QUANTITATIVE ASPECTS OF REPETITIVE FIRING OF MAMMALIAN MOTONEURONES, CAUSED BY INJECTED CURRENTS

Single-cell optogenetics reveals attenuation-by-suppression in visual cortical neurons

The relationship between neurons' input and spiking output is central to brain computation. Studies in vitro and in anesthetized animals suggest nonlinearities emerge in cells' input-output (activation) functions as network activity increases, yet how neurons transform inputs in vivo has been unclear. Here, we characterize cortical principal neurons' activation functions in awake mice using two-photon optogenetics. We deliver fixed inputs at the soma while neurons' activity varies with sensory stimuli. We find responses to fixed optogenetic input are nearly unchanged as neurons are excited, reflecting a linear response regime above neurons' resting point. In contrast, responses are dramatically attenuated by suppression. This attenuation is a powerful means to filter inputs arriving to suppressed cells, privileging other inputs arriving to excited neurons. These results have two major implications. First, somatic neural activation functions in vivo accord with the activation functions used in recent machine learning systems. Second, neurons' IO functions can filter sensory inputs - not only do sensory stimuli change neurons' spiking outputs, but these changes also affect responses to input, attenuating responses to some inputs while leaving others unchanged.

Acute and prolonged competing effects of activation history on human motor unit firing rates during contractile impairment and recovery

The purpose of this study was to investigate the effect of inducing post-activation potentiation (PAP) during prolonged low-frequency force depression (PLFFD) on motor unit (MU) firing rates. In 10 participants, grouped firing rates of 3027 MUs from the tibialis anterior were recorded with tungsten microelectrodes. Baseline MU firing rates at 25% isometric maximal voluntary contraction (MVC) were ∼14 Hz. A 1 min dorsiflexion MVC reduced torque and maximal MU firing rates (36 Hz) by 49% and 52%, respectively. Following task completion, firing rates at 25% of baseline MVC torque and torque in response to electrically evoked (single twitch, 10 Hz and 50 Hz) stimulation were assessed before and after a 5 s MVC (to induce PAP) every 10 min for 60 min. From 10 to 60 min after task completion, the torque ratios (twitch:50 Hz and 10:50 Hz) were depressed (∼30%) relative to baseline (P < 0.001), indicating PLFFD; and firing rates were higher by ∼15% relative to baseline (P < 0.001). This occurred despite recovery of MVC rates (∼99%) and torque (∼95%) by 10 min (P > 0.3). Inducing PAP during PLFFD increased both low to high torque ratios (twitch and 10:50 Hz) by ∼200% and ∼135%, respectively (P < 0.001) and firing rates were ∼18% lower relative to PLFFD rates (P < 0.001), despite a speeding of evoked contractile properties (P = 0.001). Thus, firing rates appear strongly matched to alterations in torque, rather than contractile speed when modified by contractile history, and lower rates during PAP may be a mechanism to mitigate effects of PLFFD. The effect of activation history on contractile function demonstrates acute compensatory responses of motoneuron output. KEY POINTS: Prolonged low frequency force depression (PLFFD) following a sustained 1 min isometric maximal voluntary contraction causes an increase in submaximal mean motor unit (MU) firing rates. Inducing post-activation potentiation (PAP) during PLFFD, however, causes a reduction in mean submaximal MU firing rates to a level below those at baseline. The mean firing rate reduction during PAP occurs despite a speeding of evoked contractile properties and thus firing rates are more strongly matched to alterations in torque, rather than contractile speed when modified by various contractile histories. The reductions in firing rates during PAP may mitigate the effects of PLFFD during voluntary contractions. These results demonstrate that firing rates are highly responsive to opposing influences on the contractile state and can make rapid compensatory rate adjustments dependent on the active state of the muscle.

Spinal motoneurons respond aberrantly to serotonin in a rabbit model of cerebral palsy

Cerebral palsy (CP) is caused by a variety of factors that damage the developing central nervous system. Impaired motor control, including muscle stiffness and spasticity, is the hallmark of spastic CP. Rabbits that experience hypoxic-ischaemic (HI) injury in utero (at 70%-83% gestation) are born with muscle stiffness, hyperreflexia and, as recently discovered, increased 5-HT in the spinal cord. To determine whether serotonergic modulation of spinal motoneurons (MNs) contributes to motor deficits, we performed ex vivo whole cell patch clamp in neonatal rabbit spinal cord slices at postnatal day (P) 0-5. HI MNs responded to the application of α-methyl 5-HT (a 5-HT1 /5-HT2 receptor agonist) and citalopram (a selective 5-HT reuptake inhibitor) with increased amplitude and hyperpolarization of persistent inward currents and hyperpolarized threshold voltage for action potentials, whereas control MNs did not exhibit any of these responses. Although 5-HT similarly modulated MN properties of HI motor-unaffected and motor-affected kits, it affected sag/hyperpolarization-activated cation current (Ih ) and spike frequency adaptation only in HI motor-affected MNs. To further explore the differential sensitivity of MNs to 5-HT, we performed immunostaining for inhibitory 5-HT1A receptors in lumbar spinal MNs at P5. Fewer HI MNs expressed the 5-HT1A receptor compared to age-matched control MNs. This suggests that HI MNs may lack a normal mechanism of central fatigue, mediated by 5-HT1A receptors. Altered expression of other 5-HT receptors (including 5-HT2 ) likely also contributes to the robust increase in HI MN excitability. In summary, by directly exciting MNs, the increased concentration of spinal 5-HT in HI-affected rabbits can cause MN hyperexcitability, muscle stiffness and spasticity characteristic of CP. Therapeutic strategies that target serotonergic neuromodulation may be beneficial to individuals with CP. KEY POINTS: We used whole cell patch clamp electrophysiology to test the responsivity of spinal motoneurons (MNs) from neonatal control and hypoxia-ischaemia (HI) rabbits to 5-HT, which is elevated in the spinal cord after prenatal HI injury. HI rabbit MNs showed a more robust excitatory response to 5-HT than control rabbit MNs, including hyperpolarization of the persistent inward current and threshold voltage for action potentials. Although most MN properties of HI motor-unaffected and motor-affected kits responded similarly to 5-HT, 5-HT caused larger sag/hyperpolarization-activated cation current (Ih ) and altered repetitive firing patterns only in HI motor-affected MNs. Immunostaining revealed that fewer lumbar MNs expressed inhibitory 5-HT1A receptors in HI rabbits compared to controls, which could account for the more robust excitatory response of HI MNs to 5-HT. These results suggest that elevated 5-HT after prenatal HI injury could trigger a cascade of events that lead to muscle stiffness and altered motor unit development.

Spinal motoneurons respond aberrantly to serotonin in a rabbit model of cerebral palsy

Cerebral palsy (CP) is caused by a variety of factors that damage the developing central nervous system. Impaired motor control, including muscle stiffness and spasticity, is the hallmark of spastic CP. Rabbits that experience hypoxic-ischemic (HI) injury in utero (at 70-80% gestation) are born with muscle stiffness, hyperreflexia, and, as recently discovered, increased serotonin (5-HT) in the spinal cord. To determine whether serotonergic modulation of spinal motoneurons (MNs) contributes to motor deficits, we performed ex vivo whole cell patch clamp in neonatal rabbit spinal cord slices at postnatal day (P) 0-5. HI MNs responded to application of α-methyl 5-HT (a 5-HT 1 /5-HT 2 receptor agonist) and citalopram (a selective 5-HT reuptake inhibitor) with hyperpolarization of persistent inward currents and threshold voltage for action potentials, reduced maximum firing rate, and an altered pattern of spike frequency adaptation while control MNs did not exhibit any of these responses. To further explore the differential sensitivity of MNs to 5-HT, we performed immunohistochemistry for inhibitory 5-HT 1A receptors in lumbar spinal MNs at P5. Fewer HI MNs expressed the 5-HT 1A receptor compared to age-matched controls. This suggests many HI MNs lack a normal mechanism of central fatigue mediated by 5-HT 1A receptors. Other 5-HT receptors (including 5-HT 2 ) are likely responsible for the robust increase in HI MN excitability. In summary, by directly exciting MNs, the increased concentration of spinal 5-HT in HI rabbits can cause MN hyperexcitability, muscle stiffness, and spasticity characteristic of CP. Therapeutic strategies that target serotonergic neuromodulation may be beneficial to individuals with CP.

