Reductions in Motor Unit Firing are Associated with Clinically Meaningful Leg Extensor Weakness in Older Adults

Weakness, one of the key characteristics of sarcopenia, is a significant risk factor for functional limitations and disability in older adults. It has long been suspected that reductions in motor unit firing rates (MUFRs) are one of the mechanistic causes of age-related weakness. However, prior work has not investigated the extent to which MUFR is associated with clinically meaningful weakness in older adults. Forty-three community-dwelling older adults (mean: 75.4 ± 7.4 years; 46.5% female) and 24 young adults (mean: 22.0 ± 1.8 years; 58.3% female) performed torque matching tasks at varying submaximal intensities with their non-dominant leg extensors. Decomposed surface electromyographic recordings were used to quantify MUFRs from the vastus lateralis muscle. Computational modeling was subsequently used to independently predict how slowed MUFRs would negatively impact strength in older adults. Bivariate correlations between MUFRs and indices of lean mass, voluntary activation, and physical function/mobility were also assessed in older adults. Weak older adults (n = 14) exhibited an approximate 1.5 and 3 Hz reduction in MUFR relative to non-weak older adults (n = 29) at 50% and 80% MVC, respectively. Older adults also exhibited an approximate 3 Hz reduction in MUFR relative to young adults at 80% MVC only. Our model predicted that a 3 Hz reduction in MUFR results in a strength decrement of 11-26%. Additionally, significant correlations were found between slower MUFRs and poorer neuromuscular quality, voluntary activation, chair rise time performance, and stair climb power (r's = 0.31 to 0.43). These findings provide evidence that slowed MUFRs are mechanistically linked with clinically meaningful leg extensor weakness in older adults.

Bifurcation analysis of motoneuronal excitability mechanisms under normal and ALS conditions

Introduction

Bifurcation analysis allows the examination of steady-state, non-linear dynamics of neurons and their effects on cell firing, yet its usage in neuroscience is limited to single-compartment models of highly reduced states. This is primarily due to the difficulty in developing high-fidelity neuronal models with 3D anatomy and multiple ion channels in XPPAUT, the primary bifurcation analysis software in neuroscience.

Methods

To facilitate bifurcation analysis of high-fidelity neuronal models under normal and disease conditions, we developed a multi-compartment model of a spinal motoneuron (MN) in XPPAUT and verified its firing accuracy against its original experimental data and against an anatomically detailed cell model that incorporates known MN non-linear firing mechanisms. We used the new model in XPPAUT to study the effects of somatic and dendritic ion channels on the MN bifurcation diagram under normal conditions and after amyotrophic lateral sclerosis (ALS) cellular changes.

Results

Our results show that somatic small-conductance Ca2+-activated K (SK) channels and dendritic L-type Ca2+ channels have the strongest effects on the bifurcation diagram of MNs under normal conditions. Specifically, somatic SK channels extend the limit cycles and generate a subcritical Hopf bifurcation node in the V-I bifurcation diagram of the MN to replace a supercritical node Hopf node, whereas L-type Ca2+ channels shift the limit cycles to negative currents. In ALS, our results show that dendritic enlargement has opposing effects on MN excitability, has a greater overall impact than somatic enlargement, and dendritic overbranching offsets the dendritic enlargement hyperexcitability effects.

Discussion

Together, the new multi-compartment model developed in XPPAUT facilitates studying neuronal excitability in health and disease using bifurcation analysis.

Fast Blue and Cholera Toxin-B Survival Guide for Alpha-Motoneurons Labeling: Less Is Better in Young B6SJL Mice, but More Is Better in Aged C57Bl/J Mice

Fast Blue (FB) and Cholera Toxin-B (CTB) are two retrograde tracers extensively used to label alpha-motoneurons (α-MNs). The overall goals of the present study were to (1) assess the effectiveness of different FB and CTB protocols in labeling α-MNs, (2) compare the labeling quality of these tracers at standard concentrations reported in the literature (FB 2% and CTB 0.1%) versus lower concentrations to overcome tracer leakage, and (3) determine an optimal protocol for labeling α-MNs in young B6SJL and aged C57Bl/J mice (when axonal transport is disrupted by aging). Hindlimb muscles of young B6SJL and aged C57Bl/J mice were intramuscularly injected with different FB or CTB concentrations and then euthanized at either 3 or 5 days after injection. Measurements were performed to assess labeling quality via seven different parameters. Our results show that tracer protocols of lower concentration and shorter labeling durations were generally better in labeling young α-MNs, whereas tracer protocols of higher tracer concentration and longer labeling durations were generally better in labeling aged α-MNs. A 0.2%, 3-day FB protocol provided optimal labeling of young α-MNs without tracer leakage, whereas a 2%, 5-day FB protocol or 0.1% CTB protocol provided optimal labeling of aged α-MNs. These results inform future studies on the selection of optimal FB and CTB protocols for α-MNs labeling in normal, aging, and neurodegenerative disease conditions.

Motoneuron excitability dysfunction in ALS: Pseudo-mystery or authentic conundrum?

