Intrinsic properties of mouse lumbar motoneurons revealed by intracellular recording in vivo

Intracellular neural control of an active feeding structure in Aplysia using a carbon fiber electrode array

BACKGROUND

To study neural control of behavior, intracellular recording and stimulation of many neurons in freely moving animals would be ideal. However, current technologies limit the number of neurons that can be monitored and manipulated. A new technology has become available for intracellular recording and stimulation which we demonstrate in the tractable nervous system of Aplysia.

NEW METHOD

Carbon fiber electrode arrays (whose tips are coated with platinum-iridium) were used with an in vitro feeding preparation to intracellularly record from and to control the activity of multiple neurons during feeding movements.

RESULTS

In an in vitro feeding preparation, the carbon fiber electrode arrays recorded action potentials and subthreshold synaptic potentials during feeding movements. Depolarizing or hyperpolarizing currents activated or inhibited identified neurons (respectively), manipulating the movements of the feeding apparatus.

COMPARISON WITH EXISTING METHOD(S)

Standard glass microelectrodes that are commonly used for intracellular recording are stiff, liable to break in response to movement, and require many micromanipulators to be precisely positioned. In contrast, carbon fiber arrays are less sensitive to movement, but are capable of multiple channels of intracellular recording and stimulation.

CONCLUSIONS

Carbon fiber arrays are a novel technology for intracellular recording that can be used in moving preparations. They can record both action potentials and synaptic activity in multiple neurons and can be used to stimulate multiple neurons in complex patterns.

On the origin of F-wave: involvement of central synaptic mechanisms

Neurophysiological methods are used widely to gain information about motor neuron excitability and axon conduction in neurodegenerative diseases. The F-wave is a common biomarker used to test motor neuron properties in the diagnosis of neurological diseases. Although the origin of the F-wave is a subject of debate, the most widely accepted mechanism posits that the F-wave is generated by the backfiring of motor neurons stimulated antidromically from the periphery. In this study, we developed an ex vivo mouse sciatic nerve-attached spinal cord preparation with sensory axons severed. In this preparation, stimulation of the whole sciatic nerve or its tibial branch evoked responses with the electrophysiological signatures of F-waves. Manipulations of synaptic transmission by either removal of extracellular calcium or block of post-synaptic glutamate receptors abolished these responses. These results suggest that F-waves are mediated by spinal microcircuits activated by recurrent motor axon collaterals via glutamatergic synapses.

In vitro longitudinal lumbar spinal cord preparations to study sensory and recurrent motor microcircuits of juvenile mice

In vitro spinal cord preparations have been extensively used to study microcircuits involved in the control of movement. By allowing precise control of experimental conditions coupled with state-of-the-art genetics, imaging, and electrophysiological techniques, isolated spinal cords from mice have been an essential tool in detailing the identity, connectivity, and function of spinal networks. The majority of the research has arisen from in vitro spinal cords of neonatal mice, which are still undergoing important postnatal maturation. Studies from adults have been attempted in transverse slices, however, these have been quite challenging due to the poor motoneuron accessibility and viability, as well as the extensive damage to the motoneuron dendritic trees. In this work, we describe two types of coronal spinal cord preparations with either the ventral or the dorsal horn ablated, obtained from mice of different postnatal ages, spanning from preweaned to 1 mo old. These semi-intact preparations allow recordings of sensory-afferent and motor-efferent responses from lumbar motoneurons using whole cell patch-clamp electrophysiology. We provide details of the slicing procedure and discuss the feasibility of whole cell recordings. The in vitro dorsal and ventral horn-ablated spinal cord preparations described here are a useful tool to study spinal motor circuits in young mice that have reached the adult stages of locomotor development.NEW & NOTEWORTHY In the past 20 years, most of the research into the mammalian spinal circuitry has been limited to in vitro preparations from embryonic and neonatal mice. We describe two in vitro longitudinal lumbar spinal cord preparations from juvenile mice that allow the study of motoneuron properties and respective afferent or efferent spinal circuits through whole cell patch clamp. These preparations will be useful to those interested in the study of microcircuits at mature stages of motor development.

