Evidence for plateau potentials in tail motoneurons of awake chronic spinal rats with spasticity

Intrinsic motoneuron properties in typical human development

Motoneuron properties and their firing patterns undergo significant changes throughout development and in response to neuromodulators such as serotonin. Here, we examined the age-related development of self-sustained firing and general excitability of tibialis anterior motoneurons in a young development (7-17 years), young adult (18-28 years) and adult (32-53 years) group, as well as in a separate group of participants taking selective serotonin reuptake inhibitors (SSRIs, aged 11-28 years). Self-sustained firing, as measured by ΔF, was larger in the young development (∼5.8 Hz, n = 20) compared to the young adult (∼4.9 Hz, n = 13) and adult (∼4.8 Hz, n = 8) groups, consistent with a developmental decrease in self-sustained firing mediated by persistent inward currents (PIC). ΔF was also larger in participants taking SSRIs (∼6.5 Hz, n = 9) compared to their age-matched controls (∼5.3 Hz, n = 26), consistent with increased levels of spinal serotonin facilitating the motoneuron PIC. Participants in the young development and SSRI groups also had higher firing rates and a steeper acceleration in initial firing rates (secondary ranges), consistent with the PIC producing a steeper acceleration in membrane depolarization at the onset of motoneuron firing. In summary, both the young development and SSRI groups exhibited increased intrinsic motoneuron excitability compared to the adults, which, in the young development group, was also associated with a larger unsteadiness in the dorsiflexion torque profiles. We propose several intrinsic and extrinsic factors that affect both motoneuron PICs and cell discharge which vary during development, with a time course similar to the changes in motoneuron firing behaviour observed in the present study. KEY POINTS: Neurons in the spinal cord that activate muscles in the limbs (motoneurons) undergo increases in excitability shortly after birth to help animals stand and walk. We examined whether the excitability of human ankle flexor motoneurons also continues to change from child to adulthood by recording the activity of the muscle fibres they innervate. Motoneurons in children and adolescents aged 7-17 years (young development group) had higher signatures of excitability that included faster firing rates and more self-sustained activity compared to adults aged ≥18 years. Participants aged 11-28 years of age taking serotonin reuptake inhibitors had the highest measures of motoneuron excitability compared to their age-matched controls. The young development group also had more unstable contractions, which might partly be related to the high excitability of the motoneurons.

Knockdown of calpain1 in lumbar motoneurons reduces spasticity after spinal cord injury in adult rats

Spasticity, affecting ∼75% of patients with spinal cord injury (SCI), leads to hyperreflexia, muscle spasms, and cocontractions of antagonist muscles, greatly affecting their quality of life. Spasticity primarily stems from the hyperexcitability of motoneurons below the lesion, driven by an upregulation of the persistent sodium current and a downregulation of chloride extrusion. This imbalance results from the post-SCI activation of calpain1, which cleaves Nav1.6 channels and KCC2 cotransporters. Our study was focused on mitigating spasticity by specifically targeting calpain1 in spinal motoneurons. We successfully transduced lumbar motoneurons in adult rats with SCI using intrathecal administration of adeno-associated virus vector serotype 6, carrying a shRNA sequence against calpain1. This approach significantly reduced calpain1 expression in transduced motoneurons, leading to a noticeable decrease in spasticity symptoms, including hyperreflexia, muscle spasms, and cocontractions in hindlimb muscles, which are particularly evident in the second month post-SCI. In addition, this decrease, which prevented the escalation of spasticity to a severe grade, paralleled the restoration of KCC2 levels in transduced motoneurons, suggesting a reduced proteolytic activity of calpain1. These findings demonstrate that inhibiting calpain1 in motoneurons is a promising strategy for alleviating spasticity in SCI patients.

Activated Human Adipose Tissue Transplantation Promotes Sensorimotor Recovery after Acute Spinal Cord Contusion in Rats

Traumatic spinal cord injuries (SCIs) often result in sensory, motor, and vegetative function loss below the injury site. Although preclinical results have been promising, significant solutions for SCI patients have not been achieved through translating repair strategies to clinical trials. In this study, we investigated the effective potential of mechanically activated lipoaspirated adipose tissue when transplanted into the epicenter of a thoracic spinal contusion. Male Sprague Dawley rats were divided into three experimental groups: SHAM (uninjured and untreated), NaCl (spinal cord contusion with NaCl application), and AF (spinal cord contusion with transplanted activated human fat). Pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) were measured to assess endogenous inflammation levels 14 days after injury. Sensorimotor recovery was monitored weekly for 12 weeks, and gait and electrophysiological analyses were performed at the end of this observational period. The results indicated that AF reduced endogenous inflammation post-SCI and there was a significant improvement in sensorimotor recovery. Moreover, activated adipose tissue also reinstated the segmental sensorimotor loop and the communication between supra- and sub-lesional spinal cord regions. This investigation highlights the efficacy of activated adipose tissue grafting in acute SCI, suggesting it is a promising therapeutic approach for spinal cord repair after traumatic contusion in humans.

Effects of jaw clenching and mental stress on persistent inward currents estimated by two different methods

Spinal motoneuron firing depends greatly on persistent inward currents (PICs), which in turn are facilitated by the neuromodulators serotonin and noradrenaline. The aim of this study was to determine whether jaw clenching (JC) and mental stress (MS), which may increase neuromodulator release, facilitate PICs in human motoneurons. The paired motor unit (MU) technique was used to estimate PIC contribution to motoneuron firing. Surface electromyograms were collected using a 32-channel matrix on gastrocnemius medialis (GM) during voluntary, ramp, plantar flexor contractions. MU discharges were identified, and delta frequency (ΔF), a measure of recruitment-derecruitment hysteresis, was calculated. Additionally, another technique was used (VibStim) that evokes involuntary contractions that persist after cessation of combined Achilles tendon vibration and triceps surae neuromuscular electrical stimulation. VibStim measures of plantar flexor torque and soleus activity may reflect PIC activation. ΔF was not significantly altered by JC (p = .679, n = 18, 9 females) or MS (p = .147, n = 14, 5 females). However, all VibStim variables quantifying involuntary torque and muscle activity during and after vibration cessation were significantly increased in JC (p < .011, n = 20, 10 females) and some, but not all, increased in MS (p = .017-.05, n = 19, 10 females). JC and MS significantly increased the magnitude of involuntary contractions (VibStim) but had no effect on GM ΔF during voluntary contractions. Effects of increased neuromodulator release on PIC contribution to motoneuron firing might differ between synergists or be context dependent. Based on these data, the background level of voluntary contraction and, hence, both neuromodulation and ionotropic inputs could influence neuromodulatory PIC enhancement.

Can local vibration alter the contribution of persistent inward currents to human motoneuron firing?

The response of spinal motoneurons to synaptic input greatly depends on the activation of persistent inward currents (PICs), which in turn are enhanced by the neuromodulators serotonin and noradrenaline. Local vibration (LV) induces excitatory Ia input onto motoneurons and may alter neuromodulatory inputs. Therefore, we investigated whether LV influences the contribution of PICs to motoneuron firing. This was assessed in voluntary contractions with concurrent, ongoing LV, as well as after a bout of prolonged LV. High-density surface electromyograms (HD-EMG) of the tibialis anterior were recorded with a 64-electrode matrix. Twenty males performed isometric, triangular, dorsiflexion contractions to 20% and 50% of maximal torque at baseline, during LV of the tibialis anterior muscle, and after 30-min of LV. HD-EMG signals were decomposed, and motor units tracked across time points to estimate PICs through a paired motor unit analysis, which quantifies motor unit recruitment-derecruitment hysteresis (ΔF). During ongoing LV, ΔF was lower for both 20% and 50% ramps. Although significant changes in ΔF were not observed after prolonged LV, a differential effect across the motoneuron pool was observed. This study demonstrates that PICs can be non-pharmacologically modulated by LV. Given that LV leads to reflexive motor unit activation, it is postulated that lower PIC contribution to motoneuron firing during ongoing LV results from decreased neuromodulatory inputs associated with lower descending corticospinal drive. A differential effect in motoneurons of different recruitment thresholds after prolonged LV is provocative, challenging the interpretation of previous observations and motivating future investigations. KEY POINTS: Neuromodulatory inputs from the brainstem influence motoneuron intrinsic excitability through activation of persistent inward currents (PICs). PICs make motoneurons more responsive to excitatory input. We demonstrate that vibration applied on the muscle modulates the contribution of PICs to motoneuron firing, as observed through analysis of the firing of single motor units. The effects of PICs on motoneuron firing were lower when vibration was concurrently applied during voluntary ramp contractions, likely due to lower levels of neuromodulation. Additionally, prolonged exposure to vibration led to differential effects of lower- vs. higher-threshold motor units on PICs, with lower-threshold motor units tending to present an increased and higher-threshold motor units a decreased contribution of PICs to motoneuron firing. These results demonstrate that muscle vibration has the potential to influence the effects of neuromodulation on motoneuron firing. The potential of using vibration as a non-pharmacological neuromodulatory intervention should be further investigated.

