls. Postactivation depression remained unaltered in all subject groups. Lastly, transspinal stimulation decreased the severity of spasms and ankle clonus. The results indicate decreased reflex hyperexcitability and recovery of spinal inhibitory control in the injured human spinal cord with repeated transspinal stimulation. Transspinal stimulation is a noninvasive neuromodulation method for restoring spinally-mediated afferent reflex actions after SCI in humans.

Recruitment gain of spinal motor neuron pools in cat and human

The output from a motor nucleus is determined by the synaptic input to the motor neurons and their intrinsic properties. Here, we explore whether the source of synaptic inputs to the motor neurons (cats) and the age or post-stroke conditions (humans) may change the recruitment gain of the motor neuron pool. In cats, the size of Ia EPSPs in triceps surae motor neurons (input) and monosynaptic reflexes (MSRs; output) was recorded in the soleus and medial gastrocnemius motor nerves following graded stimulation of dorsal roots. The MSR was plotted against the EPSP thereby obtaining a measure of the recruitment gain. Conditioning stimulation of sural and peroneal cutaneous afferents caused significant increase in the recruitment gain of the medial gastrocnemius, but not the soleus motor neuron pool. In humans, the discharge probability of individual soleus motor units (input) and soleus H-reflexes (output) was performed. With graded stimulation of the tibial nerve, the gain of the motor neuron pool was assessed as the slope of the relation between probability of firing and the reflex size. The gain in young subjects was higher than in elderly subjects. The gain in post-stroke survivors was higher than in age-matched neurologically intact subjects. These findings provide experimental evidence that recruitment gain of a motor neuron pool contributes to the regulation of movement at the final output stage from the spinal cord and should be considered when interpreting changes in reflex excitability in relation to movement or injuries of the nervous system.

Transcutaneous Spinal Cord Stimulation Induces Temporary Attenuation of Spasticity in Individuals with Spinal Cord Injury

Epidural spinal cord stimulation (SCS) is currently regarded as a breakthrough procedure for enabling movement after spinal cord injury (SCI), yet one of its original applications was for spinal spasticity. An emergent method that activates similar target neural structures non-invasively is transcutaneous SCS. Its clinical value for spasticity control would depend on inducing carry-over effects, because the surface-electrode-based approach cannot be applied chronically. We evaluated single-session effects of transcutaneous lumbar SCS in 12 individuals with SCI by a test-battery approach, before, immediately after and 2 h after intervention. Stimulation was applied for 30 min at 50 Hz with an intensity sub-threshold for eliciting reflexes in lower extremity muscles. The tests included evaluations of stretch-induced spasticity (Modified Ashworth Scale [MAS] sum score, pendulum test, electromyography-based evaluation of tonic stretch reflexes), clonus, cutaneous-input-evoked spasms, and the timed 10 m walk test. Across participants, the MAS sum score, clonus, and spasms were significantly reduced immediately after SCS, and all spasticity measures were improved 2 h post-intervention, with large effect sizes and including clinically meaningful improvements. The effect on walking speed varied across individuals. We further conducted a single-case multi-session study over 6 weeks to explore the applicability of transcutaneous SCS as a home-based therapy. Self-application of the intervention was successful; weekly evaluations suggested progressively improving therapeutic effects during the active period and carry-over effects for 7 days. Our results suggest that transcutaneous SCS can be a viable non-pharmacological option for managing spasticity, likely working through enhancing pre- and post-synaptic spinal inhibitory mechanisms, and may additionally serve to identify responders to treatments with epidural SCS.

Effects of training with a neuro-mechano stimulator rehabilitation bicycle on functional recovery and paired-reflex depression of the soleus in individuals with incomplete paralysis: a proof-of-principle study