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.

Diversity of Mammalian Motoneurons and Motor Units

Although they share the common function of controlling muscle fiber contraction, spinal motoneurons display a remarkable diversity. Alpha-motoneurons are the "final common pathway", which relay all the information from spinal and supraspinal centers and allow the organism to interact with the outside world by controlling the contraction of muscle fibers in the muscles. On the other hand, gamma-motoneurons are specialized motoneurons that do not generate force and instead specifically innervate muscle fibers inside muscle spindles, which are proprioceptive organs embedded in the muscles. Beta-motoneurons are hybrid motoneurons that innervate both extrafusal and intrafusal muscle fibers. Even among alpha-motoneurons, there exists an exquisite diversity in terms of motoneuron electrical and molecular properties, physiological and structural properties of their neuromuscular junctions, and molecular and contractile properties of the innervated muscle fibers. This diversity, across species, across muscles, and across muscle fibers in a given muscle, underlie the vast repertoire of movements that one individual can perform.

Maturation of persistent and hyperpolarization-activated inward currents shapes the differential activation of motoneuron subtypes during postnatal development

The size principle underlies the orderly recruitment of motor units; however, motoneuron size is a poor predictor of recruitment amongst functionally defined motoneuron subtypes. Whilst intrinsic properties are key regulators of motoneuron recruitment, the underlying currents involved are not well defined. Whole-cell patch-clamp electrophysiology was deployed to study intrinsic properties, and the underlying currents, that contribute to the differential activation of delayed and immediate firing motoneuron subtypes. Motoneurons were studied during the first three postnatal weeks in mice to identify key properties that contribute to rheobase and may be important to establish orderly recruitment. We find that delayed and immediate firing motoneurons are functionally homogeneous during the first postnatal week and are activated based on size, irrespective of subtype. The rheobase of motoneuron subtypes becomes staggered during the second postnatal week, which coincides with the differential maturation of passive and active properties, particularly persistent inward currents. Rheobase of delayed firing motoneurons increases further in the third postnatal week due to the development of a prominent resting hyperpolarization-activated inward current. Our results suggest that motoneuron recruitment is multifactorial, with recruitment order established during postnatal development through the differential maturation of passive properties and sequential integration of persistent and hyperpolarization-activated inward currents.

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.

The Involvement of CaV1.3 Channels in Prolonged Root Reflexes and Its Potential as a Therapeutic Target in Spinal Cord Injury

Spinal cord injury (SCI) results in not only the loss of voluntary muscle control, but also in the presence of involuntary movement or spasms. These spasms post-SCI involve hyperexcitability in the spinal motor system. Hyperactive motor commands post SCI result from enhanced excitatory postsynaptic potentials (EPSPs) and persistent inward currents in voltage-gated L-type calcium channels (LTCCs), which are reflected in evoked root reflexes with different timings. To further understand the contributions of these cellular mechanisms and to explore the involvement of LTCC subtypes in SCI-induced hyperexcitability, we measured root reflexes with ventral root recordings and motoneuron activities with intracellular recordings in an in vitro preparation using a mouse model of chronic SCI (cSCI). Specifically, we explored the effects of 1-(3-chlorophenethyl)-3-cyclopentylpyrimidine-2,4,6-(1H,3H,5H)-trione (CPT), a selective negative allosteric modulator of Ca_V1.3 LTCCs. Our results suggest a hyperexcitability in the spinal motor system in these SCI mice. Bath application of CPT displayed slow onset but dose-dependent inhibition of the root reflexes with the strongest effect on LLRs. However, the inhibitory effect of CPT is less potent in cSCI mice than in acute SCI (aSCI) mice, suggesting changes either in composition of Ca_V1.3 or other cellular mechanisms in cSCI mice. For intracellular recordings, the intrinsic plateau potentials, was observed in more motoneurons in cSCI mice than in aSCI mice. CPT inhibited the plateau potentials and reduced motoneuron firings evoked by intracellular current injection. These results suggest that the LLR is an important target and that CPT has potential in the therapy of SCI-induced muscle spasms.