Key points

After prenatal hypoxia-ischemia (HI), neonatal rabbits that show hypertonia are known to have higher levels of spinal serotoninWe tested responsivity of spinal motoneurons (MNs) in neonatal control and HI rabbits to serotonin using whole cell patch clampMNs from HI rabbits showed a more robust excitatory response to serotonin than control MNs, including hyperpolarization of the persistent inward current and threshold for action potentials, larger post-inhibitory rebound, and less spike frequency adaptation Based on immunohistochemistry of lumbar MNs, fewer HI MNs express inhibitory 5HT 1A receptors than control MNs, which could account for the more robust excitatory response of HI MNs. These results suggest that after HI injury, the increased serotonin could trigger a cascade of events leading to muscle stiffness and altered motor unit development.

Biologically-inspired neuronal adaptation improves learning in neural networks

Since humans still outperform artificial neural networks on many tasks, drawing inspiration from the brain may help to improve current machine learning algorithms. Contrastive Hebbian learning (CHL) and equilibrium propagation (EP) are biologically plausible algorithms that update weights using only local information (without explicitly calculating gradients) and still achieve performance comparable to conventional backpropagation. In this study, we augmented CHL and EP with Adjusted Adaptation, inspired by the adaptation effect observed in neurons, in which a neuron's response to a given stimulus is adjusted after a short time. We add this adaptation feature to multilayer perceptrons and convolutional neural networks trained on MNIST and CIFAR-10. Surprisingly, adaptation improved the performance of these networks. We discuss the biological inspiration for this idea and investigate why Neuronal Adaptation could be an important brain mechanism to improve the stability and accuracy of learning.

Neural data-driven model of spinal excitability changes induced by transcutaneous electrical stimulation in spinal cord injury subjects

The efficacy of trans-spinal direct current stimulation (tsDCS) as neurorehabilitation technology remains sub-optimal, partly due to the variability introduced by subject-specific neurophysiological features and stimulation conditions (e.g. electrode placement, stimulating amplitude, polarity, etc.) Hence, current therapies apply tsDCS in an open-loop fashion, resulting in a lack of standardized protocols for controlling elicited neuronal adaptations in closed-loop. Through the combination of high-density electromyogram (HD-EMG) decomposition, biophysical neuronal modelling and metaheuristic optimization, this work presents a novel neural data-driven framework for estimating subject-specific features and quantifying acute neuronal adaptations elicited by tsDCS on incomplete spinal cord injury subjects. This approach consists of calibrating the anatomical parameters (e.g. soma diameter) of in silico $\alpha-$motoneuron (MN) models for firing similarly to in vivo MNs decoded from HD-EMG. Assuming that cathodal-tsDCS elicits excitability changes in the MN pool, while preserving their anatomical parameters, optimization of an excitability gain common to the entire pool was performed to minimize discrepancies in firing rate and recruitment time between in vivo and in silico MNs after cathodal-tsDCS. This quantification of excitability changes on MN models calibrated in a person specific way enables closing the loop with neuro-modulation devices to tailor neurorehabilitation therapies. Clinical Relevance - This framework addresses a key limitation in non-invasive neuro-modulative technologies via a novel model-assisted framework that enables quantifying acute excitability changes induced on a person-specific in silico MN pool calibrated using in vivo neural data. This will enable the development of advanced controllers for modulating targeted neuronal adaptations in closed-loop.

Cell maturation: Hallmarks, triggers, and manipulation

How cells become specialized, or "mature," is important for cell and developmental biology. While maturity is usually deemed a terminal fate, it may be more helpful to consider maturation not as a switch but as a dynamic continuum of adaptive phenotypic states set by genetic and environment programing. The hallmarks of maturity comprise changes in anatomy (form, gene circuitry, and interconnectivity) and physiology (function, rhythms, and proliferation) that confer adaptive behavior. We discuss efforts to harness their chemical (nutrients, oxygen, and growth factors) and physical (mechanical, spatial, and electrical) triggers in vitro and in vivo and how maturation strategies may support disease research and regenerative medicine.

Electrical Properties of Adult Mammalian Motoneurons

Motoneurons are the 'final common path' between the central nervous system (that intends, selects, commands, and organises movement) and muscles (that produce the behaviour). Motoneurons are not passive relays, but rather integrate synaptic activity to appropriately tune output (spike trains) and therefore the production of muscle force. In this chapter, we focus on studies of mammalian motoneurons, describing their heterogeneity whilst providing a brief historical account of motoneuron recording techniques. Next, we describe adult motoneurons in terms of their passive, transition, and active (repetitive firing) properties. We then discuss modulation of these properties by somatic (C-boutons) and dendritic (persistent inward currents) mechanisms. Finally, we briefly describe select studies of human motor unit physiology and relate them to findings from animal preparations discussed earlier in the chapter. This interphyletic approach to the study of motoneuron physiology is crucial to progress understanding of how these diverse neurons translate intention into behaviour.

Optimization framework for the model-based estimation of in vivo α-motoneuron properties in the intact human

The in vivo estimation of α-motoneuron (MN) properties in humans is crucial to characterize the effect that neurorehabilitation technologies may elicit over the composite neuro-musculoskeletal system. Here, we combine biophysical neuronal modelling, high-density electromyography and convolutive blind-source separation along with numerical optimization to estimate geometrical and electrophysiological properties of in vivo decoded human MNs. The proposed methodology implements multi-objective optimization to automatically tune ionic channels conductance and soma size of MN models for minimizing the error between several features of simulated and in vivo decoded MN spike trains. This approach will open new avenues for the closed-loop control of motor restorative technologies such as wearable robots and neuromodulation devices.Clinical Relevance- This work proposes a non-invasive framework for the in vivo estimation of person-specific α-motoneuron properties. This will enable predicting neuronal adaptations in response to neurorehabilitation therapies in the intact human.

Firing rate trajectories of human occipitofrontalis motor units in response to triangular voluntary contraction intensity

During voluntary contractions, limb muscle motor unit (MU) firing rates accelerate over a small force range and saturate in response to increasing contraction intensity. In comparison, facial muscles are cranially innervated, and some function without crossing joints. Therefore, the MU firing rate behaviour and characteristics of saturation were explored in a facial muscle that moves skin and facia during voluntary contractions. We evaluated the firing rate trajectory in response to triangular voluntary contraction ramps in the occipitofrontalis muscle of 11 adult participants. Intramuscular electromyography of the frontalis aspect was used to record single MU trains followed up to maximal voluntary contraction intensities. Firing rates were measured from each MU sample, with the firing rate trajectory fit as both exponential (i.e., saturation) and linear models that were compared statistically. The rate coding behaviour of frontalis MUs was broad, as the peak firing rate (mean 76 Hz) was ninefold greater than the firing rate at recruitment threshold (mean 8 Hz). Across 20 MU samples, only 40% (8 MU samples) were determined to have a firing rate trajectory that saturated and had slow acceleration in response to increasing voluntary drive until maximum. The exponential curve of the firing rate trajectory had ~ tenfold lower acceleration as compared to prior reports in limb muscles. These results across all MU samples indicated that voluntary control of the frontalis muscle requires relatively slower accelerating or linear MU firing rate trajectories, suggesting that movements of facial muscles may be directly representative of extrinsic synaptic inputs.