In amyotrophic lateral sclerosis (ALS), abnormalities in motoneuronal excitability are seen in early pathogenesis and throughout disease progression. Fully understanding motoneuron excitability dysfunction may lead to more effective treatments. Yet decades of research have not produced consensus on the nature, role or underlying mechanisms of motoneuron excitability dysfunction in ALS. For example, contrary to Ca excitotoxicity theory, predictions of motoneuronal hyper-excitability, normal and hypo-excitability have also been seen at various disease stages and in multiple ALS lines. Accordingly, motoneuron excitability dysfunction in ALS is a disputed topic in the field. Specifically, the form (hyper, hypo or unchanged) and what role excitability dysfunction plays in the disease (pathogenic or downstream of other pathologies; neuroprotective or detrimental) are currently unclear. Although several motoneuron properties that determine cellular excitability change in the disease, some of these changes are pro-excitable, whereas others are anti-excitable, making dynamic fluctuations in overall 'net' excitability highly probable. Because various studies assess excitability via differing methods and at differing disease stages, the conflicting reports in the literature are not surprising. Hence, the overarching process of excitability degradation and motoneuron degeneration is not fully understood. Consequently, the discrepancies on motoneuron excitability dysfunction in the literature represent a substantial barrier to our understanding of the disease. Emerging studies suggest that biological variables, variations in experimental protocols, issues of rigor and sampling/analysis strategies are key factors that may underlie conflicting data in the literature. This review highlights potential confounding factors for researchers to consider and also offers ideas on avoiding pitfalls and improving robustness of data.

Non-Invasive Transcutaneous Spinal DC Stimulation as a Neurorehabilitation ALS Therapy in Awake G93A Mice: The First Step to Clinical Translation

Spinal direct current stimulation (sDCS) modulates motoneuron (MN) excitability beyond the stimulation period, making it a potential neurorehabilitation therapy for amyotrophic lateral sclerosis (ALS), a MN degenerative disease in which MN excitability dysfunction plays a critical and complex role. Recent evidence confirms induced changes in MN excitability via measured MN electrophysiological properties in the SOD1 ALS mouse during and following invasive subcutaneous sDCS (ssDCS). The first aim of our pilot study was to determine the clinical potential of these excitability changes at symptom onset (P90-P105) in ALS via a novel non-invasive transcutaneous sDCS (tsDCS) treatment paradigm on un-anesthetized SOD1-G93A mice. The primary outcomes were motor function and survival. Unfortunately, skin damage avoidance limited the strength of applied stimulation intensity, likewise limiting measurable primary effects. The second aim of this study was to determine which orientation of stimulation (anodal vs cathodal, which are expected to have opposing effects) is beneficial vs harmful in ALS. Despite the lack of measured primary effects, strong trends in survival of the anodal stimulation group, combined with an analysis of survival variance and correlations among symptoms, suggest anodal stimulation is harmful at symptom onset. Therefore, cathodal stimulation may be beneficial at symptom onset if a higher stimulation intensity can be safely achieved via subcutaneously implanted electrodes or alternative methods. Importantly, the many logistical, physical, and stimulation parameters explored in developing this novel non-invasive treatment paradigm on unanesthetized mice provide insight into an appropriate and feasible methodology for future tsDCS study designs and potential clinical translation.

In-silico development and assessment of a Kalman filter motor decoder for prosthetic hand control

Up to 50% of amputees abandon their prostheses, partly due to rapid degradation of the control systems, which require frequent recalibration. The goal of this study was to develop a Kalman filter-based approach to decoding motoneuron activity to identify movement kinematics and thereby provide stable, long-term, accurate, real-time decoding. The Kalman filter-based decoder was examined via biologically varied datasets generated from a high-fidelity computational model of the spinal motoneuron pool. The estimated movement kinematics controlled a simulated MuJoCo prosthetic hand. This clear-box approach showed successful estimation of hand movements under eight varied physiological conditions with no retraining. The mean correlation coefficient of 0.98 and mean normalized root mean square error of 0.06 over these eight datasets provide proof of concept that this decoder would improve long-term integrity of performance while performing new, untrained movements. Additionally, the decoder operated in real-time (~0.3 ms). Further results include robust performance of the Kalman filter when re-trained to more severe post-amputation limitations in the type and number of motoneurons remaining. An additional analysis shows that the decoder achieves better accuracy when using the firing of individual motoneurons as input, compared to using aggregate pool firing. Moreover, the decoder demonstrated robustness to noise affecting both the trained decoder parameters and the decoded motoneuron activity. These results demonstrate the utility of a proof of concept Kalman filter decoder that can support prosthetics' control systems to maintain accurate and stable real-time movement performance.

Adaptive Neural Decoder for Prosthetic Hand Control

The overarching goal was to resolve a major barrier to real-life prosthesis usability-the rapid degradation of prosthesis control systems, which require frequent recalibrations. Specifically, we sought to develop and test a motor decoder that provides (1) highly accurate, real-time movement response, and (2) unprecedented adaptability to dynamic changes in the amputee's biological state, thereby supporting long-term integrity of control performance with few recalibrations. To achieve that, an adaptive motor decoder was designed to auto-switch between algorithms in real-time. The decoder detects the initial aggregate motoneuron spiking activity from the motor pool, then engages the optimal parameter settings for decoding the motoneuron spiking activity in that particular state. "Clear-box" testing of decoder performance under varied physiological conditions and post-amputation complications was conducted by comparing the movement output of a simulated prosthetic hand as driven by the decoded signal vs. as driven by the actual signal. Pearson's correlation coefficient and Normalized Root Mean Square Error were used to quantify the accuracy of the decoder's output. Our results show that the decoder algorithm extracted the features of the intended movement and drove the simulated prosthetic hand accurately with real-time performance (<10 ms) (Pearson's correlation coefficient >0.98 to >0.99 and Normalized Root Mean Square Error <13-5%). Further, the decoder robustly decoded the spiking activity of multi-speed inputs, inputs generated from reversed motoneuron recruitment, and inputs reflecting substantial biological heterogeneity of motoneuron properties, also in real-time. As the amputee's neuromodulatory state changes throughout the day and the electrical properties and ratio of slower vs. faster motoneurons shift over time post-amputation, the motor decoder presented here adapts to such changes in real-time and is thus expected to greatly enhance and extend the usability of prostheses.