Aim: The present study describes the training effects of a novel motorized bicycle-like device for individuals with incomplete spinal cord injury. Methods: Participants were five individuals with motor incomplete spinal cord injury (56 ± 7 years). Four of five participants received two 30-min sessions of training: one with, and one without, mechanical stimulation on the plantar surface of the foot; soleus paired H-reflex depression was examined before and after each session. Three of five participants received 24 sessions of 30-min of training (long-training). Following the long-training, balance, walking and spasticity improvements were assessed using validated clinical outcome measures, in addition to the H-reflex assessment. Results: One cycling session with mechanical stimulation yielded 14% and 32% more reflex depression in participants with moderate spasticity (n = 2/4). The same trend was not observed in non-spastic participants (n = 2/4). All participants who participated in the long-training had spasticity and showed reduced spasticity, improved walking speed, endurance and balance. Conclusions: Overall, participants with spasticity showed increased soleus H-reflex suppression after one training session with mechanical stimulation and reduced spasticity scores after long training. We interpret this as evidence that the training influenced both presynaptic and postsynaptic inhibitory mechanisms acting on soleus motoneurons. Therefore, this training has the potential to be a non-invasive complementary therapy to reduce spasticity after incomplete spinal cord injury.

Sustained submaximal contraction yields biphasic modulation of soleus Post-activation depression in healthy humans

The amplitude of the H-reflex during the development and progression of fatigue reflects a complex interplay between central and peripheral factors. The purpose of this study is to characterize H-reflex homosynaptic post-activation depression (PAD) in an online fashion during a sustained submaximal fatigue task. The task required a high motor output in order to increase the likelihood of creating partial muscle ischemia with accumulation of fatigue metabolites, an important potential inhibitory influence upon the H-reflex during the progression of fatigue. Eleven subjects without neurologic impairment maintained volitional, isometric plantar flexion at 60% of maximal voluntary contraction until exhaustion. A paired-pulse stimulus (2 Hz) was delivered to the tibial nerve to elicit paired H-reflexes before, during, and after the fatigue protocol. The normalized amplitude of the second H-reflex (depression ratio) served as an estimate of PAD. Depression ratio increased during the first half of the fatigue protocol (P < 0.001), indicating a diminution of PAD, and then returned as exhaustion approached. The biphasic behavior of homosynaptic H-reflex depression during fatigue to exhaustion suggests a role for metabolic mediators of post-activation depression during fatigue.

Prochlorperazine Increases KCC2 Function and Reduces Spasticity after Spinal Cord Injury

In mature neurons, low intracellular chloride level required for inhibition is maintained by the potassium-chloride cotransporter, KCC2. Impairment of Cl- extrusion after KCC2 dysfunction has been involved in many central nervous system disorders, such as seizures, neuropathic pain, or spasticity, after a spinal cord injury (SCI). This makes KCC2 an appealing drug target for restoring Cl- homeostasis and inhibition in pathological conditions. In the present study, we screen the Prestwick Chemical Library® and identify conventional antipsychotics phenothiazine derivatives as enhancers of KCC2 activity. Among them, prochlorperazine hyperpolarizes the Cl- equilibrium potential in motoneurons of neonatal rats and restores the reciprocal inhibition post-SCI. The compound alleviates spasticity in chronic adult SCI rats with an efficacy equivalent to the antispastic agent, baclofen, and rescues the SCI-induced downregulation of KCC2 in motoneurons below the lesion. These pre-clinical data support prochlorperazine for a new therapeutic indication in the treatment of spasticity post-SCI and neurological disorders involving a KCC2 dysfunction.

Limb Segment Load Inhibits the Recovery of Soleus H-Reflex After Segmental Vibration in Humans

We investigated the effects of vertical vibration and compressive load on soleus H-reflex amplitude and postactivation depression. We hypothesized that, in the presence of a compressive load, limb vibration induces a longer suppression of soleus H-reflex. Eleven healthy adults received vibratory stimulation at a fixed frequency (30 Hz) over two loading conditions (0% and 50% of individual's body weight). H-reflex amplitude was depressed ∼88% in both conditions during vibration. Cyclic application of compression after cessation of the vibration caused a persistent reduction in H-reflex excitability and postactivation depression for > 2.5 min. A combination of limb segment vibration and compression may offer a nonpharmacologic method to modulate spinal reflex excitability in people after CNS injury.