A geometric approach to quantifying the neuromodulatory effects of persistent inward currents on individual motor unit discharge patterns

Objective.All motor commands flow through motoneurons, which entrain control of their innervated muscle fibers, forming a motor unit (MU). Owing to the high fidelity of action potentials within MUs, their discharge profiles detail the organization of ionotropic excitatory/inhibitory as well as metabotropic neuromodulatory commands to motoneurons. Neuromodulatory inputs (e.g. norepinephrine, serotonin) enhance motoneuron excitability and facilitate persistent inward currents (PICs). PICs introduce quantifiable properties in MU discharge profiles by augmenting depolarizing currents upon activation (i.e. PIC amplification) and facilitating discharge at lower levels of excitatory input than required for recruitment (i.e. PIC prolongation).Approach. Here, we introduce a novel geometric approach to estimate neuromodulatory and inhibitory contributions to MU discharge by exploiting discharge non-linearities introduced by PIC amplification during time-varying linear tasks. In specific, we quantify the deviation from linear discharge ('brace height') and the rate of change in discharge (i.e. acceleration slope, attenuation slope, angle). We further characterize these metrics on a simulated motoneuron pool with known excitatory, inhibitory, and neuromodulatory inputs and on human MUs (number of MUs; Tibialis Anterior: 1448, Medial Gastrocnemius: 2100, Soleus: 1062, First Dorsal Interosseus: 2296).Main results. In the simulated motor pool, we found brace height and attenuation slope to consistently indicate changes in neuromodulation and the pattern of inhibition (excitation-inhibition coupling), respectively, whereas the paired MU analysis (ΔF) was dependent on both neuromodulation and inhibition pattern. Furthermore, we provide estimates of these metrics in human MUs and show comparable variability in ΔFand brace height measures for MUs matched across multiple trials.Significance. Spanning both datasets, we found brace height quantification to provide an intuitive method for achieving graded estimates of neuromodulatory and inhibitory drive to individual MUs. This complements common techniques and provides an avenue for decoupling changes in the level of neuromodulatory and pattern of inhibitory motor commands.

Riluzole for treating spasticity in patients with chronic traumatic spinal cord injury: Study protocol in the phase ib/iib adaptive multicenter randomized controlled RILUSCI trial

BACKGROUND

Satisfactory treatment is often lacking for spasticity, a highly prevalent motor disorder in patients with spinal cord injury (SCI). Low concentrations of riluzole potently reduce the persistent sodium current, the post-SCI increase in which contributes to spasticity. The repurposing of this drug may therefore constitute a useful potential therapeutic option for relieving SCI patients suffering from chronic traumatic spasticity.

OBJECTIVE

RILUSCI is a phase 1b-2b trial designed to assess whether riluzole is a safe and biologically effective means of managing spasticity in adult patients with traumatic chronic SCI.

METHODS

In this multicenter double-blind trial, adults (aged 18-65 years) suffering from spasticity after SCI (target enrollment: 90 participants) will be randomly assigned to be given either a placebo or a recommended daily oral dose of riluzole for two weeks. The latter dose will be previously determined in phase 1b of the study by performing double-blind dose-finding tests using a Bayesian continuous reassessment method. The primary endpoint of the trial will be an improvement in the Modified Ashworth Score (MAS) or the Numerical Rating Score (NRS) quantifying spasticity. The secondary outcomes will be based on the safety and pharmacokinetics of riluzole as well as its impact on muscle spasms, pain, bladder dysfunction and quality of life. Analyses will be performed before, during and after the treatment and the placebo-controlled period.

CONCLUSION

To the best of our knowledge, this clinical trial will be the first to document the safety and efficacy of riluzole as a means of reducing spasticity in patients with chronic SCI.

TRIAL REGISTRATION

The clinical trial, which is already in progress, was registered on the ClinicalTrials.gov website on August 9, 2016 under the registration number NCT02859792.

TRIAL SPONSOR

Assistance Publique-Hôpitaux de Marseille.

Changes in 5-HT1F receptor expression in rats with spasticity following spinal cord injury

Spasticity is a common complication in patients with spinal cord injury (SCI) and adversely affects patients' quality of life. Little is known about the distribution of the serotonin 1F receptor (5-HT1FR) in the spinal cord, especially in relation to the spasticity caused by SCI. Adult male Wistar rats were divided into a sham-operation group and spinalized group. SCI-induced spasticity was caused by spinal transection at the second sacral segment. The spinal cord below the transection was obtained at the end of the experiment. The expression and distribution of 5-HT1FR in the spinal cord were analyzed. The results showed that the expression of 5-HT1FR (mRNA and protein) exhibited the same downward trend after spinal transection and reached the lowest expression level at 2 and 5 days, respectively. The expression of 5-HT1FR (mRNA and protein) thereafter gradually approached the levels in the sham-operation group after 60 days. Immunostaining suggested that 5-HT1FR showed particularly strong expression in the ventral horn (VH) region. The time course of 5-HT1FR mRNA downregulation is positively correlated with the development of tail spasticity after sacral spinal cord transection. There may be a connection between 5-HT1FR and the occurrence of spasticity, but elucidation of the specific mechanism needs further experimental verification.

Treatment of spasticity

Spasticity is characterized by an enhanced size and reduced threshold for activation of stretch reflexes and is associated with "positive signs" such as clonus and spasms, as well as "negative features" such as paresis and a loss of automatic postural responses. Spasticity develops over time after a lesion and can be associated with reduced speed of movement, cocontraction, abnormal synergies, and pain. Spasticity is caused by a combination of damage to descending tracts, reductions in inhibitory activity within spinal cord circuits, and adaptive changes within motoneurons. Increased tone, hypertonia, can also be caused by changes in passive stiffness due to, for example, increase in connective tissue and reduction in muscle fascicle length. Understanding the cause of hypertonia is important for determining the management strategy as nonneural, passive causes of stiffness will be more amenable to physical rather than pharmacological interventions. The management of spasticity is determined by the views and goals of the patient, family, and carers, which should be integral to the multidisciplinary assessment. An assessment, and treatment, of trigger factors such as infection and skin breakdown should be made especially in people with a recent change in tone. The choice of management strategies for an individual will vary depending on the severity of spasticity, the distribution of spasticity (i.e., whether it affects multiple muscle groups or is more prominent in one or two groups), the type of lesion, and the potential for recovery. Management options include physical therapy, oral agents; focal therapies such as botulinum injections; and peripheral nerve blocks. Intrathecal baclofen can lead to a reduction in required oral antispasticity medications. When spasticity is severe intrathecal phenol may be an option. Surgical interventions, largely used in the pediatric population, include muscle transfers and lengthening and selective dorsal root rhizotomy.

Adaptive synapse-based neuron model with heterogeneous multistability and riddled basins

Biological neurons can exhibit complex coexisting multiple firing patterns dependent on initial conditions. To this end, this paper presents a novel adaptive synapse-based neuron (ASN) model with sine activation function. The ASN model has time-varying equilibria with the variation of externally applied current and its equilibrium stability involves transitions between stable and unstable points through fold and Hopf bifurcations, resulting in complex distributions of attractive regions with heterogeneous multi-stability. Globally coexisting heterogeneous behaviors are studied by bifurcation diagram, phase portrait, dynamical distribution, and basin of attraction. The results show that the number of coexisting heterogeneous attractors can be up to 12, but for a simple neuron model, such a large number of coexisting heterogeneous attractors has not been reported in the relevant literature. Most interestingly, the ASN model also has riddled-like complex basins of attraction and four illustrative examples are depicted by the phase portraits with small changes of the initial conditions. Besides, the ASN model is implemented using a simple microcontroller platform, and various heterogeneous coexisting attractors are acquired experimentally to validate the numerical results.

GABA facilitates spike propagation through branch points of sensory axons in the spinal cord

Movement and posture depend on sensory feedback that is regulated by specialized GABAergic neurons (GAD2⁺) that form axo-axonic contacts onto myelinated proprioceptive sensory axons and are thought to be inhibitory. However, we report here that activating GAD2⁺ neurons directly with optogenetics or indirectly by cutaneous stimulation actually facilitates sensory feedback to motor neurons in rodents and humans. GABA_A receptors located at or near nodes of Ranvier of sensory axons cause this facilitation by preventing spike propagation failure at the many axon branch points, which is otherwise common without GABA. In contrast, GABA_A receptors are generally lacking from axon terminals and so cannot inhibit transmitter release onto motor neurons, unlike GABA_B receptors that cause presynaptic inhibition. GABAergic innervation near nodes and branch points allows individual branches to function autonomously, with GAD2⁺ neurons regulating which branches conduct, adding a computational layer to the neuronal networks generating movement and likely generalizing to other central nervous system axons.

Spinal cord stimulation: Beyond pain management

INTRODUCTION

The gate control theory of pain was the starting point of the development of spinal cord stimulation (SCS). We describe the indications for the treatment in pain management and other uses not related to pain.