Estimating the effects of slicing on the electrophysiological properties of spinal motoneurons under normal and disease conditions

Although slice recordings from spinal motoneurons (MNs) are being widely used, the effects of slicing on the measured MN electrical properties under normal and disease conditions have not been assessed. Using high-fidelity cell models of neonatal wild-type (WT) and superoxide dismutase-1 (SOD) cells, we examined the effects of slice thickness, soma position within the slice, and slice orientation to estimate the error induced in measured MN electrical properties from spinal slices. Our results show that most MN electrical properties are not adversely affected by slicing, except for cell time constant, cell capacitance, and Ca²⁺ persistent inward current (PIC), which all exhibited large errors, regardless of the slice condition. Among the examined factors, soma position within the slice appears to be the strongest factor in influencing the magnitude of error in measured MN electrical properties. Transverse slices appear to have the least impact on measured MN electrical properties. Surprisingly, and despite their anatomical enlargement, we found that G85R-SOD MNs experience similar error in their measured electrical properties to those of WT MNs, but their errors are more sensitive to the soma position within the slice than WT MNs. Unless in thick and symmetrical slices, slicing appears to reduce motoneuron type differences. Accordingly, slice studies should attempt to record from MNs at the slice center to avoid large and inconsistent errors in measured cell properties and have valid cell measurements' comparisons. Our results, therefore, offer information that would enhance the rigor of MN electrophysiological data measured from the slice preparation under normal and disease conditions.NEW & NOTEWORTHY Although slice recordings from motoneurons are being widely used, the effects of slicing on the measured motoneuron electrical properties under normal and disease conditions have not been assessed. Using high-fidelity cell models of neonatal WT and SOD cells, we examined the effects of slice thickness, soma position within the slice, and slice orientation. Our results offer information that enhances the rigor of MN electrophysiological data measured from the slice preparation under normal and disease conditions.

A Classification Approach to Recognize the Firing of Spinal Motoneurons in Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease that affects the nervous system causing muscle weakness, paralysis, leading to death. Given that abnormalities in spinal motoneuron (MN) excitability begin long before symptoms manifest, developing an approach that could recognize fluctuations in MN firing could help in early diagnosis of ALS. This paper introduces a machine learning approach to discriminate between ALS and normal MN firing. The approach is based on two electrophysiological markers; namely, spiking latency and the spike-triggered average signal. The method is examined using data generated from a computational model under systematic variation of MN properties. Such variations mimic the differential dynamic changes in cellular properties that different MN types experience during ALS progression. Our results demonstrate the ability of the approach to accurately recognize ALS firing patterns across the spectrum of examined variations in MN properties.Clinical Relevance- These results represent a proof of concept for using the proposed machine-learning approach in early diagnosis of ALS.

Dendritic distributions of L-type Ca2+ and SKL channels in spinal motoneurons: a simulation study

Persistent inward currents are important to motoneuron excitability and firing behaviors and also have been implicated in excitotoxicity. In particular, L-type Ca2+ channels, usually located on motoneuron dendrites, play a primary role in amplification of synaptic inputs. However, recent experimental findings on L-type Ca2+ channel behaviors challenge some fundamental assumptions that have been used in interpreting experimental and computational modeling data. Thus, the objectives of this study were to incorporate recent experimental data into an updated, high-fidelity computational model in order to explain apparent inconsistencies and to better elucidate the spatial distributions, expression patterns, and functional roles of L-type Ca2+ and SKL channels. Specifically, the updated model incorporated asymmetric channel activation/deactivation kinetics, depolarization-dependent facilitation, randomness in channel gating, and coactivation of SKL channels. Our simulation results suggest that L-type Ca2+ and SKL channels colocalize primarily on distal dendrites of motoneurons in a punctate expression. Also, punctate expression, as opposed to a homogeneous expression, provides high synaptic current amplification, limits bistability and firing rates, and robustly regulates the Ca2+ persistent inward current, thereby reducing risk of excitotoxicity. The hysteresis and bistability observed experimentally in current-voltage and frequency-current relationships result from the L-type Ca2+ channels' distal location and intrinsic warm-up. Accordingly, our results indicate that punctate expression of L-type Ca2+ and SKL channels is a potent mechanism for regulating excitability, which would provide a strong neuroprotective effect. Our results could provide broader insights into the functional significance of warm-up and punctate expression of ion channels to regulation of cell excitability.NEW & NOTEWORTHY Recent experimental findings on L-type Ca2+ channels challenge fundamental assumptions used in interpreting experimental and computational modeling data. Here, we incorporated recent experimental data into an updated, high-fidelity computational model to explain apparent inconsistencies and better elucidate the distributions, expression patterns, and functional roles of L-type Ca2+ and SKL channels. Our results indicate that punctate expression of L-type Ca2+ and SKL channels is a potent mechanism for regulating motoneuron excitability, providing a strong neuroprotective effect.