Anconeus motor unit firing rates during isometric and muscle-shortening contractions comparing young and very old adults

With effects of aging, voluntary neural drive to the muscle, measured as motor unit (MU) firing rate, is lower in older adults during sustained isometric contractions compared with young adults, but differences remain unknown during limb movements. Therefore, our purpose was to compare MU firing rates during both isometric and shortening contractions between two adult age groups. We analyzed intramuscular electromyography of single-MU recordings in the anconeus muscle of young (n = 8, 19-33 yr) and very old (n = 13, 78-93 yr) male adults during maximal voluntary contractions (MVCs). In sustained isometric and muscle-shortening contractions during limb movement, MU trains were linked with elbow joint kinematic parameters throughout the contraction time course. The older group was 33% weaker and 10% slower during movements than the young group (P < 0.01). In isometric contractions, median firing rates were 42% lower (P < 0.01) in the older group (18 Hz) compared with the young group (31 Hz), but during shortening contractions firing rates were higher for both age groups and not statistically different between groups. As a function of contraction time, firing rates at MU recruitment threshold were 39% lower in the older group, but the firing rate decrease was attenuated threefold throughout shortening contraction compared with the young group. At the single-MU level, age-related differences during isometric contractions (i.e., pre-movement initiation) do not remain constant throughout movement that comprises greater effects of muscle shortening. Results indicate that neural drive is task dependent and during movement in older adults it is decreased minimally.NEW & NOTEWORTHY Changes of neural drive to the muscle with adult aging, measured as motor unit firing rates during limb movements, are unknown. Throughout maximal voluntary efforts we found that, in comparison with young adults, firing rates were lower during isometric contraction in older adults but not different during elbow extension movements. Despite the older group being ∼33% weaker across contractions, their muscles can receive neural drive during movements that are similar to that of younger adults.

Motor unit firing rates during constant isometric contraction: establishing and comparing an age-related pattern among muscles

Motor unit (MU) firing rates (FRs) are lower in aged adults, compared with young, at relative voluntary contraction intensities. However, from a variety of independent studies of disparate muscles, the age-related degree of difference in FR among muscles is unclear. Using a standardized statistical approach with data derived from primary studies, we quantified differences in FRs across several muscles between younger and older adults. The data set included 12 different muscles in young (18-35 yr) and older adults (62-93 yr) from 18 published and one unpublished study. Experiments recorded single MU activity from intramuscular electromyography during constant isometric contraction at different (step-like) voluntary intensities. For each muscle, FR ranges and FR variance explained by voluntary contraction intensity were determined using bootstrapping. Dissimilarity of FR variance among muscles was calculated by Euclidean distances. There were threefold differences in the absolute frequency of FR ranges across muscles in the young (soleus 8-16 and superior trapezius 20-49 Hz), but in the old, FR ranges were more similar and lower for nine out of 12 muscles. In contrast, the explained FR variance from voluntary contraction intensity in the older group had 1.6-fold greater dissimilarity among muscles than the young (P < 0.001), with FR variance differences being muscle dependent. Therefore, differences between muscle FR ranges were not explained by how FRs scale to changes in voluntary contraction intensity within each muscle. Instead, FRs were muscle dependent but were more dissimilar among muscles in the older group in their responsiveness to voluntary contraction intensity.NEW & NOTEWORTHY The mean frequency of motor unit firing rates were compared systematically among several muscles and between young and older adults from new and published data sets. Firing rates among muscles were lower and more similar during voluntary isometric contraction in older than younger adults. Firing rate responses from voluntary contraction intensity were muscle dependent and more dissimilar among muscles in the older than young adults.

Neurophysiological Mechanisms Underlying Cortical Hyper-Excitability in Amyotrophic Lateral Sclerosis: A Review

Amyotrophic lateral sclerosis (ALS) is a progressive neuromotor disease characterized by the loss of upper and lower motor neurons (MNs), resulting in muscle paralysis and death. Early cortical hyper-excitability is a common pathological process observed clinically and in animal disease models. Although the mechanisms that underlie cortical hyper-excitability are not completely understood, the molecular and cellular mechanisms that cause enhanced neuronal intrinsic excitability and changes in excitatory and inhibitory synaptic activity are starting to emerge. Here, we review the evidence for an anterograde glutamatergic excitotoxic process, leading to cortical hyper-excitability via intrinsic cellular and synaptic mechanisms and for the role of interneurons in establishing disinhibition in clinical and experimental settings. Understanding the mechanisms that lead to these complex pathological processes will likely produce key insights towards developing novel therapeutic strategies to rescue upper MNs, thus alleviating the impact of this fatal disease.

Force variability is mostly not motor noise: Theoretical implications for motor control

Variability in muscle force is a hallmark of healthy and pathological human behavior. Predominant theories of sensorimotor control assume 'motor noise' leads to force variability and its 'signal dependence' (variability in muscle force whose amplitude increases with intensity of neural drive). Here, we demonstrate that the two proposed mechanisms for motor noise (i.e. the stochastic nature of motor unit discharge and unfused tetanic contraction) cannot account for the majority of force variability nor for its signal dependence. We do so by considering three previously underappreciated but physiologically important features of a population of motor units: 1) fusion of motor unit twitches, 2) coupling among motoneuron discharge rate, cross-bridge dynamics, and muscle mechanics, and 3) a series-elastic element to account for the aponeurosis and tendon. These results argue strongly against the idea that force variability and the resulting kinematic variability are generated primarily by 'motor noise.' Rather, they underscore the importance of variability arising from properties of control strategies embodied through distributed sensorimotor systems. As such, our study provides a critical path toward developing theories and models of sensorimotor control that provide a physiologically valid and clinically useful understanding of healthy and pathologic force variability.

Spinal motoneuron firing properties mature from rostral to caudal during postnatal development of the mouse

Key points

Many mammals are born with immature motor systems that develop through a critical period of postnatal development.

In rodents, postnatal maturation of movement occurs from rostral to caudal, correlating with maturation of descending supraspinal and local spinal circuits.

We asked whether development of fundamental electrophysiological properties of spinal motoneurons follows the same rostro‐caudal sequence.

We show that in both regions, repetitive firing parameters increase and excitability decreases with development; however, these characteristics mature earlier in cervical motoneurons.

We suggest that in addition to autonomous mechanisms, motoneuron development depends on activity resulting from their circuit milieu.

Abstract

Altricial mammals are born with immature nervous systems comprised of circuits that do not yet have the neuronal properties and connectivity required to produce future behaviours. During the critical period of postnatal development, neuronal properties are tuned to participate in functional circuits. In rodents, cervical motoneurons are born prior to lumbar motoneurons, and spinal cord development follows a sequential rostro‐caudal pattern. Here we asked whether birth order is reflected in the postnatal development of electrophysiological properties. We show that motoneurons of both regions have similar properties at birth and follow the same developmental profile, with maximal firing increasing and excitability decreasing into the third postnatal week. However, these maturative processes occur in cervical motoneurons prior to lumbar motoneurons, correlating with the maturation of premotor descending and local spinal systems. These results suggest that motoneuron properties do not mature by cell autonomous mechanisms alone, but also depend on developing premotor circuits.

Many mammals are born with immature motor systems that develop through a critical period of postnatal development.

In rodents, postnatal maturation of movement occurs from rostral to caudal, correlating with maturation of descending supraspinal and local spinal circuits.

We asked whether development of fundamental electrophysiological properties of spinal motoneurons follows the same rostro‐caudal sequence.

We show that in both regions, repetitive firing parameters increase and excitability decreases with development; however, these characteristics mature earlier in cervical motoneurons.