Spinal motoneurones are intrinsically more responsive in the adult G93A SOD1 mouse model of amyotrophic lateral sclerosis

KEY POINTS

Although in vitro recordings using neonatal preparations from mouse models of amyotrophic lateral sclerosis (ALS) suggest increased motoneurone excitability, in vivo recordings in adult ALS mouse models have been conflicting. In adult G93A SOD1 models, spinal motoneurones have previously been shown to have deficits in repetitive firing, in contrast to the G127X SOD1 mouse model. Our in vivo intracellular recordings in barbiturate-anaesthetized adult male G93A SOD1 mice reveal that the incidence of failure to fire with current injection was equally low in control and ALS mice (∼2%). We show that failure to fire repetitively can be a consequence of experimental protocol and should not be used alone to classify otherwise normal motoneurones as hypo-excitable. Motoneurones in the G93A SOD1 mice showed an increased response to inputs, with lower rheobase, higher input-output gains and increased activation of persistent inward currents.

ABSTRACT

In vitro studies from transgenic amyotrophic lateral sclerosis models have suggested an increased excitability of spinal motoneurones. However, in vivo intracellular recordings from adult amyotrophic lateral sclerosis mice models have produced conflicting findings. Previous investigations using barbiturate anaesthetized G93A SOD1 mice have suggested that some motoneurones are hypo-excitable, defined by deficits in repetitive firing. Our own previous recordings in G127X SOD1 mice using different anaesthesia, however, showed no repetitive firing deficits and increased persistent inward currents at symptom onset. These discrepancies may be a result of differences between models, symptomatic stage, anaesthesia or technical differences. To investigate this, we repeated our original experiments, but in adult male G93A SOD1 mice, at both presymptomatic and symptomatic stages, under barbiturate anaesthesia. In vivo intracellular recordings from antidromically identified spinal motoneurones revealed that the incidence of failure to fire with current injection was equally low in control and G93A SOD1 mice (∼2%). Motoneurones in G93A SOD1 mice fired significantly more spontaneous action potentials. Rheobase was significantly lower and the input resistance and input-output gain were significantly higher in both presymptomatic and symptomatic G93A SOD1 mice. This was despite a significant increase in the duration of the post-spike after-hyperpolarization in both presymptomatic and symptomatic G93A SOD1 mice. Finally, evidence of increased activation of persistent inward currents was seen in both presymptomatic and symptomatic G93A SOD1 mice. Our results do not confirm previous reports of hypo-excitability of spinal motoneurones in the G93A SOD1 mouse and demonstrate that the motoneurones show an increased response to inputs.

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.

Shorter axon initial segments do not cause repetitive firing impairments in the adult presymptomatic G127X SOD-1 Amyotrophic Lateral Sclerosis mouse

Increases in axonal sodium currents in peripheral nerves are some of the earliest excitability changes observed in Amyotrophic Lateral Sclerosis (ALS) patients. Nothing is known, however, about axonal sodium channels more proximally, particularly at the action potential initiating region - the axon initial segment (AIS). Immunohistochemistry for Nav1.6 sodium channels was used to investigate parameters of AISs of spinal motoneurones in the G127X SOD1 mouse model of ALS in adult mice at presymptomatic time points (~190 days old). In vivo intracellular recordings from lumbar spinal motoneurones were used to determine the consequences of any AIS changes. AISs of both alpha and gamma motoneurones were found to be significantly shorter (by 6.6% and 11.8% respectively) in G127X mice as well as being wider by 9.8% (alpha motoneurones). Measurements from 20–23 day old mice confirmed that this represented a change during adulthood. Intracellular recordings from motoneurones in presymptomatic adult mice, however, revealed no differences in individual action potentials or the cells ability to initiate repetitive action potentials. To conclude, despite changes in AIS geometry, no evidence was found for reduced excitability within the functional working range of firing frequencies of motoneurones in this model of ALS.