DEVELOPMENT

There are currently several paradigms for SCS: tonic, burst, and high frequency. The main difference lies in the presence of paraesthesias. SCS is most beneficial for treating neuropathic pain. Patients with failed back surgery syndrome show the best response rates, although a considerable reduction in pain is also observed in patients with complex regional pain syndrome, diabetic neuropathy, radiculopathy, and low back pain without previous surgery. Phantom pain or pain related to cardiovascular or peripheral vascular disease may improve, although there is a lack of robust evidence supporting generalisation of its use. SCS also improves cancer-related pain, although research on this issue is scarce. Non-pain-related indications for SCS are movement disorders, spasticity, and sequelae of spinal cord injury. The main limiting factors for the use of SCS are mechanical complications and the cost of the treatment.

CONCLUSION

In its 50-year history, SCS has progressed enormously. The perfection of hardware and software may improve its effectiveness and reduce the rate of complications. Indications for SCS could include other diseases, and its use could be expanded, if the costs of the technology are reduced.

Activity-dependent plasticity and spinal cord stimulation for motor recovery following spinal cord injury

Spinal cord injuries lead to permanent physical impairment despite most often being anatomically incomplete disruptions of the spinal cord. Remaining connections between the brain and spinal cord create the potential for inducing neural plasticity to improve sensorimotor function, even many years after injury. This narrative review provides an overview of the current evidence for spontaneous motor recovery, activity-dependent plasticity, and interventions for restoring motor control to residual brain and spinal cord networks via spinal cord stimulation. In addition to open-loop spinal cord stimulation to promote long-term neuroplasticity, we also review a more targeted approach: closed-loop stimulation. Lastly, we review mechanisms of spinal cord neuromodulation to promote sensorimotor recovery, with the goal of advancing the field of rehabilitation for physical impairments following spinal cord injury.

Effects of reciprocal inhibition and whole-body relaxation on persistent inward currents estimated by two different methods

Abstract

Persistent inward currents (PICs) are crucial for initiation, acceleration, and maintenance of motoneuron firing. As PICs are highly sensitive to synaptic inhibition and facilitated by serotonin and noradrenaline, we hypothesised that both reciprocal inhibition (RI) induced by antagonist nerve stimulation and whole‐body relaxation (WBR) would reduce PICs in humans. To test this, we estimated PICs using the well‐established paired motor unit (MU) technique. High‐density surface electromyograms were recorded from gastrocnemius medialis during voluntary, isometric 20‐s ramp, plantarflexor contractions and decomposed into MU discharges to calculate delta frequency (ΔF). Moreover, another technique (VibStim), which evokes involuntary contractions proposed to result from PIC activation, was used. Plantarflexion torque and soleus activity were recorded during 33‐s Achilles tendon vibration and simultaneous 20‐Hz bouts of neuromuscular electrical stimulation (NMES) of triceps surae. ΔF was decreased by RI (n = 15, 5 females) and WBR (n = 15, 7 females). In VibStim, torque during vibration at the end of NMES and sustained post‐vibration torque were reduced by WBR (n = 19, 10 females), while other variables remained unchanged. All VibStim variables remained unaltered in RI (n = 20, 10 females). Analysis of multiple human MUs in this study demonstrates the ability of local, focused inhibition to attenuate the effects of PICs on motoneuron output during voluntary motor control. Moreover, it shows the potential to reduce PICs through non‐pharmacological, neuromodulatory interventions such as WBR. The absence of a consistent effect in VibStim might be explained by a floor effect resulting from low‐magnitude involuntary torque combined with the negative effects of the interventions.

Key points

Spinal motoneurons transmit signals to skeletal muscles to regulate their contraction. Motoneuron firing partly depends on their intrinsic properties such as the strength of persistent (long‐lasting) inward currents (PICs) that make motoneurons more responsive to excitatory input.

In this study, we demonstrate that both reciprocal inhibition onto motoneurons and whole‐body relaxation reduce the contribution of PICs to human motoneuron firing. This was observed through analysis of the firing of single motor units during voluntary contractions.

However, an alternative technique that involves tendon vibration and neuromuscular electrical stimulation to evoke involuntary contractions showed less effect. Thus, it remains unclear whether this alternative technique can be used to estimate PICs under all physiological conditions.

These results improve our understanding of the mechanisms of PIC depression in human motoneurons. Potentially, non‐pharmacological interventions such as electrical stimulation or relaxation could attenuate unwanted PIC‐induced muscle contractions in conditions characterised by motoneuron hyperexcitability.

The Cellular Basis for the Generation of Firing Patterns in Human Motor Units

Motor units, which comprise a motoneuron and the set of muscle fibers it innervates, are the fundamental neuromuscular transducers for all motor commands. The one to one relationship between a motoneuron and its innervated muscle fibers allow motoneuron firing patterns to be readily measured in humans. In this chapter, we summarize the current understanding of the cellular basis for the generation of firing patterns in human motor units. We provide a brief review of landmark insights from classic studies and then proceed to consider the features of motor unit firing patterns that are most likely to be sensitive estimators of motoneuron inputs and properties. In addition, we discuss recent advances in technology for recording human motor unit firing patterns and highly realistic computer simulations of motoneurons. The final section presents our recent efforts to use the power of supercomputers for implementation of the motoneuron models, with a goal of achieving a true "reverse engineering" approach that maximizes the insights from motor unit firing patterns into the synaptic structure of motor commands.

A computational approach for generating continuous estimates of motor unit discharge rates and visualizing population discharge characteristics

OBJECTIVE

Successive improvements in high density surface electromyography and decomposition techniques have facilitated an increasing yield in decomposed motor unit (MU) spike times. Though these advancements enhance the generalizability of findings and promote the application of MU discharge characteristics to inform the neural control of motor output, limitations remain. Specifically, 1) common approaches for generating smooth estimates of MU discharge rates introduce artifacts in quantification, which may bias findings, and 2) discharge characteristics of large MU populations are often difficult to visualize.

APPROACH

In the present study, we propose support vector regression (SVR) as an improved approach for generating smooth continuous estimates of discharge rate and compare the fit characteristics of SVR to traditionally used methods, including Hanning window filtering and polynomial regression. Furthermore, we introduce ensembles as a method to visualize the discharge characteristics of large MU populations. We define ensembles as the average discharge profile of a subpopulation of MUs, composed of a time normalized ensemble average of all units within this subpopulation. Analysis was conducted with MUs decomposed from the tibialis anterior (N = 2128), medial gastrocnemius (N = 2673), and soleus (N = 1190) during isometric plantarflexion and dorsiflexion contractions.

MAIN RESULT

Compared to traditional approaches, we found SVR to alleviate commonly observed inaccuracies and produce significantly less absolute fit error in the initial phase of MU discharge and throughout the entire duration of discharge. Additionally, we found the visualization of MU populations as ensembles to intuitively represent population discharge characteristics with appropriate accuracy for visualization.

SIGNIFICANCE

The results and methods outlined here provide an improved method for generating estimates of MU discharge rate with SVR and present a unique approach to visualizing MU populations with ensembles. In combination, the use of SVR and generation of ensembles represent an efficient method for rendering population discharge characteristics.

Concurrent Achilles tendon vibration and tibial nerve stimulation to estimate persistent inward current strength in motoneurons

Vibratory (Tvib) and sustained (Tsust) torque responses to concurrent Achilles tendon vibration and neuromuscular electrical stimulation applied over the muscle belly (vib+stim) are used as indicators of motoneuron facilitation and, theoretically, persistent inward current strength. However, neuromuscular electrical stimulation (NMES) applied to the nerve trunk may potentiate motoneuronal excitability more than muscle belly NMES, yet it remains unclear whether NMES applied over the nerve evokes robust Tvib and Tsust responses when used during the vib+stim protocol. This study tested whether a nerve-targeted vib+stim protocol elicits Tvib and Tsust responses in the ankle plantar flexors with acceptable intra- and inter-session reliability. Fifteen men performed the vib+stim protocol with NMES applied over the tibial nerve three times across two sessions; twice in a single session (5-min apart) to test intrasession reliability and then again after 48 h to test intersession reliability. Intraclass correlation coefficients (ICC3,1), within-participant coefficients of variation (CV) and pairwise comparisons were used to verify relative and absolute reliability as well as systematic bias. Thirteen men presented Tvib and Tsust responses (response rate of 87%). Intrasession Tvib and Tsust ICCs were >0.73 but inter-session ICCs were <0.5. Although no systematic bias was detected (p>0.05), both intra- and inter-session CVs were large (>10%) for Tvib and Tsust. The Vib+stim protocol with NMES applied over the nerve evoked Tvib and Tsust in almost all participants, but presented a large intra- and inter-session variability. The method does not appear to be effective for assessing motoneuron facilitation in the plantar flexors.