Meta-analysis of biological variables' impact on spinal motoneuron electrophysiology data

Experimental, methodological, and biological variables must be accounted for statistically to maximize accuracy and comparability of published neuroscience data. However, accounting for all variables is nigh impossible. Thus we aimed to identify particularly influential variables within published neurological data, from cat, rat, and mouse studies, via a robust statistical process. Our goal was to develop tools to improve rigor in the collection and analysis of data. We strictly constrained experimental and methodological variables and then assessed four key biological variables within motoneuron research: species, age, sex, and cell type. We quantified intraexperimental and interexperimental variances in 11 commonly reported electrophysiological properties of spinal motoneurons. We first assessed variances without accounting for biological variables and then reassessed them while accounting for all four variables. We next assessed variances with all possible combinations of these four variables. We concluded that some motoneuron properties have low intraexperimental, but high interexperimental, variance; that individual motoneuron properties are impacted differently by biological variables; and that some unexplained variances still remain. We report here the optimal combinations of biological variables to reduce interexperimental variance for all 11 parameters. We also rank each parameter by intra- and interexperimental consistency. We expect these results to assist with design of experimental and analytical methods, and to support accuracy in simulations. Furthermore, although demonstrated on spinal motoneuron electrophysiology literature, our approach is applicable to biological data from all fields of neuroscience. This approach represents an important aid to experimental design, comparison of reported data, and reduction of unexplained variance in neuroscience data.NEW & NOTEWORTHY Our meta-analysis shows the impact of species, age, sex, and cell type on lumbosacral motoneuron electrophysiological properties by thoroughly quantifying variances across literature for the first time. We quantify the variances of 11 motoneuron properties with consideration of biological variables, thus providing specific insights for motoneuron modelers and experimenters, and providing a general methodological template for the quantification of variance in neurological data with the consideration of any experimental, methodological, or biological variables of interest.

The Mechanistic Basis for Successful Spinal Cord Stimulation to Generate Steady Motor Outputs

Electrical stimulation of the spinal cord is a promising rehabilitation intervention to restore/augment motor function after spinal cord injury (SCI). Combining sensory feedback with stimulation of remaining motor circuits has been shown to be a prerequisite for the functional improvement of SCI patients. However, little is known about the cellular mechanisms potentially underlying this functional benefit in the injured spinal cord. Here, we combine computer simulations with an isolated whole-tissue adult mouse spinal cord preparation to examine synaptic, cellular, and system potentials measured from single motoneurons and ventral roots. The stimulation protocol included separate and combined activation of the sensory inputs (evoked by dorsal root stimulation) and motor inputs (evoked by stimulation of spinal cord tissue) at different frequencies, intensities, and neuromodulatory states. Our data show that, while sensory inputs exhibit short-term depression in response to a train of stimulation, motor inputs exhibit short-term facilitation. However, the concurrent activation of both inputs elicits a stronger and steadier motor output. This effect is enhanced by the application of pharmacological neuromodulators. Furthermore, sensorimotor excitatory postsynaptic potentials (EPSPs) summate sublinearly (i.e., their combination produces an excitatory potential smaller than the sum of the excitatory potentials they would individually produce). However, ventral root compound action potentials (CoAPs) summate supralinearly generating much higher outputs. Computer simulations revealed that the contrasting summation and disproportionality in plasticity between the excitatory postsynaptic potentials (EPSPs) and CoAPs result from the motoneuronal firing threshold acting as an amplitude-selective filter. Together, these results provide the mechanistic basis for the cellular processes contributing to the generation of steady motor outputs using spinal stimulation. This data has great potential to guide the design of more effective stimulation protocols in SCI patients.

The effects of model composition design choices on high-fidelity simulations of motoneuron recruitment and firing behaviors

OBJECTIVE

Computational models often require tradeoffs, such as balancing detail with efficiency; yet optimal balance should incorporate sound design features that do not bias the results of the specific scientific question under investigation. The present study examines how model design choices impact simulation results.

APPROACH

We developed a rigorously-validated high-fidelity computational model of the spinal motoneuron pool to study three long-standing model design practices which have yet to be examined for their impact on motoneuron recruitment, firing rate, and force simulations. The practices examined were the use of: (1) generic cell models to simulate different motoneuron types, (2) discrete property ranges for different motoneuron types, and (3) biological homogeneity of cell properties within motoneuron types.

MAIN RESULTS

Our results show that each of these practices accentuates conditions of motoneuron recruitment based on the size principle, and minimizes conditions of mixed and reversed recruitment orders, which have been observed in animal and human recordings. Specifically, strict motoneuron orderly size recruitment occurs, but in a compressed range, after which mixed and reverse motoneuron recruitment occurs due to the overlap in electrical properties of different motoneuron types. Additionally, these practices underestimate the motoneuron firing rates and force data simulated by existing models.

SIGNIFICANCE

Our results indicate that current modeling practices increase conditions of motoneuron recruitment based on the size principle, and decrease conditions of mixed and reversed recruitment order, which, in turn, impacts the predictions made by existing models on motoneuron recruitment, firing rate, and force. Additionally, mixed and reverse motoneuron recruitment generated higher muscle force than orderly size motoneuron recruitment in these simulations and represents one potential scheme to increase muscle efficiency. The examined model design practices, as well as the present results, are applicable to neuronal modeling throughout the nervous system.