Postactivation depression of soleus H-reflex increase with recovery of lower extremities motor functions in patients with subacute stroke

[Purpose] The soleus H-reflex is depressed at stimulation rates greater than 0.1 Hz. This reflex depression is referred to as postactivation depression. Postactivation depression reflects the reduced efficacy of the Ia-motoneurons synapses when they are evaluated after a previous activation. The aim of this study was to determine whether the recovery of motor functions in the lower extremities affects the PAD of the soleus H-reflex in patients with subacute stroke undergoing rehabilitation. [Subjects and Methods] Eight patients with subacute stroke patients were recruited. Postactivation depression, Fugl-Meyer score (lower-limb portion), walking velocity, the Modified Ashworth Scale, and center of pressure sway during standing were measured within three days of admission to rehabilitation and 50 days later. [Results] After rehabilitation, Fugl-Meyer scores, center of pressure path length, and walking velocity were significantly improved, and postactivation depression had significantly increased. There was a significant positive correlation between the rates of change of postactivation depression and center of pressure path length. [Conclusion] The results demonstrated that postactivation depression is partially normalized after rehabilitation in patients with subacute stroke, and suggested that the recovery in lower extremity function after stroke particularly standing stability is affected by spinal synaptic plasticity.

How plastic are human spinal cord motor circuitries?

Human and animal studies have documented that neural circuitries in the spinal cord show adaptive changes caused by altered supraspinal and/or afferent input to the spinal circuitry in relation to learning, immobilization, injury and neurorehabilitation. Reversible adaptations following, e.g. the acquisition or refinement of a motor skill rely heavily on the functional integration between supraspinal and sensory inputs to the spinal cord networks. Accordingly, what is frequently conceived as a change in the spinal circuitry may be a change in either descending or afferent input or in the relative integration of these, i.e. a change in the neuronal weighting. This is evident from findings documenting only task-specific functional changes after periods of altered inputs whereas resting responses remain unaffected. In fact, the proximity of the spinal circuitry to the outer world may demand a more rigid organization compared to the highly flexible cortical circuits. The understanding of all of this is important for the planning and execution of neurorehabilitation.

Muscle disuse caused by botulinum toxin injection leads to increased central gain of the stretch reflex in the rat

Botulinum toxin (Btx) is used in children with cerebral palsy and in other neurological patients to diminish spasticity and reduce the risk of development of contractures. We investigated changes in the central gain of the stretch reflex circuitry in response to Btx injection in the triceps surae muscle in rats. Experiments were performed in 21 rats. Eight rats were a control group, and 13 rats were injected with 6 IU of Btx in the left triceps surae muscle. Two weeks after Btx injection, larger monosynaptic reflexes (MSR) were recorded from the left (injected) than the right (noninjected) L4 + L5 ventral roots following stimulation of the corresponding dorsal roots. A similar increase on the left side was observed in response to stimulation of descending motor tracts, suggesting that increased excitability of spinal motor neurons may at least partly explain the increased reflexes. However, significant changes were also observed in postactivation depression of the MSR, suggesting that plastic changes in transmission from Ia afferent to the motor neurons also may be involved. The data demonstrate that muscle paralysis induced by Btx injection is accompanied by plastic adaptations in the central stretch reflex circuitry, which counteract the antispastic effect of Btx.NEW & NOTEWORTHY Injection of botulinum toxin into ankle muscles causes increased gain of stretch reflex. This is caused by adaptive changes in regulation of transmitter release from Ia afferents and increased excitability of spinal motor neurons.

Modeling trans-spinal direct current stimulation for the modulation of the lumbar spinal motor pathways

OBJECTIVE

Trans-spinal direct current stimulation (tsDCS) is a potential new technique for the treatment of spinal cord injury (SCI). TsDCS aims to facilitate plastic changes in the neural pathways of the spinal cord with a positive effect on SCI recovery. To establish tsDCS as a possible treatment option for SCI, it is essential to gain a better understanding of its cause and effects. We seek to understand the acute effect of tsDCS, including the generated electric field (EF) and its polarization effect on the spinal circuits, to determine a cellular target. We further ask how these findings can be interpreted to explain published experimental results.

APPROACH

We use a realistic full body finite element volume conductor model to calculate the EF of a 2.5 mA direct current for three different electrode configurations. We apply the calculated electric field to realistic motoneuron models to investigate static changes in membrane resting potential. The results are combined with existing knowledge about the theoretical effect on a neuronal level and implemented into an existing lumbar spinal network model to simulate the resulting changes on a network level.

MAIN RESULTS

Across electrode configurations, the maximum EF inside the spinal cord ranged from 0.47 V m^-1 to 0.82 V m^-1. Axon terminal polarization was identified to be the dominant cellular target. Also, differences in electrode placement have a large influence on axon terminal polarization. Comparison between the simulated acute effects and the electrophysiological long-term changes observed in human tsDCS studies suggest an inverse relationship between the two.