We suggest that in addition to autonomous mechanisms, motoneuron development depends on activity resulting from their circuit milieu.

Neural and muscular determinants of maximal rate of force development

The ability to produce rapid forces requires quick motor unit recruitment, high motor unit discharge rates, and fast motor unit force twitches. The relative importance of these parameters for maximum rate of force development (RFD), however, is poorly understood. In this study, we systematically investigated these relationships using a computational model of motor unit pool activity and force. Across simulations, neural and muscular properties were systematically varied in experimentally observed ranges. Motor units were recruited over an interval starting from contraction onset (range: 22–233 ms). Upon recruitment, discharge rates declined from an initial rate (range: 89–212 pulses per second), with varying likelihood of doublet (interspike interval of 3 ms; range: 0–50%). Finally, muscular adaptations were modeled by changing average twitch contraction time (range: 42–78 ms). Spectral analysis showed that the effective neural drive to the simulated muscle had smaller bandwidths than the average motor unit twitch, indicating that the bandwidth of the motor output, and thus the capacity for explosive force, was limited mainly by neural properties. The simulated RFD increased by 1,050 ± 281% maximal voluntary contraction force per second from the longest to the shortest recruitment interval. This effect was more than fourfold higher than the effect of increasing the initial discharge rate, more than fivefold higher than the effect of increasing the chance of doublets, and more than sixfold higher than the effect of decreasing twitch contraction times. The simulated results suggest that the physiological variation of the rate by which motor units are recruited during ballistic contractions is the main determinant for the variability in RFD across individuals.

NEW & NOTEWORTHY An important limitation of human performance is the ability to generate explosive movements by means of rapid development of muscle force. The physiological determinants of this ability, however, are poorly understood. In this study, we show using extensive simulations that the rate by which motor units are recruited is the main limiting factor for maximum rate of force development.

Curated Model Development Using NEUROiD: A Web-Based NEUROmotor Integration and Design Platform

Decades of research on neuromotor circuits and systems has provided valuable information on neuronal control of movement. Computational models of several elements of the neuromotor system have been developed at various scales, from sub-cellular to system. While several small models abound, their structured integration is the key to building larger and more biologically realistic models which can predict the behavior of the system in different scenarios. This effort calls for integration of elements across neuroscience and musculoskeletal biomechanics. There is also a need for development of methods and tools for structured integration that yield larger in silico models demonstrating a set of desired system responses. We take a small step in this direction with the NEUROmotor integration and Design (NEUROiD) platform. NEUROiD helps integrate results from motor systems anatomy, physiology, and biomechanics into an integrated neuromotor system model. Simulation and visualization of the model across multiple scales is supported. Standard electrophysiological operations such as slicing, current injection, recording of membrane potential, and local field potential are part of NEUROiD. The platform allows traceability of model parameters to primary literature. We illustrate the power and utility of NEUROiD by building a simple ankle model and its controlling neural circuitry by curating a set of published components. NEUROiD allows researchers to utilize remote high-performance computers for simulation, while controlling the model using a web browser.

Primal categories of neural polarity codes

Neuronal membrane and synapse polarities have been attracting considerable interest in recent years. Certain functional roles for such polarities have been suggested, yet, they have largely remained a subject for speculation and debate. Here, we note that neural circuit polarity codes, defined as sets of polarity permutations, divide into primal-size circuit polarity subcodes, which, sharing certain connectivity attributes, are called categories. Two long-debated, seemingly competing paradigms of neuronal self-feedback, namely, axonal discharge and synaptic mediation, are shown to jointly define the distinction between these categories. However, as the second paradigm contains the first, it is mathematically sufficient for complete specification of all categories. The analysis of primal-size circuit polarity categories is found to reveal, explain and extend experimentally observed cortical information capacity values termed "magical numbers", associated with "working memory". While these have been previously argued on grounds of psychological experiments, here they are further supported on analytic grounds by the so-called Hebbian memory paradigm. The information dimensionality associated with these capacities is found to be a consequence of prime factorization of composite circuit polarity code sizes. Different categories of circuit polarity, identical in size and neuronal parameters, are shown to generate different firing rate dynamics.

You are as fast as your motor neurons: speed of recruitment and maximal discharge of motor neurons determine the maximal rate of force development in humans

KEY POINTS

We propose and validate a method for accurately identifying the activity of populations of motor neurons during contractions at maximal rate of force development in humans. The behaviour of the motor neuron pool during rapid voluntary contractions in humans is presented. We show with this approach that the motor neuron recruitment speed and maximal motor unit discharge rate largely explains the individual ability in generating rapid force contractions. The results also indicate that the synaptic inputs received by the motor neurons before force is generated dictate human potential to generate force rapidly. This is the first characterization of the discharge behaviour of a representative sample of human motor neurons during rapid contractions.

ABSTRACT

During rapid contractions, motor neurons are recruited in a short burst and begin to discharge at high frequencies (up to >200 Hz). In the present study, we investigated the behaviour of relatively large populations of motor neurons during rapid (explosive) contractions in humans, applying a new approach to accurately identify motor neuron activity simultaneous to measuring the rate of force development. The activity of spinal motor neurons was assessed by high-density electromyographic decomposition from the tibialis anterior muscle of 20 men during isometric explosive contractions. The speed of motor neuron recruitment and the instantaneous motor unit discharge rate were analysed as a function of the impulse (the time-force integral) and the maximal rate of force development. The peak of motor unit discharge rate occurred before force generation and discharge rates decreased thereafter. The maximal motor unit discharge rate was associated with the explosive force variables, at the whole population level (r2  = 0.71 ± 0.12; P < 0.001). Moreover, the peak motor unit discharge and maximal rate of force variables were correlated with an estimate of the supraspinal drive, which was measured as the speed of motor unit recruitment before the generation of afferent feedback (P < 0.05). We show for the first time the full association between the effective neural drive to the muscle and human maximal rate of force development. The results obtained in the present study indicate that the variability in the maximal contractile explosive force of the human tibialis anterior muscle is determined by the neural activation preceding force generation.

Mechanisms and functional implications of motoneuron adaptations to increased physical activity

Motoneurons demonstrate adaptations in their physiological properties to alterations in chronic activity levels. The most consistent change that appears to result from endurance-type exercise training is the reduced excitatory current required to initiate and maintain rhythmic firing. While the precise mechanisms through which these neurons adapt to activity are currently unknown, evidence exists that adaptation may involve alterations in the expression of genes that code for membrane receptors, which can influence the responses of neurons to transmitters during activation. The influence of these adaptations may also extend to the resting condition, where ambient levels of neuroactive substances may influence ion conductances at rest, and thus result in the activation or inhibition of specific ion conductances that underlie the measurements of increased excitability that have been reported for motoneurons in the anesthetised state. We have applied motoneuron excitability and muscle unit contractile changes with endurance training to a mathematical computerized model of motor unit recruitment (Heckman and Binder 1991; J. Neurophysiol. 65(4):952–967). The results from the modelling exercise demonstrate increased task efficiency at relative levels of effort during a submaximal contraction. The physiological impact that nerve and muscle adaptations have on the neuromuscular system during standardized tasks seem to fit with reported changes in motor unit behaviour in trained human subjects.