The vulnerability of spinal motoneurons and soma size plasticity in a mouse model of amyotrophic lateral sclerosis

KEY POINTS

Motoneuron soma size is a largely plastic property that is altered during amyotrophic lateral sclerosis (ALS) progression. We report evidence of systematic spinal motoneuron soma size plasticity in mutant SOD1-G93A mice at various disease stages and across sexes, spinal regions and motoneuron types. We show that disease-vulnerable motoneurons exhibit early increased soma sizes. We show via computer simulations that the measured changes in soma size have a profound impact on the excitability of disease-vulnerable motoneurons. This study reveals a novel form of plasticity in ALS and suggests a potential target for altering motoneuron function and survival.

ABSTRACT

α-Motoneuron soma size is correlated with the cell's excitability and function, and has been posited as a plastic property that changes during cellular maturation, injury and disease. This study examined whether α-motoneuron somas change in size over disease progression in the G93A mouse model of amyotrophic lateral sclerosis (ALS), a disease characterized by progressive motoneuron death. We used 2D- and 3D-morphometric analysis of motoneuron size and measures of cell density at four key disease stages: neonatal (P10 - with earliest known disease changes); young adult (P30 - presymptomatic with early motoneuron death); symptom onset (P90 - with death of 70-80% of motoneurons); and end-stage (P120+ - with full paralysis of hindlimbs). We additionally examined differences in lumbar vs. sacral vs. cervical motoneurons; in motoneurons from male vs. female mice; and in fast vs. slow motoneurons. We present the first evidence of plastic changes in the soma size of spinal α-motoneurons occurring throughout different stages of ALS with profound effects on motoneuron excitability. Somatic changes are time dependent and are characterized by early-stage enlargement (P10 and P30); no change around symptom onset; and shrinkage at end-stage. A key finding in the study indicates that disease-vulnerable motoneurons exhibit increased soma sizes (P10 and P30). This pattern was confirmed across spinal cord regions, genders and motoneuron types. This extends the theory of motoneuron size-based vulnerability in ALS: not only are larger motoneurons more vulnerable to death in ALS, but are also enlarged further in the disease. Such information is valuable for identifying ALS pathogenesis mechanisms.

Modulation of SK channels regulates locomotor alternating bursting activity in the functionally-mature spinal cord

The spinal cord contains specialized groups of cells called pattern generators, which are capable of orchestrating rhythmic firing activity in an isolated preparation. Different patterns of activity could be generated in vitro including right-left alternating bursting and bursting in which both sides are synchronized. The cellular and network mechanisms that enable these behaviors are not fully understood. We have recently shown that Ca²⁺-activated K⁺ channels (SK channels) control the initiation and amplitude of synchronized bursting in the spinal cord. It is unclear, however, whether SK channels play a similar role in the alternating rhythmic pattern. In the current study, we used a spinal cord preparation from functionally mature mice capable of weight bearing and walking. The present results extend our previous work and show that SK channel inhibition initiates and modulates the amplitude of alternating bursting. We also show that addition of methoxamine, an α₁-adrenergic agonist, to a cocktail of serotonin, dopamine, and NMDA evokes robust and consistent alternating bursting throughout the cord.

Mixed-mode oscillations in pyramidal neurons under antiepileptic drug conditions

Subthreshold oscillations in combination with large-amplitude oscillations generate mixed-mode oscillations (MMOs), which mediate various spatial and temporal cognition and memory processes and behavioral motor tasks. Although many studies have shown that canard theory is a reliable method to investigate the properties underlying the MMOs phenomena, the relationship between the results obtained by applying canard theory and conductance-based models of neurons and their electrophysiological mechanisms are still not well understood. The goal of this study was to apply canard theory to the conductance-based model of pyramidal neurons in layer V of the Entorhinal Cortex to investigate the properties of MMOs under antiepileptic drug conditions (i.e., when persistent sodium current is inhibited). We investigated not only the mathematical properties of MMOs in these neurons, but also the electrophysiological mechanisms that shape spike clustering. Our results show that pyramidal neurons can display two types of MMOs and the magnitude of the slow potassium current determines whether MMOs of type I or type II would emerge. Our results also indicate that slow potassium currents with large time constant have significant impact on generating the MMOs, as opposed to fast inward currents. Our results provide complete characterization of the subthreshold activities in MMOs in pyramidal neurons and provide explanation to experimental studies that showed MMOs of type I or type II in pyramidal neurons under antiepileptic drug conditions.