SIGNIFICANCE

We provide methods and knowledge for better understanding the effects of tsDCS and serve as a basis for a more targeted and optimized application of tsDCS.

Patellar tendon vibration reduces the increased facilitation from quadriceps to soleus in post-stroke hemiparetic individuals

BACKGROUND

Stimulation of the femoral nerve in healthy people can facilitate soleus H-reflex and electromyography (EMG) activity. In stroke patients, such facilitation of transmission in spinal pathways linking the quadriceps and soleus muscles is enhanced and related to co-activation of knee and ankle extensors while sitting and walking. Soleus H-reflex facilitation can be depressed by vibration of the quadriceps in healthy people, but the effects of such vibration have never been studied on the abnormal soleus facilitation observed in people after stroke.

OBJECTIVES

To determine whether vibration of the quadriceps can modify the enhanced heteronymous facilitation of the soleus muscle observed in people with spastic stroke after femoral nerve stimulation and compare post-vibration effects on soleus facilitation in control and stroke individuals.

METHODS

Modulation of voluntary soleus EMG activity induced by femoral nerve stimulation (2×motor threshold) was assessed before, during and after vibration of the patellar tendon in 10 healthy controls and 17 stroke participants.

RESULTS

Voluntary soleus EMG activity was facilitated by femoral nerve stimulation in 4/10 (40%) controls and 11/17 (65%) stroke participants. The level of facilitation was greater in the stroke than control group. Vibration significantly reduced early heteronymous facilitation in both groups (50% of pre-vibration values). However, the delay in recovery of soleus facilitation after vibration was shorter for the stroke than control group. The control condition with the vibrator turned off had no effect on the modulation.

CONCLUSIONS

Patellar tendon vibration can reduce the facilitation between knee and ankle extensors, which suggests effective presynaptic inhibition but decreased post-activation depression in the lower limb of people after chronic hemiparetic stroke. Further studies are warranted to determine whether such vibration could be used to reduce the abnormal extension synergy of knee and ankle extensors in people after hemiparetic stroke.

Vibration training after chronic spinal cord injury: Evidence for persistent segmental plasticity

H-reflex paired-pulse depression is gradually lost within the first year post-SCI, a process believed to reflect reorganization of segmental interneurons after the loss of normal descending (cortical) inhibition. This reorganization co-varies in time with the development of involuntary spasms and spasticity. The purpose of this study is to determine whether long-term vibration training may initiate the return of H-reflex paired-pulse depression in individuals with chronic, complete SCI. Five men with SCI received twice-weekly vibration training (30Hz, 0.6g) to one lower limb while seated in a wheelchair. The contra-lateral limb served as a within-subject control. Paired-pulse H-reflexes were obtained before, during, and after a session of vibration. Untrained limb H-reflex depression values were comparable to chronic SCI values from previous reports. In contrast, the trained limbs of all 5 participants showed depression values that were within the range of previously-reported Acute SCI and Non-SCI H-reflex depression. The average difference between limbs was 34.98% (p=0.016). This evidence for the return of H-reflex depression suggests that even for people with long-standing SCI, plasticity persists in segmental reflex pathways. The spinal networks involved with the clinical manifestation of spasticity may thus retain adaptive plasticity after long-term SCI. The results of this study indicate that vibration training may hold promise as an anti-spasticity rehabilitation intervention.

The swimming test is effective for evaluating spasticity after contusive spinal cord injury

Spasticity is a frequent chronic complication in individuals with spinal cord injury (SCI). However, the severity of spasticity varies in patients with SCI. Therefore, an evaluation method is needed to determine the severity of spasticity. We used a contusive SCI model that is suitable for clinical translation. In this study, we examined the feasibility of the swimming test and an EMG for evaluating spasticity in a contusive SCI rat model. Sprague-Dawley rats received an injury at the 8th thoracic vertebra. Swimming tests were performed 3 to 6 weeks after SCI induction. We placed the SCI rats into spasticity-strong or spasticity-weak groups based on the frequency of spastic behavior during the swimming test. Subsequently, we recorded the Hoffman reflex (H-reflex) and examined the immunoreactivity of serotonin (5-HT) and its receptor (5-HT_2A) in the spinal tissues of the SCI rats. The spasticity-strong group had significantly decreased rate-dependent depression of the H-reflex compared to the spasticity-weak group. The area of 5-HT_2A receptor immunoreactivity was significantly increased in the spasticity-strong group. Thus, both electrophysiological and histological evaluations indicate that the spasticity-strong group presented with a more severe upper motor neuron syndrome. We also observed the groups in their cages for 20 hours. Our results suggest that the swimming test provides an accurate evaluation of spasticity in this contusive SCI model. We believe that the swimming test is an effective method for evaluating spastic behaviors and developing treatments targeting spasticity after SCI.