Altered exocytosis in chromaffin cells from mouse models of neurodegenerative diseases

Chromaffin cells from the adrenal gland (CCs) have extensively been used to explore the molecular structure and function of the exocytotic machinery, neurotransmitter release and synaptic transmission. The CC is integrated in the sympathoadrenal axis that helps the body maintain homoeostasis during both routine life and in acute stress conditions. This function is exquisitely controlled by the cerebral cortex and the hypothalamus. We propose the hypothesis that damage undergone by the brain during neurodegenerative diseases is also affecting the neurosecretory function of adrenal medullary CCs. In this context, we review here the following themes: (i) How the discharge of catecholamines is centrally and peripherally regulated at the sympathoadrenal axis; (ii) which are the intricacies of the amperometric techniques used to study the quantal release of single-vesicle exocytotic events; (iii) which are the alterations of the exocytotic fusion pore so far reported, in CCs of mouse models of neurodegenerative diseases; (iv) how some proteins linked to neurodegenerative pathologies affect the kinetics of exocytotic events; (v) finally, we try to integrate available data into a hypothesis to explain how the centrally originated neurodegenerative diseases may alter the kinetics of single-vesicle exocytotic events in peripheral adrenal medullary CCs.

The Subprimary Range of Firing Is Present in Both Cat and Mouse Spinal Motoneurons and Its Relationship to Force Development Is Similar for the Two Species

In the motor system, force gradation is achieved by recruitment of motoneurons and rate modulation of their firing frequency. Classical experiments investigating the relationship between injected current to the soma during intracellular recording and the firing frequency (the I-f relation) in cat spinal motoneurons identified two clear ranges: a primary range and a secondary range. Recent work in mice, however, has identified an additional range proposed to be exclusive to rodents, the subprimary range (SPR), due to the presence of mixed mode oscillations of the membrane potential. Surprisingly, fully summated tetanic contractions occurred in mice during SPR frequencies. With the mouse now one of the most popular models to investigate motor control, it is crucial that such discrepancies between observations in mice and basic principles that have been widely accepted in larger animals are resolved. To do this, we have reinvestigated the I-f relation using ramp current injections in spinal motoneurons in both barbiturate-anesthetized and decerebrate (nonanesthetized) cats and mice. We demonstrate the presence of the SPR and mixed mode oscillations in both species and show that the SPR is enhanced by barbiturate anesthetics. Our measurements of the I-f relation in both cats and mice support the classical opinion that firing frequencies in the higher end of the primary range are necessary to obtain a full summation. By systematically varying the leg oil pool temperature (from 37°C to room temperature), we found that only at lower temperatures can maximal summation occur at SPR frequencies due to prolongation of individual muscle twitches.SIGNIFICANCE STATEMENT This work investigates recent revelations that mouse motoneurons behave in a fundamentally different way from motoneurons of larger animals with respect to the importance of rate modulation of motoneuron firing for force gradation. The current study systematically addresses the proposed discrepancies between mice and larger species (cats) and demonstrates that mouse motoneurons, in fact, use rate modulation as a mechanism of force modulation in a similar manner to the classical descriptions in larger animals.

Circuit Polarity Effect of Cortical Connectivity, Activity, and Memory

Experimental constraints have traditionally implied separate studies of different cortical functions, such as memory and sensory-motor control. Yet certain cortical modalities, while repeatedly observed and reported, have not been clearly identified with one cortical function or another. Specifically, while neuronal membrane and synapse polarities with respect to a certain potential value have been attracting considerable interest in recent years, the purposes of such polarities have largely remained a subject for speculation and debate. Formally identifying these polarities as on-off neuronal polarity gates, we analytically show that cortical circuit structure, behavior, and memory are all governed by the combined potent effect of these gates, which we collectively term circuit polarity. Employing widely accepted and biologically validated firing rate and plasticity paradigms, we show that circuit polarity is mathematically embedded in the corresponding models. Moreover, we show that the firing rate dynamics implied by these models are driven by ongoing circuit polarity gating dynamics. Furthermore, circuit polarity is shown to segregate cortical circuits into internally synchronous, externally asynchronous subcircuits, defining their firing rate modes in accordance with different cortical tasks. In contrast to the Hebbian paradigm, which is shown to be susceptible to mutual neuronal interference in the face of asynchrony, circuit polarity is shown to block such interference. Noting convergence of synaptic weights, we show that circuit polarity holds the key to cortical memory, having a segregated capacity linear in the number of neurons. While memory concealment is implied by complete neuronal silencing, memory is restored by reactivating the original circuit polarity. Finally, we show that incomplete deterioration or restoration of circuit polarity results in memory modification, which may be associated with partial or false recall, or novel innovation.

Driven to decay: Excitability and synaptic abnormalities in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron (MN) disease and is clinically characterised by the death of corticospinal motor neurons (CSMNs), spinal and brainstem MNs and the degeneration of the corticospinal tract. Degeneration of CSMNs and MNs leads inexorably to muscle wastage and weakness, progressing to eventual death within 3-5 years of diagnosis. The CSMNs, located within layer V of the primary motor cortex, project axons constituting the corticospinal tract, forming synaptic connections with brainstem and spinal cord interneurons and MNs. Clinical ALS may be divided into familial (∼10% of cases) or sporadic (∼90% of cases), based on apparent random incidence. The emergence of transgenic murine models, expressing different ALS-associated mutations has accelerated our understanding of ALS pathogenesis, although precise mechanisms remain elusive. Multiple avenues of investigation suggest that cortical electrical abnormalities have pre-eminence in the pathophysiology of ALS. In addition, glutamate-mediated functional and structural alterations in both CSMNs and MNs are present in both sporadic and familial forms of ALS. This review aims to promulgate debate in the field with regard to the common aetiology of sporadic and familial ALS. A specific focus on a nexus point in ALS pathogenesis, namely, the synaptic and intrinsic hyperexcitability of CSMNs and MNs and alterations to their structure are comprehensively detailed. The association of extramotor dysfunction with neuronal structural/functional alterations will be discussed. Finally, the implications of the latest research on the dying-forward and dying-back controversy are considered.

Spike threshold dynamics in spinal motoneurons during scratching and swimming

KEY POINTS

Action potential threshold can vary depending on firing history and synaptic inputs. We used an ex vivo carapace-spinal cord preparation from adult turtles to study spike threshold dynamics in motoneurons during two distinct types of functional motor behaviour - fictive scratching and fictive swimming. The threshold potential depolarizes by about 10 mV within each burst of spikes generated during scratch and swim network activity and recovers between bursts to a slightly depolarized level. Slow synaptic integration resulting in a wave of membrane potential depolarization is the factor influencing the threshold potential within firing bursts during motor behaviours. Depolarization of the threshold potential decreases the excitability of motoneurons and may provide a mechanism for stabilization of the response of a motoneuron to intense synaptic inputs to maintain the motor commands within an optimal range for muscle activation.

ABSTRACT

During functional spinal neural network activity motoneurons receive intense synaptic input, and this could modulate the threshold for action potential generation, providing the ability to dynamically adjust the excitability and recruitment order for functional needs. In the present study we investigated the dynamics of action potential threshold during motor network activity. Intracellular recordings from spinal motoneurons in an ex vivo carapace-spinal cord preparation from adult turtles were performed during two distinct types of motor behaviour - fictive scratching and fictive swimming. We found that the threshold of the first spike in episodes of scratching and swimming was the lowest. The threshold potential depolarizes by about 10 mV within each burst of spikes generated during scratch and swim network activity and recovers between bursts to a slightly depolarized level. Depolarization of the threshold potential results in decreased excitability of motoneurons. Synaptic inputs do not modulate the threshold of the first action potential during episodes of scratching or of swimming. There is no correlation between changes in spike threshold and interspike intervals within bursts. Slow synaptic integration that results in a wave of membrane potential depolarization rather than fast synaptic events preceding each spike is the factor influencing the threshold potential within firing bursts during motor behaviours.

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.