SK channel inhibition mediates the initiation and amplitude modulation of synchronized burst firing in the spinal cord

Burst firing in motoneurons represents the basis for generating meaningful movements. Neuromodulators and inhibitory receptor blocker cocktails have been used for years to induce burst firing in vitro; however, the ionic mechanisms in the motoneuron membrane that contribute to burst initiation and amplitude modulation are not fully understood. Small conductance Ca²⁺-activated potassium (SK) channels regulate excitatory inputs and firing output of motoneurons and interneurons and therefore, are a candidate for mediating bursting behavior. The present study examines the role of SK channels in the generation of synchronized bursting using an in vitro spinal cord preparation from adult mice. Our results show that SK channel inhibition is required for both initiation and amplitude modulation of burst firing. Specifically, administration of the synaptic inhibition blockers strychnine and picrotoxin amplified the spinal circuit excitatory drive but not enough to evoke bursting. However, when SK channels were inhibited using various approaches, the excitatory drive was further amplified, and synchronized bursting was always evoked. Furthermore, graded SK channel inhibition modulated the amplitude of the burst in a dose-dependent manner, which was reversed using SK channel activators. Importantly, modulation of neuronal excitability using multiple approaches failed to mimic the effects of SK modulators, suggesting a specific role for SK channel inhibition in generating bursting. Both NMDA (N-methyl-d-aspartate) and AMPA (α-amino-3-hydroxy-5-methylisoxazole-4-propionate) receptors were found to drive the synchronized bursts. The blocking of gap junctions did not disturb the burst synchrony. These results demonstrate a novel mechanistic role for SK channels in initiating and modulating burst firing of spinal motoneurons.NEW & NOTEWORTHY This study demonstrates that cholinergic inhibition or direct blockade of small conductance Ca²⁺-activated potassium (SK) channels facilitates burst firing in spinal motoneurons. The data provide a novel mechanistic explanation for synchronized bursting initiation and amplitude modulation through SK channel inhibition. Evidence also shows that synchronized bursting is driven by NMDA (N-methyl-d-aspartate) and AMPA (α-amino-3-hydroxy-5-methylisoxazole-4-propionate) receptors and that gap junctions do not mediate motoneuron synchronization in this behavior.

The transformation of synaptic to system plasticity in motor output from the sacral cord of the adult mouse

Synaptic plasticity is fundamental in shaping the output of neural networks. The transformation of synaptic plasticity at the cellular level into plasticity at the system level involves multiple factors, including behavior of local networks of interneurons. Here we investigate the synaptic to system transformation for plasticity in motor output in an in vitro preparation of the adult mouse spinal cord. System plasticity was assessed from compound action potentials (APs) in spinal ventral roots, which were generated simultaneously by the axons of many motoneurons (MNs). Synaptic plasticity was assessed from intracellular recordings of MNs. A computer model of the MN pool was used to identify the middle steps in the transformation from synaptic to system behavior. Two input systems that converge on the same MN pool were studied: one sensory and one descending. The two synaptic input systems generated very different motor outputs, with sensory stimulation consistently evoking short-term depression (STD) whereas descending stimulation had bimodal plasticity: STD at low frequencies but short-term facilitation (STF) at high frequencies. Intracellular and pharmacological studies revealed contributions from monosynaptic excitation and stimulus time-locked inhibition but also considerable asynchronous excitation sustained from local network activity. The computer simulations showed that STD in the monosynaptic excitatory input was the primary driver of the system STD in the sensory input whereas network excitation underlies the bimodal plasticity in the descending system. These results provide insight on the roles of plasticity in the monosynaptic and polysynaptic inputs converging on the same MN pool to overall motor plasticity.

Development of modified cable models to simulate accurate neuronal active behaviors

In large network and single three-dimensional (3-D) neuron simulations, high computing speed dictates using reduced cable models to simulate neuronal firing behaviors. However, these models are unwarranted under active conditions and lack accurate representation of dendritic active conductances that greatly shape neuronal firing. Here, realistic 3-D (R3D) models (which contain full anatomical details of dendrites) of spinal motoneurons were systematically compared with their reduced single unbranched cable (SUC, which reduces the dendrites to a single electrically equivalent cable) counterpart under passive and active conditions. The SUC models matched the R3D model's passive properties but failed to match key active properties, especially active behaviors originating from dendrites. For instance, persistent inward currents (PIC) hysteresis, frequency-current (FI) relationship secondary range slope, firing hysteresis, plateau potential partial deactivation, staircase currents, synaptic current transfer ratio, and regional FI relationships were not accurately reproduced by the SUC models. The dendritic morphology oversimplification and lack of dendritic active conductances spatial segregation in the SUC models caused significant underestimation of those behaviors. Next, SUC models were modified by adding key branching features in an attempt to restore their active behaviors. The addition of primary dendritic branching only partially restored some active behaviors, whereas the addition of secondary dendritic branching restored most behaviors. Importantly, the proposed modified models successfully replicated the active properties without sacrificing model simplicity, making them attractive candidates for running R3D single neuron and network simulations with accurate firing behaviors. The present results indicate that using reduced models to examine PIC behaviors in spinal motoneurons is unwarranted.

Adult spinal motoneurones are not hyperexcitable in a mouse model of inherited amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), an adult onset disease in which there is progressive degeneration of motoneurones, it has been suggested that an intrinsic hyperexcitability of motoneurones (i.e. an increase in their firing rates), contributes to excitotoxicity and to disease onset. Here we show that there is no such intrinsic hyperexcitability in spinal motoneurones. Our studies were carried out in an adult mouse model of ALS with a mutated form of superoxide dismutase 1 around the time of the first muscle fibre denervations. We showed that the recruitment current, the voltage threshold for spiking and the frequency-intensity gain in the primary range are all unchanged in most spinal motoneurones, despite an increased input conductance. On its own, increased input conductance would decrease excitability, but the homeostasis for excitability is maintained due to an upregulation of a depolarizing current that is activated just below the spiking threshold. However, this homeostasis failed in a substantial fraction of motoneurones, which became hypoexcitable and unable to produce sustained firing in response to ramps of current. We found similar results both in lumbar motoneurones recorded in anaesthetized mice, and in sacrocaudal motoneurones recorded in vitro, indicating that the lack of hyperexcitability is not caused by anaesthetics. Our results suggest that, if excitotoxicity is indeed a mechanism leading to degeneration in ALS, it is not caused by the intrinsic electrical properties of motoneurones but by extrinsic factors such as excessive synaptic excitation.