Estimates of persistent inward currents are reduced in upper limb motor units of older adults

Ageing is a natural process causing alterations in the neuromuscular system, which contributes to reduced quality of life. Motor unit (MU) contributes to weakness, but the mechanisms underlying reduced firing rates are unclear. Persistent inward currents (PICs) are crucial for initiation, gain control and maintenance of motoneuron firing, and are directly proportional to the level of monoaminergic input. Since concentrations of monoamines (i.e. serotonin and noradrenaline) are reduced with age, we sought to determine if estimates of PICs are reduced in older (>60 years old) compared to younger adults (<35 years old). We decomposed MU spike trains from high-density surface electromyography over the biceps and triceps brachii during isometric ramp contractions to 20% of maximum. Estimates of PICs (ΔFrequency; or simply ΔF) were computed using the paired MU analysis technique. Regardless of the muscle, peak firing rates of older adults were reduced by ∼1.6 pulses per second (pps) (P = 0.0292), and ΔF was reduced by ∼1.9 pps (P < 0.0001), compared to younger adults. We further found that age predicted ΔF in older adults (P = 0.0261), resulting in a reduction of ∼1 pps per decade, but there was no relationship in younger adults (P = 0.9637). These findings suggest that PICs are reduced in the upper limbs of older adults during submaximal isometric contractions. Reduced PIC magnitude represents one plausible mechanism for reduced firing rates and function in older individuals, but further work is required to understand the implications in other muscles and during a variety of motor tasks. KEY POINTS: Persistent inward currents play an important role in the neural control of human movement and are influenced by neuromodulation via monoamines originating in the brainstem. During ageing, motor unit firing rates are reduced, and there is deterioration of brainstem nuclei, which may reduce persistent inward currents in alpha motoneurons. Here we show that estimates of persistent inward currents (ΔF) of both elbow flexor and extensor motor units are reduced in older adults. Estimates of persistent inward currents have a negative relationship with age in the older adults, but not in the young. This novel mechanism may play a role in the alteration of motor firing rates that occurs with ageing, which may have consequences for motor control.

Non-uniform upregulation of the autogenic stretch reflex among hindlimb extensors following lateral spinal lesion in the cat

Successful propagation throughout the step cycle is contingent on adequate regulation of whole-limb stiffness by proprioceptive feedback. Following spinal cord injury (SCI), there are changes in the strength and organization of proprioceptive feedback that can result in altered joint stiffness. In this study, we measured changes in autogenic feedback of five hindlimb extensor muscles following chronic low thoracic lateral hemisection (LSH) in decerebrate cats. We present three features of the autogenic stretch reflex obtained using a mechanographic method. Stiffness was a measure of the resistance to stretch during the length change. The dynamic index documented the extent of adaptation or increase of the force response during the hold phase, and the impulse measured the integral of the response from initiation of a stretch to the return to the initial length. The changes took the form of variable and transient increases in the stiffness of vastus (VASTI) group, soleus (SOL), and flexor hallucis longus (FHL), and either increased (VASTI) or decreased adaptation (GAS and PLANT). The stiffness of the gastrocnemius group (GAS) was also variable over time but remained elevated at the final time point. An unexpected finding was that these effects were observed bilaterally. Potential reasons for this finding and possible sources of increased excitability to this muscle group are discussed.

Spinal Cord Injury Alters Spinal Shox2 Interneurons by Enhancing Excitatory Synaptic Input and Serotonergic Modulation While Maintaining Intrinsic Properties in Mouse

Neural circuitry generating locomotor rhythm and pattern is located in the spinal cord. Most spinal cord injuries (SCIs) occur above the level of spinal locomotor neurons; therefore, these circuits are a target for improving motor function after SCI. Despite being relatively intact below the injury, locomotor circuitry undergoes substantial plasticity with the loss of descending control. Information regarding cell type-specific plasticity within locomotor circuits is limited. Shox2 interneurons (INs) have been linked to locomotor rhythm generation and patterning, making them a potential therapeutic target for the restoration of locomotion after SCI. The goal of the present study was to identify SCI-induced plasticity at the level of Shox2 INs in a complete thoracic transection model in adult male and female mice. Whole-cell patch-clamp recordings of Shox2 INs revealed minimal changes in intrinsic excitability properties after SCI. However, afferent stimulation resulted in mixed excitatory and inhibitory input to Shox2 INs in uninjured mice which became predominantly excitatory after SCI. Shox2 INs were differentially modulated by serotonin (5-HT) in a concentration-dependent manner in uninjured conditions but following SCI, 5-HT predominantly depolarized Shox2 INs. 5-HT7 receptors mediated excitatory effects on Shox2 INs from both uninjured and SCI mice, but activation of 5-HT2B/2C receptors enhanced excitability of Shox2 INs only after SCI. Overall, SCI alters sensory afferent input pathways to Shox2 INs and 5-HT modulation of Shox2 INs to enhance excitatory responses. Our findings provide relevant information regarding the locomotor circuitry response to SCI that could benefit strategies to improve locomotion after SCI.SIGNIFICANCE STATEMENT Current therapies to gain locomotor control after spinal cord injury (SCI) target spinal locomotor circuitry. Improvements in therapeutic strategies will require a better understanding of the SCI-induced plasticity within specific locomotor elements and their controllers, including sensory afferents and serotonergic modulation. Here, we demonstrate that excitability and intrinsic properties of Shox2 interneurons, which contribute to the generation of the locomotor rhythm and pattering, remain intact after SCI. However, SCI induces plasticity in both sensory afferent pathways and serotonergic modulation, enhancing the activation and excitation of Shox2 interneurons. Our findings will impact future strategies looking to harness these changes with the ultimate goal of restoring functional locomotion after SCI.

Estimates of persistent inward currents in tibialis anterior motor units during standing ramped contraction tasks in humans

Persistent inward currents (PICs) play an essential role in setting motor neuron gain and shaping motor unit firing patterns. Estimates of PICs in humans can be made using the paired motor unit analysis technique, which quantifies the difference in discharge rate of a lower threshold motor unit at the recruitment onset and offset of a higher threshold motor unit (ΔF). Because PICs are highly dependent on the level of neuromodulatory drive, ΔF represents an estimate of level of neuromodulation at the level of the spinal cord. Most of the estimates of ΔF are performed under constrained, isometric, seated conditions. In the present study, we used high-density surface EMG arrays to discriminate motor unit firing patterns during isometric seated conditions with torque or EMG visual feedback and during unconstrained standing anterior-to-posterior movements with root mean square EMG visual feedback. We were able to apply the paired motor unit analysis technique to the decomposed motor units in each of the three conditions. We hypothesized that ΔF would be higher during unconstrained standing anterior-to-posterior movements compared with the seated conditions, reflecting an increase in the synaptic input to motoneurons drive while standing. In agreement with previous work, we found that there was no evidence of a difference in ΔF between the seated and standing postures, although slight differences in the initial and peak discharge rates were observed. Taken together, our results suggest that both the standing and seated postures are likely not sufficiently different, both being "upright" postures, to result in large changes in neuromodulatory drive.NEW & NOTEWORTHY In the present study, we show that the discharge rate of a lower threshold motor unit at the recruitment onset and offset of a higher threshold motor unit (ΔF) is similar between standing and seated conditions in human tibialis anterior motor units, suggesting that at least for these two upright postures neuromodulatory drive is similar. We also highlight a proposed technological development in using high-density EMG arrays for real-time muscle activity feedback to accomplish standing ramped contraction tasks and demonstrate the validity of the paired motor unit analysis technique during these conditions.

Early delivery and prolonged treatment with nimodipine prevents the development of spasticity after spinal cord injury in mice

Spasticity, one of the most frequent comorbidities of spinal cord injury (SCI), disrupts motor recovery and quality of life. Despite major progress in neurorehabilitative and pharmacological approaches, therapeutic strategies for treating spasticity are lacking. Here, we show in a mouse model of chronic SCI that treatment with nimodipine-an L-type calcium channel blocker already approved from the European Medicine Agency and from the U.S. Food and Drug Administration-starting in the acute phase of SCI completely prevents the development of spasticity measured as increased muscle tone and spontaneous spasms. The aberrant muscle activities associated with spasticity remain inhibited even after termination of the treatment. Constitutive and conditional silencing of the L-type calcium channel Ca_V1.3 in neuronal subtypes demonstrated that this channel mediated the preventive effect of nimodipine on spasticity after SCI. This study identifies a treatment protocol and suggests that targeting Ca_V1.3 could prevent spasticity after 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.

Differences in estimated persistent inward currents between ankle flexors and extensors in humans

Persistent inward currents (PICs) are responsible for amplifying motoneuronal synaptic inputs and contribute to generating normal motoneuron activation. Delta-F (ΔF) is a well-established method that estimates PICs in humans indirectly from firing patterns of individual motor units. Traditionally, motor unit firing patterns are obtained by manually decomposing electromyography (EMG) signals recorded through intramuscular electrodes (iEMG). A previous iEMG study has shown that in humans the elbow extensors have higher ΔF than the elbow flexors. In this study, EMG signals were collected from the ankle extensors and flexors using high-density surface array electrodes during isometric sitting and standing at 10-30% maximum voluntary contraction. The signals were then decomposed into individual motor unit firings. We hypothesized that comparable to the upper limb, the lower limb extensor muscles (soleus) would have higher ΔF than the lower limb flexor muscles [tibialis anterior (TA)]. Contrary to our expectations, ΔF was higher in the TA than the soleus during sitting and standing despite the difference in cohort of participants and body positions. The TA also had significantly higher maximum discharge rate than the soleus while there was no difference in rate increase. When only the unit pairs with similar maximum discharge rates were compared, ∆F was still higher in the TA than the soleus. Future studies will focus on investigating the functional significance of the findings.NEW & NOTEWORTHY With the use of high-density surface array electrodes and convolutive blind source separation algorithm, thousands of motor units were decomposed from the soleus and tibialis anterior muscles. Persistent inward currents were estimated under seated and standing conditions via delta-F (∆F) calculation, and the results showed that unlike the upper limb, the flexor has higher ∆F than the extensor in the lower limb. Future studies will focus on functional significance of the findings.