Motor Neurons

Motor neurons translate synaptic input from widely distributed premotor networks into patterns of action potentials that orchestrate motor unit force and motor behavior. Intercalated between the CNS and muscles, motor neurons add to and adjust the final motor command. The identity and functional properties of this facility in the path from synaptic sites to the motor axon is reviewed with emphasis on voltage sensitive ion channels and regulatory metabotropic transmitter pathways. The catalog of the intrinsic response properties, their underlying mechanisms, and regulation obtained from motoneurons in in vitro preparations is far from complete. Nevertheless, a foundation has been provided for pursuing functional significance of intrinsic response properties in motoneurons in vivo during motor behavior at levels from molecules to systems. © 2017 American Physiological Society. Compr Physiol 7:463-484, 2017.

Inhibition linearizes firing rate responses in human motor units: implications for the role of persistent inward currents

KEY POINTS

Motor neurons are the output neurons of the central nervous system and are responsible for controlling muscle contraction. When initially activated during voluntary contraction, firing rates of motor neurons increase steeply but then level out at modest rates. Activation of an intrinsic source of excitatory current at recruitment onset may underlie the initial steep increase in firing rate in motor neurons. We attempted to disable this intrinsic excitatory current by artificially activating an inhibitory reflex. When motor neuron activity was recorded while the inhibitory reflex was engaged, firing rates no longer increased steeply, suggesting that the intrinsic excitatory current was probably responsible for the initial sharp rise in motor neuron firing rate.

ABSTRACT

During graded isometric contractions, motor unit (MU) firing rates increase steeply upon recruitment but then level off at modest rates even though muscle force continues to increase. The mechanisms underlying such firing behaviour are not known although activation of persistent inward currents (PICs) might be involved. PICs are intrinsic, voltage-dependent currents that activate strongly when motor neurons (MNs) are first recruited. Such activation might cause a sharp escalation in depolarizing current and underlie the steep initial rise in MU firing rate. Because PICs can be disabled with synaptic inhibition, we hypothesized that artificial activation of an inhibitory pathway might curb this initial steep rise in firing rate. To test this, human subjects performed slow triangular ramp contractions of the ankle dorsiflexors in the absence and presence of tonic synaptic inhibition delivered to tibialis anterior (TA) MNs by sural nerve stimulation. Firing rate profiles (expressed as a function of contraction force) of TA MUs recorded during these tasks were compared for control and stimulation conditions. Under control conditions, during the ascending phase of the triangular contractions, 93% of the firing rate profiles were best fitted by rising exponential functions. With stimulation, however, firing rate profiles were best fitted with linear functions or with less steeply rising exponentials. Firing rate profiles for the descending phases of the contractions were best fitted with linear functions for both control and stimulation conditions. These results seem consistent with the idea that PICs contribute to non-linear firing rate profiles during ascending but not descending phases of contractions.

Developmental metaplasticity in neural circuit codes of firing and structure

Firing-rate dynamics have been hypothesized to mediate inter-neural information transfer in the brain. While the Hebbian paradigm, relating learning and memory to firing activity, has put synaptic efficacy variation at the center of cortical plasticity, we suggest that the external expression of plasticity by changes in the firing-rate dynamics represents a more general notion of plasticity. Hypothesizing that time constants of plasticity and firing dynamics increase with age, and employing the filtering property of the neuron, we obtain the elementary code of global attractors associated with the firing-rate dynamics in each developmental stage. We define a neural circuit connectivity code as an indivisible set of circuit structures generated by membrane and synapse activation and silencing. Synchronous firing patterns under parameter uniformity, and asynchronous circuit firing are shown to be driven, respectively, by membrane and synapse silencing and reactivation, and maintained by the neuronal filtering property. Analytic, graphical and simulation representation of the discrete iteration maps and of the global attractor codes of neural firing rate are found to be consistent with previous empirical neurobiological findings, which have lacked, however, a specific correspondence between firing modes, time constants, circuit connectivity and cortical developmental stages.

The influence of increased membrane conductance on response properties of spinal motoneurons

During functional spinal neural network activity motoneurons receive massive synaptic excitation and inhibition, and their membrane conductance increases considerably - they are switched to a high-conductance state. High-conductance states can substantially alter response properties of motoneurons. In the present study we investigated how an increase in membrane conductance affects spike frequency adaptation, the gain (i.e., the slope of the frequency-current relationship) and the threshold for action potential generation. We used intracellular recordings from adult turtle motoneurons in spinal cord slices. Membrane conductance was increased pharmacologically by extracellular application of the GABAA receptor agonist muscimol. Our findings suggest that an increase in membrane conductance of about 40-50% increases the magnitude of spike frequency adaptation, but does not change the threshold for action potential generation. Increased conductance causes a subtractive rather than a divisive effect on the initial and the early frequency-current relationships and may have not only a subtractive but also a divisive effect on the steady-state frequency-current relationship.

A leaky integrate-and-fire model with adaptation for the generation of a spike train

A model is proposed to describe the spike-frequency adaptation observed in many neuronal systems. We assume that adaptation is mainly due to a calcium-activated potassium current, and we consider two coupled stochastic differential equations for which an analytical approach combined with simulation techniques and numerical methods allow to obtain both qualitative and quantitative results about asymptotic mean firing rate, mean calcium concentration and the firing probability density. A related algorithm, based on the Hazard Rate Method, is also devised and described.

Current injection and receptor-mediated excitation produce similar maximal firing rates in hypoglossal motoneurons

The maximum firing rates of motoneurons (MNs), activated in response to synaptic drive, appear to be much lower than that elicited by current injection. It could be that the decrease in input resistance associated with increased synaptic activity (but not current injection) might blunt overall changes in membrane depolarization and thereby limit spike-frequency output. To test this idea, we recorded, in the same cells, maximal firing responses to current injection and to synaptic activation. We prepared 300 μm medullary slices in neonatal rats that contained hypoglossal MNs and used whole-cell patch-clamp electrophysiology to record their maximum firing rates in response to triangular-ramp current injections and to glutamate receptor-mediated excitation. Brief pressure pulses of high-concentration glutamate led to significant depolarization, high firing rates, and temporary cessation of spiking due to spike inactivation. In the same cells, we applied current clamp protocols that approximated the time course of membrane potential change associated with glutamate application and with peak current levels large enough to cause spike inactivation. Means (SD) of maximum firing rates obtained in response to glutamate application were nearly identical to those obtained in response to ramp current injection [glutamate 47.1 ± 12.0 impulses (imp)/s, current injection 47.5 ± 11.2 imp/s], even though input resistance was 40% less during glutamate application compared with current injection. Therefore, these data suggest that the reduction in input resistance associated with receptor-mediated excitation does not, by itself, limit the maximal firing rate responses in MNs.

Time course of human motoneuron recovery after sustained low-level voluntary activity

Motoneurons often fire repetitively and for long periods. In sustained voluntary contractions the excitability of motoneurons declines. We provide the first detailed description of the time course of human motoneuron recovery after sustained activity at a constant discharge rate. We recorded the discharge of single motor units (MUs, n = 30) with intramuscular wire electrodes inserted in triceps brachii during weak isometric contractions. Subjects ( n = 15) discharged single MUs at a constant frequency (∼10 Hz) with visual feedback for prolonged durations (3–7 min) until rectified surface electromyogram (sEMG) of triceps brachii increased by ∼100%. After a rest of 1–2, 15, 30, 60, 120, or 240 s, subjects briefly resumed the contraction with the target MU at the same discharge rate. Each MU was tested with three to four rest periods. The magnitude of sEMG was increased when contractions were resumed, and the target motoneuron discharged at the test frequency following rest intervals of 2–60 s ( P = 0.001–0.038). The increased sEMG indicates that greater excitatory drive was needed to discharge the motoneuron at the test rate. The increase in EMG recovered exponentially with a time constant of 28 s but did not return to baseline even after a rest period of ∼240 s. Thus the decline in motoneuron excitability from a weak contraction takes several minutes to recover fully.