Nonlinear Input-Output Functions of Motoneurons

All movements are generated by the activation of motoneurons, and hence their input-output properties define the final step in processing of all motor commands. A major challenge to understanding this transformation has been the striking nonlinear behavior of motoneurons conferred by the activation of persistent inward currents (PICs) mediated by their voltage-gated Na+ and Ca2+ channels. In this review, we focus on the contribution that these PICs make to motoneuronal discharge and how the nonlinearities they engender impede the construction of a comprehensive model of motor control.

Scaling of Motor Output, From Mouse to Humans

Appropriate scaling of motor output from mouse to humans is essential. The motoneurons that generate all motor output are, however, very different in rodents compared with humans, being smaller and much more excitable. In contrast, feline motoneurons are more similar to those in humans. These scaling differences need to be taken into account for the use of rodents for translational studies of motor output.

Molecular and electrophysiological properties of mouse motoneuron and motor unit subtypes

The field of motoneuron and motor unit physiology in mammals has deeply evolved the last decade thanks to the parallel development of mouse genetics and transcriptomic analysis and of in vivo mouse preparations that allow intracellular electrophysiological recordings of motoneurons. We review the efforts made to investigate the electrophysiological properties of the different functional subtypes of mouse motoneurons, to decipher the mosaic of molecular markers specifically expressed in each subtype, and to elucidate which of those factors drive the identity of motoneurons.

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.

Hypoexcitability precedes denervation in the large fast-contracting motor units in two unrelated mouse models of ALS

Hyperexcitability has been suggested to contribute to motoneuron degeneration in amyotrophic lateral sclerosis (ALS). If this is so, and given that the physiological type of a motor unit determines the relative susceptibility of its motoneuron in ALS, then one would expect the most vulnerable motoneurons to display the strongest hyperexcitability prior to their degeneration, whereas the less vulnerable should display a moderate hyperexcitability, if any. We tested this hypothesis in vivo in two unrelated ALS mouse models by correlating the electrical properties of motoneurons with their physiological types, identified based on their motor unit contractile properties. We found that, far from being hyperexcitable, the most vulnerable motoneurons become unable to fire repetitively despite the fact that their neuromuscular junctions were still functional. Disease markers confirm that this loss of function is an early sign of degeneration. Our results indicate that intrinsic hyperexcitability is unlikely to be the cause of motoneuron degeneration.

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, is a fatal disorder of the nervous system. Early symptoms include muscle weakness, unsteadiness and slurred speech. These symptoms arise because the neurons that control muscles – the motoneurons – lose their ability to make the muscles contract. Eventually, the muscles become paralyzed, with more and more muscles affected over time. Most patients die within a few years of diagnosis when the disease destroys the muscles that control breathing.

Muscles are made up of muscle fibers. Each motoneuron controls a bundle of muscle fibers, and the motoneuron and its muscle fibers together make up a motor unit. A single muscle contains hundreds of motor units. These consist of several different types, which differ in how many muscle fibers they contain, how fast those muscle fibers can contract, and how fatigable the muscle fibers are. In ALS, motoneurons become detached from their muscle fibers, causing motor units to break down. But what triggers this process? One long-standing idea is that motoneurons begin to respond excessively to commands from the brain and spinal cord. In other words, they become hyperexcitable, which ultimately leads to their death.

But some more recent studies of ALS suggest the opposite, namely that motoneurons become less active, or hypoexcitable. To distinguish between these possibilities, Martinez-Silva et al. took advantage of the fact that different types of motor unit break down at different rates in ALS. Large motor units containing fast-contracting muscle fibers break down before smaller motor units. By measuring the activity of motor units in two mouse models of ALS, Martinez-Silva et al. showed that large motoneurons are hypoexcitable. In other words, the motoneurons that are most vulnerable to ALS respond too little to commands from the nervous system, rather than too much.