Does trans-spinal and local DC polarization affect presynaptic inhibition and post-activation depression?

KEY POINTS

Trans-spinal polarization was recently introduced as a means to improve deficient spinal functions. However, only a few attempts have been made to examine the mechanisms underlying DC actions. We have now examined the effects of DC on two spinal modulatory systems, presynaptic inhibition and post-activation depression, considering whether they might weaken exaggerated spinal reflexes and enhance excessively weakened ones. Direct current effects were evoked by using local intraspinal DC application (0.3-0.4 μA) in deeply anaesthetized rats and were compared with the effects of trans-spinal polarization (0.8-1.0 mA). Effects of local intraspinal DC were found to be polarity dependent, as locally applied cathodal polarization enhanced presynaptic inhibition and post-activation depression, whereas anodal polarization weakened them. In contrast, both cathodal and anodal trans-spinal polarization facilitated them. The results suggest some common DC-sensitive mechanisms of presynaptic inhibition and post-activation depression, because both were facilitated or depressed by DC in parallel.

ABSTRACT

Direct current (DC) polarization has been demonstrated to alleviate the effects of various deficits in the operation of the central nervous system. However, the effects of trans-spinal DC stimulation (tsDCS) have been investigated less extensively than the effects of transcranial DC stimulation, and their cellular mechanisms have not been elucidated. The main objectives of this study were, therefore, to extend our previous analysis of DC effects on the excitability of primary afferents and synaptic transmission by examining the effects of DC on two spinal modulatory feedback systems, presynaptic inhibition and post-activation depression, in an anaesthetized rat preparation. Other objectives were to compare the effects of locally and trans-spinally applied DC (locDC and tsDCS). Local polarization at the sites of terminal branching of afferent fibres was found to induce polarity-dependent actions on presynaptic inhibition and post-activation depression, as cathodal locDC enhanced them and anodal locDC depressed them. In contrast, tsDCS modulated presynaptic inhibition and post-activation depression in a polarity-independent fashion because both cathodal and anodal tsDCS facilitated them. The results show that the local presynaptic actions of DC might counteract both excessively strong and excessively weak monosynaptic actions of group Ia and cutaneous afferents. However, they indicate that trans-spinally applied DC might counteract the exaggerated spinal reflexes but have an adverse effect on pathologically weakened spinal activity by additional presynaptic weakening. The results are also relevant for the analysis of the basic properties of presynaptic inhibition and post-activation depression because they indicate that some common DC-sensitive mechanisms contribute to them.

High-frequency neuromuscular electrical stimulation modulates interhemispheric inhibition in healthy humans

High-frequency neuromuscular electrical stimulation (HF NMES) induces muscular contractions through neural mechanisms that partially match physiological motor control. Indeed, a portion of the contraction arises from central mechanisms, whereby spinal motoneurons are recruited through the evoked sensory volley. However, the involvement of supraspinal centers of motor control during such stimulation remains poorly understood. Therefore, we tested whether a single HF NMES session applied to the upper limb influences interhemispheric inhibition (IHI) from left to right motor cortex (M1). Using noninvasive electrophysiology and transcranial magnetic stimulation, we evaluated the effects of a 10-min HF NMES session applied to a right wrist flexor on spinal and corticospinal excitability of both arms, as well as IHI, in healthy subjects. HF NMES induced a rapid decline in spinal excitability on the right stimulated side that closely matched the modulation of evoked force during the protocol. More importantly, IHI was significantly increased by HF NMES, and this increase was correlated to the electromyographic activity within the contralateral homologous muscle. Our study highlights a new neurophysiological mechanism, suggesting that HF NMES has an effect on the excitability of the transcallosal pathway probably to regulate the lateralization of the motor output. The data suggest that HF NMES can modify the hemispheric balance between both M1 areas. These findings provide important novel perspectives for the implementation of HF NMES in sport training and neurorehabilitation.