NMDA induces persistent inward and outward currents that cause rhythmic bursting in adult rodent motoneurons

N-methyl-d-aspartate (NMDA) receptors are of critical importance for locomotion in the developing neonatal spinal cord in rats and mice. However, due to profound changes in the expression of NMDA receptors in development between the neonatal stages and adulthood, it is unclear whether NMDA receptors are still an important component of locomotion in the adult rodent spinal cord. To shed light on this issue, we have taken advantage of recently developed preparations allowing the intracellular recording of adult motoneurons that control the tail in the sacrocaudal spinal cord of adult mice and rats. We show that in the adult sacrocaudal spinal cord, NMDA induces rhythmic activity recorded on the ventral roots, often coordinated from left to right, as in swimming motions with the tail (fictive locomotion). The adult motoneurons themselves are intrinsically sensitive to NMDA application. That is, when motoneurons are synaptically isolated with TTX, NMDA still causes spontaneous bursts of rhythmic activity, depending on the membrane potential. We show that these bursts in motoneurons depend on an NMDA-mediated persistent inward current and are terminated by the progressive activation of a persistent outward current. These results indicate that motoneurons, along with the central pattern generator, can actively participate in the production of swimminglike locomotor activity in adult rodents.

Contribution of intrinsic properties and synaptic inputs to motoneuron discharge patterns: a simulation study

Motoneuron discharge patterns reflect the interaction of synaptic inputs with intrinsic conductances. Recent work has focused on the contribution of conductances mediating persistent inward currents (PICs), which amplify and prolong the effects of synaptic inputs on motoneuron discharge. Certain features of human motor unit discharge are thought to reflect a relatively stereotyped activation of PICs by excitatory synaptic inputs; these features include rate saturation and de-recruitment at a lower level of net excitation than that required for recruitment. However, PIC activation is also influenced by the pattern and spatial distribution of inhibitory inputs that are activated concurrently with excitatory inputs. To estimate the potential contributions of PIC activation and synaptic input patterns to motor unit discharge patterns, we examined the responses of a set of cable motoneuron models to different patterns of excitatory and inhibitory inputs. The models were first tuned to approximate the current- and voltage-clamp responses of low- and medium-threshold spinal motoneurons studied in decerebrate cats and then driven with different patterns of excitatory and inhibitory inputs. The responses of the models to excitatory inputs reproduced a number of features of human motor unit discharge. However, the pattern of rate modulation was strongly influenced by the temporal and spatial pattern of concurrent inhibitory inputs. Thus, even though PIC activation is likely to exert a strong influence on firing rate modulation, PIC activation in combination with different patterns of excitatory and inhibitory synaptic inputs can produce a wide variety of motor unit discharge patterns.

Management of spasticity after spinal cord injury: current techniques and future directions

Spasticity, resulting in involuntary and sustained contractions of muscles, may evolve in patients with stroke, cerebral palsy, multiple sclerosis, brain injury, and spinal cord injury (SCI). The authors critically review the neural mechanisms that may contribute to spasticity after SCI and assess their likely degree of involvement and relative significance to its pathophysiology. Experimental data from patients and animal models of spasticity as well as computer simulations are evaluated. The current clinical methods used for the management of spasticity and the pharmacological actions of drugs are discussed in relation to their effects on spinal mechanisms. Critical assessment of experimental findings indicates that increased excitability of both motoneurons and interneurons plays a crucial role in pathophysiology of spasticity. New interventions, including forms of spinal electrical stimulation to suppress increased neuronal excitability, may reduce the severity of spasticity and its complications.

Suppressing the excitability of spinal motoneurons by extracellularly applied electrical fields: insights from computer simulations

The effect of extracellularly applied electrical fields on neuronal excitability and firing behavior is attributed to the interaction between neuronal morphology and the spatial distribution and level of differential polarization induced by the applied field in different elements of the neuron. The presence of voltage-gated ion channels that mediate persistent inward currents (PICs) on the dendrites of spinal motoneurons enhances the influence of electrical fields on the motoneuronal firing behavior. The goal of the present study was to investigate, with a realistic motoneuron computer model, the effects of extracellularly applied electrical fields on the excitability of spinal motoneurons with the aim of reducing the increased motoneuronal excitability after spinal cord injury (SCI). Our results suggest that electrical fields could suppress the excitability of motoneurons and reduce their firing rate significantly by modulating the magnitude of their dendritic PIC. This effect was achieved at different field directions, intensities, and polarities. The reduction in motoneuronal firing rate resulted from the reduction in the magnitude of the dendritic PIC reaching the soma by the effect of the applied electrical field. This reduction in PIC was attributed to the dendritic field-induced differential polarization and the nonlinear current-voltage relationship of the dendritic PIC-mediating channels. Because of the location of the motoneuronal somata and initial segment with respect to the dendrites, these structures were minimally polarized by the applied field compared with the extended dendrites. In conclusion, electrical fields could be used for suppressing the hyperexcitability of spinal motoneurons after SCI and reducing the level of spasticity.