Studies of specific proteins inside the cells confirmed that hypoexcitable motoneurons are further along in the disease process than other motoneurons. Hypoexcitability is thus a key player in the ALS disease process. Developing drugs to target this hypoexcitability may be a promising strategy for the future of this condition.

Neuronal Population Activity in Spinal Motor Circuits: Greater Than the Sum of Its Parts

The core elements of stereotypical movements such as locomotion, scratching and breathing are generated by networks in the lower brainstem and the spinal cord. Ensemble activities in spinal motor networks had until recently been merely a black box, but with the emergence of ultra-thin Silicon multi-electrode technology it was possible to reveal the spiking activity of larger parts of the network. A series of experiments revealed unexpected features of spinal networks, such as multiple spiking regimes and lognormal firing rate distributions. The lognormality renders the widespread idea of a typical firing rate ± standard deviation an ill-suited description, and therefore these findings define a new arithmetic of motor networks. Focusing on the population activity behind motor pattern generation this review summarizes this advance and discusses its implications.

Midbrain stimulation-evoked lumbar spinal activity in the adult decerebrate mouse

BACKGROUND

Genetic techniques rendering murine models a popular choice for neuroscience research has led to important insights on neural networks controlling locomotor function. Using genetically altered mouse models for in vivo, electrophysiological studies in the adult state could validate key principles of locomotor network organization that have been described in neonatal, in vitro preparations.

NEW METHOD

The experimental model presented here describes a decerebrate, in vivo adult mouse preparation in which focal, electrical midbrain stimulation was combined with monitoring lumbar neural activity and motor output after pre-collicular decerebration and neuromuscular blockade.

RESULTS

Lumbar cord dorsum potentials (in 9/10 animals) and motoneuron output (in 3/5 animals) including fictive locomotion, was achieved by focal midbrain stimulation. The stimulation electrode locations could be reconstructed (in 6/7 animals) thereby allowing anatomical identification of the stimulated supraspinal regions.

COMPARISON WITH EXISTING METHODS

This preparation allows for concomitant recording or stimulation in the spinal cord and in the mid/hindbrain of adult mice. It differs from other methods used in the past with adult mice as it does not require pharmacological manipulation of neural excitability in order to generate motor output.

CONCLUSIONS

Midbrain stimulation can consistently be used for inducing lumbar neural activity in adult mice under neuromuscular blockade. This model is suited for examination of brain-spinal connectivity and it may benefit a wide range of fields depending on the features of the genetically modified mouse models used in combination with the presented methods.

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

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

PICs in motoneurons do not scale with the size of the animal: a possible mechanism for faster speed of muscle contraction in smaller species

The majority of studies on the electrical properties of neurons are carried out in rodents, and in particular in mice. However, the minute size of this animal compared with humans potentially limits the relevance of the resulting insights. To be able to extrapolate results obtained in a small animal such as a rodent, one needs to have proper knowledge of the rules governing how electrical properties of neurons scale with the size of the animal. Generally speaking, electrical resistances of neurons increase as cell size decreases, and thus maintenance of equal depolarization across cells of different sizes requires the underlying currents to decrease in proportion to the size decrease. Thus it would generally be expected that voltage-sensitive currents are smaller in smaller animals. In this study, we used in vivo preparations to record electrical properties of spinal motoneurons in deeply anesthetized adult mice and cats. We found that PICs do not scale with size, but instead are constant in their amplitudes across these species. This constancy, coupled with the threefold differences in electrical resistances, means that PICs contribute a threefold larger depolarization in the mouse than in the cat. As a consequence, motoneuronal firing rate sharply increases as animal size decreases. These differences in firing rates are likely essential in allowing different species to control muscles with widely different contraction speeds (smaller animals have faster muscle fibers). Thus from our results we have identified a possible new mechanism for how electrical properties are tuned to match mechanical properties within the motor output system.NEW & NOTEWORTHY The small size of the mouse warrants concern over whether the properties of their neurons are a scaled version of those in larger animals or instead have unique features. Comparison of spinal motoneurons in mice to cats showed unique features. Firing rates in the mouse were much higher, in large part due to relatively larger persistent inward currents. These differences likely reflect adaptations for controlling much faster muscle fibers in mouse than cat.