Vibration attenuates spasm-like activity in humans with spinal cord injury

KEY POINTS

Cutaneous reflexes were tested to examine the neuronal mechanisms contributing to muscle spasms in humans with chronic spinal cord injury (SCI). Specifically, we tested the effect of Achilles and tibialis anterior tendon vibration on the early and late components of the cutaneous reflex and reciprocal Ia inhibition in the soleus and tibialis anterior muscles in humans with chronic SCI. We found that tendon vibration reduced the amplitude of later but not earlier cutaneous reflex in the antagonist but not in the agonist muscle relative to the location of the vibration. In addition, reciprocal Ia inhibition between antagonist ankle muscles increased with tendon vibration and participants with a larger suppression of the later component of the cutaneous reflex had stronger reciprocal Ia inhibition from the antagonistic muscle. Our study is the first to provide evidence that tendon vibration attenuates late cutaneous spasm-like reflex activity, likely via reciprocal inhibitory mechanisms, and may represent a method, when properly targeted, for controlling spasms in humans with SCI.

ABSTRACT

The neuronal mechanisms contributing to the generation of involuntary muscle contractions (spasms) in humans with spinal cord injury (SCI) remain poorly understood. To address this question, we examined the effect of Achilles and tibialis anterior tendon vibration at 20, 40, 80 and 120 Hz on the amplitude of the long-polysynaptic (LPR, from reflex onset to 500 ms) and long-lasting (LLR, from 500 ms to reflex offset) cutaneous reflex evoked by medial plantar nerve stimulation in the soleus and tibialis anterior, and reciprocal Ia inhibition between these muscles, in 25 individuals with chronic SCI. We found that Achilles tendon vibration at 40 and 80 Hz, but not other frequencies, reduced the amplitude of the LLR in the tibialis anterior, but not the soleus muscle, without affecting the amplitude of the LPR. Vibratory effects were stronger at 80 than 40 Hz. Similar results were found in the soleus muscle when the tibialis anterior tendon was vibrated. Notably, tendon vibration at 80 Hz increased reciprocal Ia inhibition between antagonistic ankle muscles and vibratory-induced increases in reciprocal Ia inhibition were correlated with decreases in the LLR, suggesting that participants with a larger suppression of later cutaneous reflex activity had stronger reciprocal Ia inhibition from the antagonistic muscle. Our study is the first to provide evidence that tendon vibration suppresses late spasm-like activity in antagonist but not agonist muscles, likely via reciprocal inhibitory mechanisms, in humans with chronic SCI. We argue that targeted vibration of antagonistic tendons might help to control spasms after SCI.

Estimation of self-sustained activity produced by persistent inward currents using firing rate profiles of multiple motor units in humans

Persistent inward calcium and sodium currents (I_P) activated during motoneuron recruitment help synaptic inputs maintain self-sustained firing until derecruitment. Here, we estimate the contribution of the I_P to self-sustained firing in human motoneurons of varying recruitment threshold by measuring the difference in synaptic input needed to maintain minimal firing once the I_P is fully activated compared with the larger synaptic input required to initiate firing before full I_P activation. Synaptic input to ≈20 dorsiflexor motoneurons simultaneously recorded during ramp contractions was estimated from firing profiles of motor units decomposed from high-density surface electromyography (EMG). To avoid errors introduced when using high-threshold units firing in their nonlinear range, we developed methods where the lowest threshold units firing linearly with force were used to construct a composite (control) unit firing rate profile to estimate synaptic input to higher threshold (test) units. The difference in the composite firing rate (synaptic input) at the time of test unit recruitment and derecruitment (ΔF = F_recruit - F_derecruit) was used to measure I_P amplitude that sustained firing. Test units with recruitment thresholds 1-30% of maximum had similar ΔF values, which likely included both slow and fast motor units activated by small and large motoneurons, respectively. This suggests that the portion of the I_P that sustains firing is similar across a wide range of motoneuron sizes.NEW & NOTEWORTHY A new method of estimating synaptic drive to multiple, simultaneously recorded motor units provides evidence that the portion of the depolarizing drive from persistent inward currents that contributes to self-sustained firing is similar across motoneurons of different sizes.

Delayed Injection of a Physically Cross-Linked PNIPAAm-g-PEG Hydrogel in Rat Contused Spinal Cord Improves Functional Recovery

Spinal cord injury is a main health issue, leading to multiple functional deficits with major consequences such as motor and sensitive impairment below the lesion. To date, all repair strategies remain ineffective. In line with the experiments showing that implanted hydrogels, immunologically inert biomaterials, from natural or synthetic origins, are promising tools and in order to reduce functional deficits, to increase locomotor recovery, and to reduce spasticity, we injected into the lesion area, 1 week after a severe T10 spinal cord contusion, a thermoresponsive physically cross-linked poly(N-isopropylacrylamide)-poly(ethylene glycol) copolymer hydrogel. The effect of postinjury intensive rehabilitation training was also studied. A group of male Sprague-Dawley rats receiving the hydrogel was enrolled in an 8 week program of physical activity (15 min/day, 5 days/week) in order to verify if the combination of a treadmill step-training and hydrogel could lead to better outcomes. The data obtained were compared to those obtained in animals with a spinal lesion alone receiving a saline injection with or without performing the same program of physical activity. Furthermore, in order to verify the biocompatibility of our designed biomaterial, an inflammatory reaction (interleukin-1β, interleukin-6, and tumor necrosis factor-α) was examined 15 days post-hydrogel injection. Functional recovery (postural and locomotor activities and sensorimotor coordination) was assessed from the day of injection, once a week, for 9 weeks. Finally, 9 weeks postinjection, the spinal reflexivity (rate-dependent depression of the H-reflex) was measured. The results indicate that the hydrogel did not induce an additional inflammation. Furthermore, we observed the same significant locomotor improvements in hydrogel-injected animals as in trained saline-injected animals. However, the combination of hydrogel with exercise did not show higher recovery compared to that evaluated by the two strategies independently. Finally, the H-reflex depression recovery was found to be induced by the hydrogel and, albeit to a lesser degree, exercise. However, no recovery was observed when the two strategies were combined. Our results highlight the effectiveness of our copolymer and its high therapeutic potential to preserve/repair the spinal cord after lesion.

Linking Motoneuron PIC Location to Motor Function in Closed-Loop Motor Unit System Including Afferent Feedback: A Computational Investigation

The goal of this study is to investigate how the activation location of persistent inward current (PIC) over motoneuron dendrites is linked to motor output in the closed-loop motor unit. Here, a physiologically realistic model of a motor unit including afferent inputs from muscle spindles was comprehensively analyzed under intracellular stimulation at the soma and synaptic inputs over the dendrites during isometric contractions over a full physiological range of muscle lengths. The motor output of the motor unit model was operationally assessed by evaluating the rate of force development, the degree of force potentiation and the capability of self-sustaining force production. Simulations of the model motor unit demonstrated a tendency for a faster rate of force development, a greater degree of force potentiation, and greater capacity for self-sustaining force production under both somatic and dendritic stimulation of the motoneuron as the PIC channels were positioned farther from the soma along the path of motoneuron dendrites. Interestingly, these effects of PIC activation location on force generation significantly differed among different states of muscle length. The rate of force development and the degree of force potentiation were systematically modulated by the variation of PIC channel location for shorter-than-optimal muscles but not for optimal and longer-than-optimal muscles. Similarly, the warm-up behavior of the motor unit depended on the interplay between PIC channel location and muscle length variation. These results suggest that the location of PIC activation over motoneuron dendrites may be distinctively reflected in the motor performance during shortening muscle contractions.

Impact of parameter selection on estimates of motoneuron excitability using paired motor unit analysis

OBJECTIVE

Noninvasive estimation of motoneuron excitability in human motoneurons is achieved through a paired motor unit analysis (ΔF) that quantifies hysteresis in the instantaneous firing rates at motor unit recruitment and de-recruitment. The ΔF technique provides insight into the magnitude of neuromodulatory synaptic input and persistent inward currents (PICs). While the ΔF technique is commonly used for estimating motoneuron excitability during voluntary contractions, computational parameters used for the technique vary across studies. A systematic investigation into the relationship between these parameters and ΔF values is necessary.

APPROACH

We assessed the sensitivity of the ΔF technique with several criteria commonly used in selecting motor unit pairs for analysis and methods used for smoothing the instantaneous motor unit firing rates. Using high-density surface EMG and convolutive blind source separation, we obtained a large number of motor unit pairs (5409) from the triceps brachii of ten healthy individuals during triangular isometric contractions.