Extensor motoneurone properties are altered immediately before and during fictive locomotion in the adult decerebrate rat

This study examined motoneurone properties during fictive locomotion in the adult rat for the first time. Fictive locomotion was induced via electrical stimulation of the mesencephalic locomotor region in decerebrate adult rats under neuromuscular blockade to compare basic and rhythmic motoneurone properties in antidromically identified extensor motoneurones during: (1) quiescence, before and after fictive locomotion; (2) the 'tonic' period immediately preceding locomotor-like activity, whereby the amplitude of peripheral flexor (peroneal) and extensor (tibial) nerves are increased but alternation has not yet occurred; and (3) locomotor-like episodes. Locomotion was identified by alternating flexor-extensor nerve activity, where the motoneurone either produced membrane oscillations consistent with a locomotor drive potential (LDP) or did not display membrane oscillation during alternating nerve activity. Cells producing LDPs were referred to as such, while those that did not were referred to as 'idle' motoneurones. LDP and idle motoneurones during locomotion had hyperpolarized spike threshold (Vth ; LDP: 3.8 mV; idle: 5.8 mV), decreased rheobase and an increased discharge rate (LDP: 64%; idle: 41%) during triangular ramp current injection even though the frequency-current slope was reduced by 70% and 55%, respectively. Modulation began in the tonic period immediately preceding locomotion, with a hyperpolarized Vth and reduced rheobase. Spike frequency adaptation did not occur in spiking LDPs or firing generated from sinusoidal current injection, but occurred during a sustained current pulse during locomotion. Input conductance showed no change. Results suggest motoneurone modulation occurs across the pool and is not restricted to motoneurones engaged in locomotion.

Motoneurons derived from induced pluripotent stem cells develop mature phenotypes typical of endogenous spinal motoneurons

Induced pluripotent cell-derived motoneurons (iPSCMNs) are sought for use in cell replacement therapies and treatment strategies for motoneuron diseases such as amyotrophic lateral sclerosis (ALS). However, much remains unknown about the physiological properties of iPSCMNs and how they compare with endogenous spinal motoneurons or embryonic stem cell-derived motoneurons (ESCMNs). In the present study, we first used a proteomic approach and compared protein expression profiles between iPSCMNs and ESCMNs to show that <4% of the proteins identified were differentially regulated. Like ESCs, we found that mouse iPSCs treated with retinoic acid and a smoothened agonist differentiated into motoneurons expressing the LIM homeodomain protein Lhx3. When transplanted into the neural tube of developing chick embryos, iPSCMNs selectively targeted muscles normally innervated by Lhx3 motoneurons. In vitro studies showed that iPSCMNs form anatomically mature and functional neuromuscular junctions (NMJs) when cocultured with chick myofibers for several weeks. Electrophysiologically, iPSCMNs developed passive membrane and firing characteristic typical of postnatal motoneurons after several weeks in culture. Finally, iPSCMNs grafted into transected mouse tibial nerve projected axons to denervated gastrocnemius muscle fibers, where they formed functional NMJs, restored contractile force. and attenuated denervation atrophy. Together, iPSCMNs possess many of the same cellular and physiological characteristics as ESCMNs and endogenous spinal motoneurons. These results further justify using iPSCMNs as a source of motoneurons for cell replacement therapies and to study motoneuron diseases such as ALS.

Physiological consequences of doublet discharges on motoneuronal firing and motor unit force

The double discharges are observed at the onset of contractions of mammalian motor units (MUs), especially during their recruitment to strong or fast movements. Doublets lead to MU force increase and improve ability of muscles to maintain high force during prolonged contractions. In this review we discuss an ability to produce doublets by fast and slow motoneurons (MNs), their influence on the course of action potential afterhyperpolarization (AHP) as well as its role in modulation of the initial stage of the firing pattern of MNs. In conclusion, a generation of doublets is an important strategy of motor control, responsible for fitting the motoneuronal firing rate to the optimal for MUs at the start of their contraction, necessary for increment of muscle force.

Variation in swim bladder drumming sounds from three doradid catfish species with similar sonic morphologies

A variety of teleost fishes produce sounds for communication by vibrating the swim bladder with fast contracting muscles. Doradid catfishes have an elastic spring apparatus (ESA) for sound production. Contractions of the ESA protractor muscle pull the anterior transverse process of the 4th vertebra or Müllerian ramus (MR) to expand the swim bladder and elasticity of the MR returns the swim bladder to the resting state. In this study, we examined the sound characteristics and associated fine structure of the protractor drumming muscles of three doradid species: Acanthodoras cataphractus, Platydoras hancockii, and Agamyxis pectinifrons. Despite important variations in sizes, sounds from all three species had similar mean dominant rates ranging from 91-131 Hz and showed frequencies related to muscle contraction speed rather than fish size. Sounds differed among species in terms of waveform shape and their rate of amplitude modulation. In addition, multiple distinguishable sound types were observed from each species: three sound types from A. cataphractus and P. hancockii, and two sound types from A. pectinifrons. Though sounds differed among species, no differences in muscle fiber fine structure were observed at the species level. Drumming muscles from each species bear features associated with fast contractions, including sarcoplasmic cores, thin radial myofibrils, abundant mitochondria, and an elaborated sarcoplasmic reticulum. These results indicate that sound differences between doradids are not due to swimbladder size, muscle anatomy, muscle length, or Müllerian ramus shape, but instead result from differences in neural activation of sonic muscles.

Influence of developmental nicotine exposure on spike-timing precision and reliability in hypoglossal motoneurons

Smoothly graded muscle contractions depend in part on the precision and reliability of motoneuron action potential generation. Whether or not a motoneuron generates spikes precisely and reliably depends on both its intrinsic membrane properties and the nature of the synaptic input that it receives. Factors that perturb neuronal intrinsic properties and/or synaptic drive may compromise the temporal precision and the reliability of action potential generation. We have previously shown that developmental nicotine exposure (DNE) alters intrinsic properties and synaptic transmission in hypoglossal motoneurons (XIIMNs). Here we show that the effects of DNE also include alterations in spike-timing precision and reliability, and spike-frequency adaptation, in response to sinusoidal current injection. Current-clamp experiments in brainstem slices from neonatal rats show that DNE lowers the threshold for spike generation but increases the variability of spike-timing mechanisms. DNE is also associated with an increase in spike-frequency adaptation and reductions in both peak and steady-state firing rate in response to brief, square wave current injections. Taken together, our data indicate that DNE causes significant alterations in the input-output efficiency of XIIMNs. These alterations may play a role in the increased frequency of obstructive apneas and altered suckling strength and coordination observed in nicotine-exposed neonatal humans.

Human stretch reflex pathways reexamined

Reflex responses of tibialis anterior motor units to stretch stimuli were investigated in human subjects. Three types of stretch stimuli were applied (tap-like, ramp-and-hold, and half-sine stretch). Stimulus-induced responses in single motor units were analyzed using the classical technique, which involved building average surface electromyogram (SEMG) and peristimulus time histograms (PSTH) from the discharge times of motor units and peristimulus frequencygrams (PSF) from the instantaneous discharge rates of single motor units. With the use of SEMG and PSTH, the tap-like stretch stimulus induced five separate reflex responses, on average. With the same single motor unit data, the PSF technique indicated that the tap stimulus induced only three reflex responses. Similar to the finding using the tap-like stretch stimuli, ramp-and-hold stimuli induced several peaks and troughs in the SEMG and PSTH. The PSF analyses displayed genuine increases in discharge rates underlying the peaks but not underlying the troughs. Half-sine stretch stimuli induced a long-lasting excitation followed by a long-lasting silent period in SEMG and PSTH. The increase in the discharge rate, however, lasted for the entire duration of the stimulus and continued during the silent period. The results are discussed in the light of the fact that the discharge rate of a motoneuron has a strong positive linear association with the effective synaptic current it receives and hence represents changes in the membrane potential more directly and accurately than the other indirect measures. This study suggests that the neuronal pathway of the human stretch reflex does not include inhibitory pathways.