Potassium channel abnormalities are consistent with early axon degeneration of motor axons in the G127X SOD1 mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disease, which selectively affects upper and lower motoneurones. The underlying pathophysiology of the disease is complex but electrophysiological studies of peripheral nerves in ALS patients as well as human autopsy studies indicate that a potassium channel dysfunction/loss is present early in the symptomatic phase. It remains unclear to what extent potassium channel abnormalities reflect a specific pathogenic mechanism in ALS. The aim of this study was therefore to investigate the temporal changes in the expression and/or function of potassium channels in motoneurones in the adult G127X SOD1 mouse model of ALS, a model which has a very long presymptomatic phase. Evidence from animal models indicates that the early progressive motoneurone dysfunction and degeneration can be largely compensated by motor unit remodeling, delaying the clinical symptom onset. Experiments were therefore performed both before and after symptom onset. Immunohistochemistry of motor axons in the ventral roots of G127X SOD1 mice, was used to investigate juxta-paranodal Kv1.2 potassium channels along with nodal Nav1.6 and the paranodal scaffolding protein Caspr. This allowed an investigation of changes in the distribution of Kv1.2 relative to the general structure of the nodal-paranodal-juxta-paranodal complex. This revealed that the motor axons in the ventral roots of presymptomatic G127X SOD1 mice, already show a disruption in juxta-paranodal Kv1.2 potassium channels. The axonal Kv1.2 disruption was preceded by abnormalities in the distribution of the paranodal scaffolding protein Caspr with the nodal arrangement of Nav1.6 appearing relatively preserved even in symptomatic mice. These changes were accompanied by axon swelling and a slowing of conduction in the peripheral motor axons in symptomatic mice. In vivo electrophysiological intracellular recordings of individual spinal motoneurones revealed that central potassium channel function was preserved or even enhanced with higher amplitude and longer duration after-hyperpolarisations in the G127X SOD1 mice. Our data suggest that the potassium channel abnormalities observed in presymptomatic G127X, rather than representing a specific pathophysiological mechanism targeting potassium channels, most likely reflect early axonal degenerative changes, consistent with the "dying-back" phenomenon observed in other ALS models.

Decerebrate mouse model for studies of the spinal cord circuits

The adult decerebrate mouse model (a mouse with the cerebrum removed) enables the study of sensory-motor integration and motor output from the spinal cord for several hours without compromising these functions with anesthesia. For example, the decerebrate mouse is ideal for examining locomotor behavior using intracellular recording approaches, which would not be possible using current anesthetized preparations. This protocol describes the steps required to achieve a low-blood-loss decerebration in the mouse and approaches for recording signals from spinal cord neurons with a focus on motoneurons. The protocol also describes an example application for the protocol: the evocation of spontaneous and actively driven stepping, including optimization of these behaviors in decerebrate mice. The time taken to prepare the animal and perform a decerebration takes ∼2 h, and the mice are viable for up to 3-8 h, which is ample time to perform most short-term procedures. These protocols can be modified for those interested in cardiovascular or respiratory function in addition to motor function and can be performed by trainees with some previous experience in animal surgery.

Gamma motor neurons survive and exacerbate alpha motor neuron degeneration in ALS

The molecular and cellular basis of selective motor neuron (MN) vulnerability in amyotrophic lateral sclerosis (ALS) is not known. In genetically distinct mouse models of familial ALS expressing mutant superoxide dismutase-1 (SOD1), TAR DNA-binding protein 43 (TDP-43), and fused in sarcoma (FUS), we demonstrate selective degeneration of alpha MNs (α-MNs) and complete sparing of gamma MNs (γ-MNs), which selectively innervate muscle spindles. Resistant γ-MNs are distinct from vulnerable α-MNs in that they lack synaptic contacts from primary afferent (I_A) fibers. Elimination of these synapses protects α-MNs in the SOD1 mutant, implicating this excitatory input in MN degeneration. Moreover, reduced I_A activation by targeted reduction of γ-MNs in SOD1^G93A mutants delays symptom onset and prolongs lifespan, demonstrating a pathogenic role of surviving γ-MNs in ALS. This study establishes the resistance of γ-MNs as a general feature of ALS mouse models and demonstrates that synaptic excitation of MNs within a complex circuit is an important determinant of relative vulnerability in ALS.