MAIN RESULTS

We found an exponential plateau relationship between ΔF and the recruitment time difference between the motor unit pairs and an exponential decay relationship between ΔF and the de-recruitment time difference between the motor unit pairs, with the plateaus occurring at approximately 1 s and 1.5 s, respectively. Reduction or removal of the minimum threshold for rate-rate correlation of the two units did not affect ΔF values or variance. Removing motor unit pairs in which the firing rate of the control unit was saturated had no significant effect on ΔF. Smoothing the filter selection had no substantial effect on ΔF values and ΔF variance; however, filter selection affected the minimum recruitment and de-recruitment time differences.

SIGNIFICANCE

Our results offer recommendations for standardized parameters for the ΔF approach and facilitate the interpretation of findings from studies that implement the ΔF analysis but use different computational parameters.

Estimates of persistent inward current in human motor neurons during postural sway

Persistent inward current (PIC) plays a critical role in setting the gain of spinal motor neurons. In humans, most estimates of PIC are made from plantarflexor or dorsiflexor motor units in a seated position. This seated and static posture negates the task-dependent nature of the monoaminergic drive and afferent inhibition that modulate PIC activation. Our purpose was to estimate PIC during both the conventional seated posture and in a more functionally relevant anterior postural sway. We hypothesized that paired motor unit estimates of PIC would be greater when during standing compared with sitting. Soleus motor neuron PIC was estimated via the paired motor unit (PMU) technique. For each motor unit pair, difference in reference unit firing frequency (ΔF) estimates of PIC were made during isometric ramps in plantarflexion force during sitting (conventional approach) and during standing anterior postural sway (new approach). Baseline reciprocal inhibition (RI) was also measured in each posture using the poststimulus time histogram technique. ΔF estimates during standing postural sway were not different [2.64 ± 0.95 pulses/s (pps), P = 0.098] from seated PIC estimates (3.15 ± 1.45 pps) measured from the same motor unit pair. Similarly, reciprocal inhibition at the onset of each task was the same in standing (-0.60 ± 0.32, P = 0.301) and seated (-0.86 ± 0.82) postures. PMU recordings made during standing postural sway met all assumptions that underlay the PMU technique, including rate modulation ≥0.5 pps (3.11 ± 1.90 pps), rate-rate correlation r ≥ 0.7 (0.84 ± 0.13), and time between reference and test unit recruitment ≥1 s (1.83 ± 0.81 s). This study presents a novel, functionally relevant standing method for investigating PIC in humans.NEW & NOTEWORTHY Paired motor unit (PMU) estimates of persistent inward current (PIC) in human soleus motor units are typically made in seated posture. Our study demonstrates that these estimates can be made during standing forward sway, a task that more accurately reflects the postural role of human soleus muscle. PMU recordings made during standing postural sway were validated using all previously published criteria used to test the assumptions of the PMU technique. Standing estimates of PIC did not differ from seated estimates made from the same motor unit pairs.

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.

Residual descending motor pathways influence spasticity after spinal cord injury

OBJECTIVE

Spasticity is one of the most common symptoms manifested in humans with spinal cord injury (SCI). The neural mechanisms contributing to its development are not yet understood. Using neurophysiological and imaging techniques, we examined the influence of residual descending motor pathways on spasticity in humans with SCI.

METHODS

We measured spasticity in 33 individuals with motor complete SCI (determined by clinical examination) without preservation of voluntary motor output in the quadriceps femoris muscle. To examine residual descending motor pathways, we used magnetic and electrical stimulation over the leg motor cortex to elicit motor evoked potentials (MEPs) in the quadriceps femoris muscle and structural magnetic resonance imaging to measure spinal cord atrophy.

RESULTS

We found that 60% of participants showed symptoms of spasticity, whereas the other 40% showed no spasticity, demonstrating the presence of 2 clear subgroups of humans with motor complete SCI. MEPs were only present in individuals who had spasticity, and MEP size correlated with the severity of spasticity. Spinal cord atrophy was greater in nonspastic compared with spastic subjects. Notably, the degree of spared tissue in the lateral regions of the spinal cord was positively correlated with the severity of spasticity, indicating preservation of white matter related to motor tracts when spasticity was present.

INTERPRETATION

These results support the hypothesis that preservation of descending motor pathways influences spasticity in humans with motor complete SCI; this knowledge might help the rehabilitation and assessment of people with SCI. ANN NEUROL 2019.

Postnatal development of persistent inward currents in rat XII motoneurons and their modulation by serotonin, muscarine and noradrenaline

KEY POINTS

Persistent inward currents (PICs) in spinal motoneurons are long-lasting, voltage-dependent currents that increase excitability; they are dramatically potentiated by serotonin, muscarine, and noradrenaline (norepinephrine). Loss of these modulators (and the PIC) during sleep is hypothesized as a major contributor to REM sleep atonia. Reduced excitability of XII motoneurons that drive airway muscles and maintain airway patency is causally implicated in obstructive sleep apnoea (OSA), but whether XII motoneurons possess a modulator-sensitive PIC that could be a factor in the reduced airway tone of sleep is unknown. Whole-cell recordings from rat XII motoneurons in brain slices indicate that PIC amplitude increases ∼50% between 1 and 23 days of age, when potentiation of the PIC by 5HT₂ , muscarinic, or α₁ noradrenergic agonists peaks at <50%, manyfold lower than the potentiation observed in spinal motoneurons. α₁ noradrenergic receptor activation produced changes in XII motoneuron firing behaviour consistent with PIC involvement, but indicators of strong PIC activation were never observed; in vivo experiments are needed to determine the role of the modulator-sensitive PIC in sleep-dependent reductions in airway tone.

ABSTRACT

Hypoglossal (XII) motoneurons play a key role in maintaining airway patency; reductions in their excitability during sleep through inhibition and disfacilitation, i.e. loss of excitatory modulation, is implicated in obstructive sleep apnoea. In spinal motoneurons, 5HT₂ , muscarinic and α₁ noradrenergic modulatory systems potentiate persistent inward currents (PICs) severalfold, dramatically increasing excitability. If the PICs in XII and spinal motoneurons are equally sensitive to modulation, loss of the PIC secondary to reduced modulatory tone during sleep could contribute to airway atonia. Modulatory systems also change developmentally. We therefore characterized developmental changes in magnitude of the XII motoneuron PIC and its sensitivity to modulation by comparing, in neonatal (P1-4) and juvenile (P14-23) rat brainstem slices, the PIC elicited by slow voltage ramps in the absence and presence of agonists for 5HT₂ , muscarinic, and α₁ noradrenergic receptors. XII motoneuron PIC amplitude increased developmentally (from -195 ± 12 to -304 ± 19 pA). In neonatal XII motoneurons, the PIC was only potentiated by α₁ receptor activation (5 ± 4%). In contrast, all modulators potentiated the juvenile XII motoneurons PIC (5HT₂ , 5 ± 5%; muscarine, 22 ± 11%; α₁ , 18 ± 5%). These data suggest that the influence of the PIC and its modulation on XII motoneuron excitability will increase with postnatal development. Notably, the modulator-induced potentiation of the PIC in XII motoneurons was dramatically smaller than the 2- to 6-fold potentiation reported for spinal motoneurons. In vivo measurements are required to determine if the modulator-sensitive, XII motoneuron PIC is an important factor in sleep-state dependent reductions in airway tone.

Understanding exercise-dependent plasticity of motoneurons using intracellular and intramuscular approaches

Spinal motoneurons (MN) exhibit exercise-dependent adaptations to increased activity, such as exercise and locomotion, as well as decreased activity associated with disuse, spinal cord injury, and aging. The development of several experimental approaches, in both human and animal models, has contributed significantly to our understanding of this plasticity. The purpose of this review is to summarize how intracellular recordings in an animal model and motor unit recordings in a human model have, together, contributed to our current understanding of exercise-dependent MN plasticity. These approaches and techniques will allow neuroscientists to continue to advance our understanding of MN physiology and the plasticity of the "final common path" of the motor system, and to design experiments to answer the critical questions that are emerging in this field.

Retracing your footsteps: developmental insights to spinal network plasticity following injury

During development of the spinal cord, a precise interaction occurs between descending projections and sensory afferents, with spinal networks that lead to expression of coordinated motor output. In the rodent, during the last embryonic week, motor output first occurs as regular bursts of spontaneous activity, progressing to stochastic patterns of episodes that express bouts of coordinated rhythmic activity perinatally. Locomotor activity becomes functionally mature in the 2nd postnatal wk and is heralded by the onset of weight-bearing locomotion on the 8th and 9th postnatal day. Concomitantly, there is a maturation of intrinsic properties and key conductances mediating plateau potentials. In this review, we discuss spinal neuronal excitability, descending modulation, and afferent modulation in the developing rodent spinal cord. In the adult, plastic mechanisms are much more constrained but become more permissive following neurotrauma, such as spinal cord injury. We discuss parallel mechanisms that contribute to maturation of network function during development to mechanisms of pathological plasticity that contribute to aberrant motor patterns, such as spasticity and clonus, which emerge following central injury.