NEW & NOTEWORTHY

High-frequency neuromuscular electrical stimulation (HF NMES) induces muscular contractions that partially match physiological motor control. Here, we tested whether HF NMES applied to the upper limb influences interhemispheric inhibition. Our results show that interhemispheric inhibition was increased after HF NMES and that this increase was correlated to the electromyographic activity within the contralateral homologous muscle. This opens up original perspectives for the implementation of HF NMES in sport training and neurorehabilitation.

A Review on Locomotor Training after Spinal Cord Injury: Reorganization of Spinal Neuronal Circuits and Recovery of Motor Function

Locomotor training is a classic rehabilitation approach utilized with the aim of improving sensorimotor function and walking ability in people with spinal cord injury (SCI). Recent studies have provided strong evidence that locomotor training of persons with clinically complete, motor complete, or motor incomplete SCI induces functional reorganization of spinal neuronal networks at multisegmental levels at rest and during assisted stepping. This neuronal reorganization coincides with improvements in motor function and decreased muscle cocontractions. In this review, we will discuss the manner in which spinal neuronal circuits are impaired and the evidence surrounding plasticity of neuronal activity after locomotor training in people with SCI. We conclude that we need to better understand the physiological changes underlying locomotor training, use physiological signals to probe recovery over the course of training, and utilize established and contemporary interventions simultaneously in larger scale research studies. Furthermore, the focus of our research questions needs to change from feasibility and efficacy to the following: what are the physiological mechanisms that make it work and for whom? The aforementioned will enable the scientific and clinical community to develop more effective rehabilitation protocols maximizing sensorimotor function recovery in people with SCI.

Interlimb Reflexes Induced by Electrical Stimulation of Cutaneous Nerves after Spinal Cord Injury

Whether interlimb reflexes emerge only after a severe insult to the human spinal cord is controversial. Here the aim was to examine interlimb reflexes at rest in participants with chronic (>1 year) spinal cord injury (SCI, n = 17) and able-bodied control participants (n = 5). Cutaneous reflexes were evoked by delivering up to 30 trains of stimuli to either the superficial peroneal nerve on the dorsum of the foot or the radial nerve at the wrist (5 pulses, 300 Hz, approximately every 30 s). Participants were instructed to relax the test muscles prior to the delivery of the stimuli. Electromyographic activity was recorded bilaterally in proximal and distal arm and leg muscles. Superficial peroneal nerve stimulation evoked interlimb reflexes in ipsilateral and contralateral arm and contralateral leg muscles of SCI and control participants. Radial nerve stimulation evoked interlimb reflexes in the ipsilateral leg and contralateral arm muscles of control and SCI participants but only contralateral leg muscles of control participants. Interlimb reflexes evoked by superficial peroneal nerve stimulation were longer in latency and duration, and larger in magnitude in SCI participants. Interlimb reflex properties were similar for both SCI and control groups for radial nerve stimulation. Ascending interlimb reflexes tended to occur with a higher incidence in participants with SCI, while descending interlimb reflexes occurred with a higher incidence in able-bodied participants. However, the overall incidence of interlimb reflexes in SCI and neurologically intact participants was similar which suggests that the neural circuitry underlying these reflexes does not necessarily develop after central nervous system injury.

Post-activation depression of the lower extremities in stroke patients with spasticity and spastic equinovarus deformity

This study aimed to investigate changes of post-activation depression in two groups of patients with or without spastic equinovarus deformity (SED). Paired and independent t-tests were used to compare post-activation depression within and between the groups, respectively. There was a significant positive correlation between diminished post-activation depression and spasticity severity. The soleus and tibialis anterior (TA) post-activation depression values were significantly decreased on the affected sides of patients with SED compared to those without. In patients without SED, the soleus post-activation depression was significantly decreased on the affected side; however, TA post-activation depression was higher on the affected side. Both the soleus and TA become active, but the onset time may be different. The imbalanced muscle tone between the soleus and TA in the early stage after stroke may be related to equinus deformity.