Development of Electrophysiological Properties of Nucleus Gigantocellularis Neurons Correlated with Increased CNS Arousal

Many types of data have suggested that neurons in the nucleus gigantocellularis (NGC) in the medullary reticular formation are critically important for CNS arousal and behavioral responsiveness. To extend this topic to a developmental framework, whole-cell patch-recorded characteristics of NGC neurons in brainstem slices and measures of arousal-dependent locomotion of postnatal day 3 (P3) to P6 mouse pups were measured and compared. These neuronal characteristics developed in an orderly, statistically significant monotonic manner over the course of P3-P6: (1) proportion of neurons capable of firing action potential (AP) trains, (2) AP amplitude, (3) AP threshold, (4) amplitude of inward and outward currents, (5) amplitude of negative peak currents, and (6) steady state currents (in I-V plot). These measurements reflect the maturation of sodium and certain potassium channels. Similarly, all measures of locomotion, latency to first movement, total locomotion duration, net locomotion distance, and total quiescence time also developed monotonically over P3-P6. Most importantly, electrophysiological and behavioral measures were significantly correlated. Interestingly, the behavioral measures were not correlated with frequency of excitatory postsynaptic currents or the proportion of neurons showing these currents, responses to a battery of neurotransmitter agents, or rapid activating potassium currents (including IA). Considering the results here in the context of a large body of literature on NGC, we hypothesize that the developmental increase in NGC neuronal excitability participates in causing the increased behavioral responsivity during the postnatal period from P3 to P6.

Lognormal firing rate distribution reveals prominent fluctuation-driven regime in spinal motor networks

When spinal circuits generate rhythmic movements it is important that the neuronal activity remains within stable bounds to avoid saturation and to preserve responsiveness. Here, we simultaneously record from hundreds of neurons in lumbar spinal circuits of turtles and establish the neuronal fraction that operates within either a 'mean-driven' or a 'fluctuation-driven' regime. Fluctuation-driven neurons have a 'supralinear' input-output curve, which enhances sensitivity, whereas the mean-driven regime reduces sensitivity. We find a rich diversity of firing rates across the neuronal population as reflected in a lognormal distribution and demonstrate that half of the neurons spend at least 50 % of the time in the 'fluctuation-driven' regime regardless of behavior. Because of the disparity in input-output properties for these two regimes, this fraction may reflect a fine trade-off between stability and sensitivity in order to maintain flexibility across behaviors.

Potassium currents dynamically set the recruitment and firing properties of F-type motoneurons in neonatal mice

In neonatal mice, fast- and slow-type motoneurons display different patterns of discharge. In response to a long liminal current pulse, the discharge is delayed up to several seconds in fast-type motoneurons and their firing frequency accelerates. In contrast, slow-type motoneurons discharge immediately, and their firing frequency decreases at the beginning of the pulse. Here, we identify the ionic currents that underlie the delayed firing of fast-type motoneurons. We find that the firing delay is caused by a combination of an A-like potassium current that transiently suppresses firing on a short time scale and a slowly-inactivating potassium current that inhibits the discharge over a much longer time scale. We then show how these intrinsic currents dynamically shape the discharge threshold and the frequency-input function of fast-type motoneurons. These currents contribute to the orderly recruitment of motoneurons in neonates and might play a role in the postnatal maturation of motor units.