Impact of the localization of dendritic calcium persistent inward current on the input-output properties of spinal motoneuron pool: a computational study

The goal of this study is to investigate how the dendritic Ca-PIC location influences nonlinear input-output properties and depends on the type of motoneurons across the motoneuron pool. A model motoneuron pool consisting of 10 motoneurons was constructed using a recently developed two-compartment modeling approach that reflected key cell type-associated properties experimentally identified. The dendritic excitability and firing output depended systematically on both the PIC location and the motoneuron type. The PIC onset and offset in the current-voltage (I-V) relationship tended to occur at more hyperpolarized voltages as the path length to the PIC channels from the soma increased and as the cell type shifted from high- to low-threshold motoneurons. At the same time, the firing acceleration and frequency hysteresis in the frequency-current (F-I) relationship became faster and larger, respectively. However, the PIC onset-offset hysteresis increased as the path length and the recruitment threshold increased. Furthermore, the gain of frequency-current function before full PIC activation was larger for PIC channels located over distal dendritic regions in low- compared with high-threshold motoneurons. When compared with previously published experimental observations, the modeling concurred when Ca-PIC channels were placed closer to the soma in high- than low-threshold motoneurons in the model motoneuron pool. All of these results suggest that the negative relationship of Ca-PIC location and cell recruitment threshold may underlie the systematic variation in I-V and F-I transformation across the motoneuron pool.NEW & NOTEWORTHY How does the dendritic location of calcium persistent inward current (Ca-PIC) influence dendritic excitability and firing behavior across the spinal motoneuron pool? This issue was investigated developing a model motoneuron pool that reflected key motoneuron type-specific properties experimentally identified. The simulation results point out the negative relationship between the distance of Ca-PIC source from the soma and cell recruitment threshold as a basis underlying the systematic variation in input-output properties of motoneurons over the motoneuron pool.

Spinal Cord Stimulation for Spasticity: Historical Approaches, Current Status, and Future Directions

INTRODUCTION

Millions of people worldwide suffer with spasticity related to irreversible damage to the brain or spinal cord. Typical antecedent events include stroke, traumatic brain injury, and spinal cord injury, although insidious onset is also common. Regardless of the cause, the resulting spasticity leads to years of disability and reduced quality of life. Many treatments are available to manage spasticity; yet each is fraught with drawbacks including incomplete response, high cost, limited duration, dose-limiting side effects, and periodic maintenance. Spinal cord stimulation (SCS), a once promising therapy for spasticity, has largely been relegated to permanent experimental status.

METHODS

In this review, our goal is to document and critique the history and assess the development of SCS as a treatment of lower limb spasticity. By incorporating recent discoveries with the insights gained from the early pioneers in this field, we intend to lay the groundwork needed to propose testable hypotheses for future studies.

RESULTS

SCS has been tested in over 25 different conditions since a potentially beneficial effect was first reported in 1973. However, the lack of a fully formed understanding of the pathophysiology of spasticity, archaic study methodology, and the early technological limitations of implantable hardware limit the validity of many studies. SCS offers a measure of control for spasticity that cannot be duplicated with other interventions.

CONCLUSIONS

With improved energy-source miniaturization, tailored control algorithms, novel implant design, and a clearer picture of the pathophysiology of spasticity, we are poised to reintroduce and test SCS in this population.

Electroacupuncture improves gait locomotion, H-reflex and ventral root potentials of spinal compression injured rats

This study explored the effect of electroacupuncture stimulation (EA) on alterations in the Hoffman reflex (H-reflex) response and gait locomotion provoked by spinal cord injury (SCI) in the rat. A compression lesion of the spinal cord was evoked by insufflating a Fogarty balloon located in the epidural space at the T8-9 spinal level of adult Wistar male rats (200-250 gr; n=60). In different groups of SCI rats, EA (frequencies: 2, 50 and 100Hz) was applied simultaneously to Huantiao (GB30), Yinmen (BL37), Jizhong (GV6) and Zhiyang (GV9) acupoints from the third post-injury day until the experimental session. At 1, 2, 3 and 4 post-injury weeks, the BBB scores of the SCI group of rats treated with EA at 50Hz showed a gradual but greater enhancement of locomotor activity than the other groups of rats. Unrestrained gait kinematic analysis of SCI rats treated with EA-50Hz stimulation showed a significant improvement in stride duration, length and speed (p<0.05), whereas a discrete recovery of gait locomotion was observed in the other groups of animals. After four post-injury weeks, the H-reflex amplitude and H-reflex/M wave amplitude ratio obtained in SCI rats had a noticeable enhancement (217%) compared to sham rats (n=10). Meanwhile, SCI rats treated with EA at 50Hz manifested a decreased facilitation of the H-reflex amplitude and H/M amplitude ratio (154%) and a reduced frequency-dependent amplitude depression of the H-reflex (66%). In addition, 50 Hz-EA treatment induced a recovery of the presynaptic depression of the Gs-VRP evoked by PBSt conditioning stimulation in the SCI rat (63.2±8.1%; n=9). In concordance with the latter, it could be suggested that 50 Hz-EA stimulation reduced the hyper-excitability of motoneurons and provokes a partial improvement of the locomotive performance and H reflex responses by a possible recovery of presynaptic mechanisms in the spinal cord of experimentally injured rats.

Spatiotemporal correlation of spinal network dynamics underlying spasms in chronic spinalized mice

Spasms after spinal cord injury (SCI) are debilitating involuntary muscle contractions that have been associated with increased motor neuron excitability and decreased inhibition. However, whether spasms involve activation of premotor spinal excitatory neuronal circuits is unknown. Here we use mouse genetics, electrophysiology, imaging and optogenetics to directly target major classes of spinal interneurons as well as motor neurons during spasms in a mouse model of chronic SCI. We find that assemblies of excitatory spinal interneurons are recruited by sensory input into functional circuits to generate persistent neural activity, which interacts with both the graded expression of plateau potentials in motor neurons to generate spasms, and inhibitory interneurons to curtail them. Our study reveals hitherto unrecognized neuronal mechanisms for the generation of persistent neural activity under pathophysiological conditions, opening up new targets for treatment of muscle spasms after SCI.

DOI:http://dx.doi.org/10.7554/eLife.23011.001

Characterization of Involuntary Contractions after Spinal Cord Injury Reveals Associations between Physiological and Self-Reported Measures of Spasticity

Correlations between physiological, clinical and self-reported assessments of spasticity are often weak. Our aims were to quantify functional, self-reported and physiological indices of spasticity in individuals with thoracic spinal cord injury (SCI; 3 women, 9 men; 19–52 years), and to compare the strength and direction of associations between these measures. The functional measure we introduced involved recording involuntary electromyographic activity during a transfer from wheelchair to bed which is a daily task necessary for function. High soleus (SL) and tibialis anterior (TA) F-wave/M-wave area ratios were the only physiological measures that distinguished injured participants from the uninjured (6 women, 13 men, 19–67 years). Hyporeflexia (decreased SL H/M ratio) was unexpectedly present in older participants after injury. During transfers, the duration and intensity of involuntary electromyographic activity varied across muscles and participants, but coactivity was common. Wide inter-participant variability was seen for self-reported spasm frequency, severity, pain and interference with function, as well as tone (resistance to imposed joint movement). Our recordings of involuntary electromyographic activity during transfers provided evidence of significant associations between physiological and self-reported measures of spasticity. Reduced low frequency H-reflex depression in SL and high F-wave/M-wave area ratios in TA, physiological indicators of reduced inhibition and greater motoneuron excitability, respectively, were associated with long duration SL and biceps femoris (BF) electromyographic activity during transfers. In turn, participants reported high spasm frequency when transfers involved short duration TA EMG, decreased co-activation between SL and TA, as well as between rectus femoris (RF) vs. BF. Thus, the duration of muscle activity and/or the time of agonist-antagonist muscle coactivity may be used by injured individuals to count spasms. Intense electromyographic activity and high tone related closely (possibly from joint stabilization), while intense electromyographic activity in one muscle of an agonist-antagonist pair (especially in TA vs. SL, and RF vs. BF) likely induced joint movement and was associated with severe spasms. These data support the idea that individuals with SCI describe their spasticity by both the duration and intensity of involuntary agonist-antagonist muscle coactivity during everyday tasks.

Rehabilitation Strategies after Spinal Cord Injury: Inquiry into the Mechanisms of Success and Failure

Body-weight supported locomotor training (BWST) promotes recovery of load-bearing stepping in lower mammals, but its efficacy in individuals with a spinal cord injury (SCI) is limited and highly dependent on injury severity. While animal models with complete spinal transections recover stepping with step-training, motor complete SCI individuals do not, despite similarly intensive training. In this review, we examine the significant differences between humans and animal models that may explain this discrepancy in the results obtained with BWST. We also summarize the known effects of SCI and locomotor training on the muscular, motoneuronal, interneuronal, and supraspinal systems in human and non-human models of SCI and address the potential causes for failure to translate to the clinic. The evidence points to a deficiency in neuronal activation as the mechanism of failure, rather than muscular insufficiency. While motoneuronal and interneuronal systems cannot be directly probed in humans, the changes brought upon by step-training in SCI animal models suggest a beneficial re-organization of the systems' responsiveness to descending and afferent feedback that support locomotor recovery. The literature on partial lesions in humans and animal models clearly demonstrate a greater dependency on supraspinal input to the lumbar cord in humans than in non-human mammals for locomotion. Recent results with epidural stimulation that activates the lumbar interneuronal networks and/or increases the overall excitability of the locomotor centers suggest that these centers are much more dependent on the supraspinal tonic drive in humans. Sensory feedback shapes the locomotor output in animal models but does not appear to be sufficient to drive it in humans.