Global attractor alphabet of neural firing modes

The elementary set, or alphabet, of neural firing modes is derived from the widely accepted conductance-based rectified firing-rate model. The firing dynamics of interacting neurons are shown to be governed by a multidimensional bilinear threshold discrete iteration map. The parameter-dependent global attractors of the map morph into 12 attractor types. Consistent with the dynamic modes observed in biological neuronal firing, the global attractor alphabet is highly visual and intuitive in the scalar, single-neuron case. As synapse permeability varies from high depression to high potentiation, the global attractor type varies from chaotic to multiplexed, oscillatory, fixed, and saturated. As membrane permeability decreases, the global attractor transforms from active to passive state. Under the same activation, learning and retrieval end at the same global attractor. The bilinear threshold structure of the multidimensional map associated with interacting neurons generalizes the global attractor alphabet of neuronal firing modes to multineuron systems. Selective positive or negative activation and neural interaction yield combinatorial revelation and concealment of stored neuronal global attractors.

Maturation of spinal motor neurons derived from human embryonic stem cells

Our understanding of motor neuron biology in humans is derived mainly from investigation of human postmortem tissue and more indirectly from live animal models such as rodents. Thus generation of motor neurons from human embryonic stem cells and human induced pluripotent stem cells is an important new approach to model motor neuron function. To be useful models of human motor neuron function, cells generated in vitro should develop mature properties that are the hallmarks of motor neurons in vivo such as elaborated neuronal processes and mature electrophysiological characteristics. Here we have investigated changes in morphological and electrophysiological properties associated with maturation of neurons differentiated from human embryonic stem cells expressing GFP driven by a motor neuron specific reporter (Hb9::GFP) in culture. We observed maturation in cellular morphology seen as more complex neurite outgrowth and increased soma area over time. Electrophysiological changes included decreasing input resistance and increasing action potential firing frequency over 13 days in vitro. Furthermore, these human embryonic stem cell derived motor neurons acquired two physiological characteristics that are thought to underpin motor neuron integrated function in motor circuits; spike frequency adaptation and rebound action potential firing. These findings show that human embryonic stem cell derived motor neurons develop functional characteristics typical of spinal motor neurons in vivo and suggest that they are a relevant and useful platform for studying motor neuron development and function and for modeling motor neuron diseases.

Gain of spinal motoneurons measured from square and ramp current pulses

The gain of motoneurons (MNs) characterizes how variations in synaptic input are transformed in to variations in output firing and muscle contraction. Experimentally gain is often defined as the frequency-current relation observed in response to injected suprathreshold square current pulses or current ramps during intracellular recording. The gain of MNs is strongly affected by adaptation: transient gain in response to depolarization is usually higher than steady state gain measured during sustained depolarization. The transient and the stationary gain of neurons are separate entities that can be selectively modified. Here we investigated how the transient and the stationary gain of spinal MNs obtained from responses to square current pulses are related to gain estimated from the responses to the current ramps. We found, that the gain in response to current ramps is identical to the steady state gain during sustained depolarization. Therefore, gain modulation is more fully characterized with square current pulses than with current ramps.

Hierarchical control of motor units in voluntary contractions

For the past five decades there has been wide acceptance of a relationship between the firing rate of motor units and the afterhyperpolarization of motoneurons. It has been promulgated that the higher-threshold, larger-soma, motoneurons fire faster than the lower-threshold, smaller-soma, motor units. This relationship was based on studies on anesthetized cats with electrically stimulated motoneurons. We questioned its applicability to motor unit control during voluntary contractions in humans. We found that during linearly force-increasing contractions, firing rates increased as exponential functions. At any time and force level, including at recruitment, the firing rate values were inversely related to the recruitment threshold of the motor unit. The time constants of the exponential functions were directly related to the recruitment threshold. From the Henneman size principle it follows that the characteristics of the firing rates are also related to the size of the soma. The "firing rate spectrum" presents a beautifully simple control scheme in which, at any given time or force, the firing rate value of earlier-recruited motor units is greater than that of later-recruited motor units. This hierarchical control scheme describes a mechanism that provides an effective economy of force generation for the earlier-recruited lower force-twitch motor units, and reduces the fatigue of later-recruited higher force-twitch motor units-both characteristics being well suited for generating and sustaining force during the fight-or-flight response.

The beginning of intracellular recording in spinal neurons: facts, reflections, and speculations

Intracellular (IC) recording of action potentials in neurons of the vertebrate central nervous system (CNS) was first reported by John Eccles and two colleagues, Walter Brock and John Coombs, in Dunedin, NZL in 1951/1952 and by Walter Woodbury and Harry Patton in Seattle, WA, USA in 1952. Both groups studied spinal cord neurons of the adult cat. In this review, we discuss the precedents to their notable achievement and reflect and speculate on some of the scientific and personal nuances of their work and its immediate and later impact. We then briefly discuss early achievements in IC recording in the study of CNS neurobiology in other laboratories around the world, and some of the methods that led to enhancement of CNS IC-recording techniques. Our modern understanding of CNS neurophysiology directly emanates from the pioneering endeavors of the five who wrote the seminal 1951/1952 articles.

Effects of persistent inward currents, accommodation, and adaptation on motor unit behavior: a simulation study

Motor neurons are often assumed to generate spikes in proportion to the excitatory synaptic input received. There are, however, many intrinsic properties of motor neurons that might affect this relationship, such as persistent inward currents (PICs), spike-threshold accommodation, or spike-frequency adaptation. These nonlinear properties have been investigated in reduced animal preparation but have not been well studied during natural motor behaviors because of the difficulty in characterizing synaptic input in intact animals. Therefore, we studied the influence of each of these intrinsic properties on spiking responses and muscle force using a population model of motor units that simulates voluntary contractions in human subjects. In particular, we focused on the difference in firing rate of low-threshold motor units when higher threshold motor units were recruited and subsequently derecruited, referred to as ΔF. Others have used ΔF to evaluate the extent of PIC activation during voluntary behavior. Our results showed that positive ΔF values could arise when any one of these nonlinear properties was included in the simulations. Therefore, a positive ΔF should not be considered as exclusive evidence for PIC activation. Furthermore, by systematically varying contraction duration and speed in our simulations, we identified a means that might be used experimentally to distinguish among PICs, accommodation, and adaptation as contributors to ΔF.

Reversal of the late phase of spike frequency adaptation in cat spinal motoneurons during fictive locomotion

In spinal motoneurons, late spike frequency adaptation (SFA) is defined as the slowing of the firing rate over tens of seconds and can be seen during sustained or intermittent current injection. Although the function of late SFA is not known, it may result in a decrease in force production over time, or muscle fatigue. Because locomotion can persist for long periods of time without fatigue, late SFA was studied using intracellular recordings from adult cat motoneurons during fictive locomotion. Of eight lumbar motoneurons studied, all showed late adaptation during control conditions, but none demonstrated late adaptation during locomotor activity. The most consistent properties that correlated with the presence or absence of late SFA were those related to availability of fast, inactivating sodium channels, particularly action potential rate of rise. Evidence of the reversal of late SFA during locomotion was present for several minutes following locomotor trials, consistent with the suggestion that SFA is modulated through slow metabotropic pathways. The abolition of late adaptation in spinal motoneurons during fictive locomotion is an example of a state-dependent change in the "intrinsic" properties of mammalian motoneurons. This change contributes to increased excitability of motoneurons during locomotion and results in robust firing during sustained locomotion.