Modulation of motoneuronal firing behavior after spinal cord injury using intraspinal microstimulation current pulses: a modeling study

We simulated the effects of delivering focal electrical stimuli to the central nervous system to modulate the firing rate of neurons and alleviate motor disorders. Application of these stimuli to the spinal cord to reduce the increased excitability of motoneurons and resulting spasticity after spinal cord injury (SCI) was examined by means of a morphologically detailed computer model of a spinal motoneuron. High-frequency sinusoidal and rectangular pulses as well as biphasic charge-balanced and charge-imbalanced pulses were examined. Our results suggest that suprathreshold high-frequency sinusoidal or rectangular current pulses could inactivate the Na+ channels in the soma and initial segment, and block action potentials from propagating through the axon. Subthreshold biphasic charge-imbalanced pulses reduced the motoneuronal firing rate significantly (up to approximately 25% reduction). The reduction in firing rate was achieved through stimulation-induced hyperpolarization generated in the first node of Ranvier. Because of their low net DC current, these pulses could be tolerated safely by the tissue. To deliver charge-imbalanced pulses with the lowest net DC current and induce the largest reduction in motoneuronal firing rate, we studied the effect of various charge-imbalanced pulse parameters. Short pulse durations were found to induce the largest reduction in firing rate for the same net DC level. Subthreshold high-frequency sinusoidal and rectangular current pulses and low-frequency biphasic charge-balanced pulses, on the other hand, were ineffective in reducing the motoneuronal firing rate. In conclusion, the proposed electrical stimulation paradigms could provide potential rehabilitation interventions for suppressing the excitability of neurons to reduce the severity of motor disorders after injury to the central nervous system.

Simulation of Ca2+ persistent inward currents in spinal motoneurones: mode of activation and integration of synaptic inputs

The goal of this study was to investigate the nature of activation of the dendritic calcium persistent inward current (Ca(2+) PIC) and its contribution to the enhancement and summation of synaptic inputs in spinal motoneurones. A compartmental cable model of a cat alpha-motoneurone was developed comprising the realistic dendritic distribution of Ia-afferent synapses and low-voltage-activated L-type calcium (Ca(v)1.3) channels distributed over the dendrites in a manner that was previously shown to match a wide set of experimental measurements. The level of synaptic activation was systematically increased and the resulting firing rate, somatic and dendritic membrane potentials, dendritic Ca(v)1.3 channel conductance, and dendritic Ca(2+) PIC were measured. Our simulation results suggest that during cell firing the dendritic Ca(2+) PIC is not activated in an all-or-none manner. Instead, it is initially activated in a graded manner with increasing synaptic input until it reaches its full activation level, after which additional increases in synaptic input result in minimal changes in the Ca(2+) PIC (PIC saturated). The range of graded activation of Ca(2+) PIC occurs when the cell is recruited and causes a steep increase in the firing frequency as the synaptic current is increased, coinciding with the secondary range of the synaptic frequency-current (F-I) relationship. Once the Ca(2+) PIC is saturated the slope of the F-I relationship is reduced, corresponding to the tertiary range of firing. When the post-spike afterhyperpolarization (AHP) is blocked, either directly by blocking the calcium-activated potassium channels, or indirectly by blocking the sodium spikes, the PIC is activated in an all-or-none manner with increasing synaptic input. Thus, the AHP serves to limit the depolarization of the cell during firing and enables graded, rather than all-or-none, activation of the Ca(2+) PIC. The graded activation of the Ca(2+) PIC with increasing synaptic input results in a graded (linear) enhancement and linear summation of synaptic inputs. In contrast, the saturated Ca(2+) PIC enhances synaptic inputs by a constant amount (constant current), and leads to less-than linear summation of multiple synaptic inputs. These model predictions improve our understanding of the mode of activation of the dendritic Ca(2+) PIC and its role in the enhancement and integration of synaptic inputs.

Simulation of dendritic CaV1.3 channels in cat lumbar motoneurons: spatial distribution

We used computer simulations to study the dendritic spatial distribution of low voltage-activated L-type calcium (Ca(V)1.3 type) channels, which mediate hysteretic persistent inward current (PIC) in spinal motoneurons. This study was prompted by the growing experimental evidence of the functional interactions between synaptic inputs and active conductances over the motoneuron dendritic tree. A compartmental cable model of an adult cat alpha-motoneuron was developed in NEURON simulation environment constituting the detailed morphology of type-identified triceps surae alpha-motoneuron and realistic distribution of group Ia afferent-to-motoneuron contacts. Simulations of different distributions of Ca(V)1.3 channels were conducted and the resultant behavior was compared to experimental data. Our results suggest that Ca(V)1.3 channels do not uniformly cover the whole motoneuron dendritic tree. Instead, their distribution is similar to that of synaptic contacts. We found that Ca(V)1.3 channels are primarily localized to a wide intermediate band overlapping with the dendritic Ia-synaptic territory at dendritic distances of 300 to 850 microm (0.62 +/- 0.21lambda) from the soma in triceps surae alpha-motoneurons. These findings explain the functional interaction between synaptic inputs and the Ca(V)1.3 channels over the motoneuron dendritic tree.