Intramuscular Neurotrophin-3 normalizes low threshold spinal reflexes, reduces spasms and improves mobility after bilateral corticospinal tract injury in rats

Brain and spinal injury reduce mobility and often impair sensorimotor processing in the spinal cord leading to spasticity. Here, we establish that complete transection of corticospinal pathways in the pyramids impairs locomotion and leads to increased spasms and excessive mono- and polysynaptic low threshold spinal reflexes in rats. Treatment of affected forelimb muscles with an adeno-associated viral vector (AAV) encoding human Neurotrophin-3 at a clinically-feasible time-point after injury reduced spasticity. Neurotrophin-3 normalized the short latency Hoffmann reflex to a treated hand muscle as well as low threshold polysynaptic spinal reflexes involving afferents from other treated muscles. Neurotrophin-3 also enhanced locomotor recovery. Furthermore, the balance of inhibitory and excitatory boutons in the spinal cord and the level of an ion co-transporter in motor neuron membranes required for normal reflexes were normalized. Our findings pave the way for Neurotrophin-3 as a therapy that treats the underlying causes of spasticity and not only its symptoms.

DOI:http://dx.doi.org/10.7554/eLife.18146.001

Injuries to the brain and spinal cord cause disability in millions of people worldwide. Physical rehabilitation can restore some muscle control and improve mobility in affected individuals. However, no current treatments provide long-term relief from the unwanted muscle contractions and spasms that affect as many as 78% of people with a spinal cord injury. These spasms can seriously hamper a person’s ability to carry out day-to-day tasks and get around independently. A few treatments can help in the short term but have side effects; indeed while Botox injections are used to paralyse the muscle, these also reduce the chances of useful improvements. As such, better therapies for muscle spasms are needed; especially ones that reduce spasms in the arms.

Rats with injuries to the spinal cord between their middle to lower back typically develop spasms in their legs or tail, and rat models have helped scientists begin to understand why these involuntary movements occur. Now, Kathe et al. report that cutting one specific pathway that connects the brain to the spinal cord in anesthetised rats leads to the development of spasms in the forelimbs as well. Several months after the surgery, the rats had spontaneous muscle contractions in their forelimbs and walked abnormally. Further experiments showed that some other neural pathways in the rats became incorrectly wired and hyperactive and that this resulted in the abnormal movements.

Next, Kathe et al. asked whether using gene therapy to deliver a protein that is required for neural circuits to form between muscles and the spinal cord (called neurotrophin-3) would stop the involuntary movements in the forelimbs. Delivering the gene therapy directly into the forelimb muscles of the disabled rats a day after their injury increased the levels of neurotrophin-3 in these muscles. Rats that received this treatment had fewer spasms and walked better than those that did not. Further experiments confirmed that this was because the rats’ previously hyperactive and abnormally wired neural circuits became more normal after the treatment.

Together these results suggest that neurotrophin-3 might be a useful treatment for muscle spasms in people with spinal injury. There have already been preliminary studies in people showing that treatment with neurotrophin-3 is safe and well tolerated. Future studies are needed to confirm that it could be useful in humans.

DOI:http://dx.doi.org/10.7554/eLife.18146.002

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.

Awake behaving electrophysiological correlates of forelimb hyperreflexia, weakness and disrupted muscular synchronization following cervical spinal cord injury in the rat

Spinal cord injury usually occurs at the level of the cervical spine and results in profound impairment of forelimb function. In this study, we recorded awake behaving intramuscular electromyography (EMG) from the biceps and triceps muscles of the impaired forelimb during volitional and reflexive forelimb movements before and after unilateral cervical spinal cord injury (cSCI) in rats. C5/C6 hemicontusion reduced volitional forelimb strength by more than 50% despite weekly rehabilitation for one month post-injury. Triceps EMG during volitional strength assessment was reduced by more than 60% following injury, indicating reduced descending drive. Biceps EMG during reflexive withdrawal from a thermal stimulus was increased by 500% following injury, indicating flexor withdrawal hyperreflexia. The reduction in volitional forelimb strength was significantly correlated with volitional and reflexive biceps EMG activity. Our results support the hypothesis that biceps hyperreflexia and descending volitional drive both significantly contribute to forelimb strength deficits after cSCI and provide new insight into dynamic muscular dysfunction after cSCI. The use of multiple automated quantitative measures of forelimb dysfunction in the rodent cSCI model will likely aid the search for effective regenerative, pharmacological, and neuroprosthetic treatments for spinal cord injury.

Foundational dendritic processing that is independent of the cell type-specific structure in model primary neurons

It has long been known that primary neurons in the brain and spinal cord exhibit very distinctive dendritic structures. However, it remains unclear whether dendritic processing for signal propagation and channel activation over dendrites is a function of the cell type-specific dendritic structure. By applying an extended analysis of signal attenuation for the physiological distributions of synaptic inputs and active channels on dendritic branches, we first demonstrate that regardless of their specific structure, all anatomically reconstructed models of primary neurons display a similar pattern of directional signal attenuation and locational channel activation over their dendrites. Then, using a novel modeling approach that allows direct comparison of the anatomically reconstructed primary neurons with their reduced models that exclusively retain anatomical dendritic signaling without being associated with structural specificity, we show that the reduced model can accurately predict dendritic excitability of the anatomical model in both passive and active mode. These results indicate that the directional signaling, locational excitability and their relationship are foundational features of dendritic processing that are independent of the cell type-specific structure across primary neurons.

Contribution of intrinsic motoneuron properties to discharge hysteresis and its estimation based on paired motor unit recordings: a simulation study

Motoneuron activity is strongly influenced by the activation of persistent inward currents (PICs) mediated by voltage-gated sodium and calcium channels. However, the amount of PIC contribution to the activation of human motoneurons can only be estimated indirectly. Simultaneous recordings of pairs of motor units have been used to provide an estimate of the PIC contribution by using the firing rate of the lower threshold unit to provide an estimate of the common synaptic drive to both units, and the difference in firing rate (ΔF) of this lower threshold unit at recruitment and de-recruitment of the higher threshold unit to estimate the PIC contribution to activation of the higher threshold unit. It has recently been suggested that a number of factors other than PIC can contribute to ΔF values, including mechanisms underlying spike frequency adaptation and spike threshold accommodation. In the present study, we used a set of compartmental models representing a sample of 20 motoneurons with a range of thresholds to investigate how several different intrinsic motoneuron properties can potentially contribute to variations in ΔF values. We drove the models with linearly increasing and decreasing noisy conductance commands of different rate of rise and duration and determined the influence of different intrinsic mechanisms on discharge hysteresis (the difference in excitatory drive at recruitment and de-recruitment) and ΔF. Our results indicate that, although other factors can contribute, variations in discharge hysteresis and ΔF values primarily reflect the contribution of dendritic PICs to motoneuron activation.

Homeostatic dysregulation in membrane properties of masticatory motoneurons compared with oculomotor neurons in a mouse model for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative motoneuron disease with presently no cure. Motoneuron (MN) hyperexcitability is commonly observed in ALS and is suggested to be a precursor for excitotoxic cell death. However, it is unknown whether hyperexcitability also occurs in MNs that are resistant to degeneration. Second, it is unclear whether all the MNs within homogeneous motor pools would present similar susceptibility to excitability changes since high-threshold MNs innervating fast fatigable muscle fibers selectively degenerate compared with low-threshold MNs innervating fatigue resistant slow muscle fibers. Therefore, we concurrently examined the excitability of ALS-vulnerable trigeminal motoneurons (TMNs) controlling jaw musculature and ALS-resistant oculomotor neurons (OMNs) controlling eye musculature in a well studied SOD1(G93A) ALS mouse model using in vitro patch-clamp electrophysiology at presymptomatic ages P8-P12. Our results show that hyperexcitability is not a global change among all the MNs, although mutant SOD1 is ubiquitously expressed. Instead, complex changes occur in ALS-vulnerable TMNs based on motor unit type and discharge characteristics. Firing threshold decreases among high-threshold TMNs and increases in a subpopulation of low-threshold TMNs. The latter group was identified based on their linear frequency-current responses to triangular ramp current injections. Such complex changes in MN recruitment were absent in ALS-resistant OMNs. We simulated the observed complex changes in TMN excitability using a computer-based jaw closer motor pool model. Model results suggest that hypoexcitability may indeed represent emerging disease symptomology that causes resistance in muscle force initiation. Identifying the cellular and molecular properties of these hypoexcitable cells may guide effective therapeutic strategies in ALS.