Postactivation depression of the Ia EPSP in motoneurons is reduced in both the G127X SOD1 model of amyotrophic lateral sclerosis and in aged mice

Postactivation depression (PActD) of Ia afferent excitatory postsynaptic potentials (EPSPs) in spinal motoneurons results in a long-lasting depression of the stretch reflex. This phenomenon (PActD) is of clinical interest as it has been shown to be reduced in a number of spastic disorders. Using in vivo intracellular recordings of Ia EPSPs in adult mice, we demonstrate that PActD in adult (100-220 days old) C57BL/6J mice is both qualitatively and quantitatively similar to that which has been observed in larger animals with respect to both the magnitude (with ∼20% depression of EPSPs at 0.5 ms after a train of stimuli) and the time course (returning to almost normal amplitudes by 5 ms after the train). This validates the use of mouse models to study PActD. Changes in such excitatory inputs to spinal motoneurons may have important implications for hyperreflexia and/or glutamate-induced excitotoxicity in the neurodegenerative disease amyotrophic lateral sclerosis (ALS). With the use of the G127X SOD1 mutant mouse, an ALS model with a prolonged asymptomatic phase and fulminant symptom onset, we observed that PActD is significantly reduced at both presymptomatic (16% depression) and symptomatic (17.3% depression) time points compared with aged-matched controls (22.4% depression). The PActD reduction was not markedly altered by symptom onset. Comparing these PActD changes at the EPSP with the known effect of the depression on the monosynaptic reflex, we conclude that this is likely to have a much larger effect on the reflex itself (a 20-40% difference). Nevertheless, it should also be accounted that in aged (580 day old) C57BL/6J mice there was also a reduction in PActD although, aging is not usually associated with spasticity.

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.

Neuromodulation impact on nonlinear firing behavior of a reduced model motoneuron with the active dendrite

Neuromodulatory inputs from brainstem systems modulate the normal function of spinal motoneurons by altering the activation properties of persistent inward currents (PICs) in their dendrites. However, the effect of the PIC on firing outputs also depends on its location in the dendritic tree. To investigate the interaction between PIC neuromodulation and PIC location dependence, we used a two-compartment model that was biologically realistic in that it retains directional and frequency-dependent electrical coupling between the soma and the dendrites, as seen in multi-compartment models based on full anatomical reconstructions of motoneurons. Our two-compartment approach allowed us to systematically vary the coupling parameters between the soma and the dendrite to accurately reproduce the effect of location of the dendritic PIC on the generation of nonlinear (hysteretic) motoneuron firing patterns. Our results show that as a single parameter value for PIC activation was either increased or decreased by 20% from its default value, the solution space of the coupling parameter values for nonlinear firing outputs was drastically reduced by approximately 80%. As a result, the model tended to fire only in a linear mode at the majority of dendritic PIC sites. The same results were obtained when all parameters for the PIC activation simultaneously changed only by approximately ±10%. Our results suggest the democratization effect of neuromodulation: the neuromodulation by the brainstem systems may play a role in switching the motoneurons with PICs at different dendritic locations to a similar mode of firing by reducing the effect of the dendritic location of PICs on the firing behavior.

Asymmetry in signal propagation between the soma and dendrites plays a key role in determining dendritic excitability in motoneurons

It is widely recognized that propagation of electrophysiological signals between the soma and dendrites of neurons differs depending on direction, i.e. it is asymmetric. How this asymmetry influences the activation of voltage-gated dendritic channels, and consequent neuronal behavior, remains unclear. Based on the analysis of asymmetry in several types of motoneurons, we extended our previous methodology for reducing a fully reconstructed motoneuron model to a two-compartment representation that preserved asymmetric signal propagation. The reduced models accurately replicated the dendritic excitability and the dynamics of the anatomical model involving a persistent inward current (PIC) dispersed over the dendrites. The relationship between asymmetric signal propagation and dendritic excitability was investigated using the reduced models while varying the asymmetry in signal propagation between the soma and the dendrite with PIC density constant. We found that increases in signal attenuation from soma to dendrites increased the activation threshold of a PIC (hypo-excitability), whereas increases in signal attenuation from dendrites to soma decreased the activation threshold of a PIC (hyper-excitability). These effects were so strong that reversing the asymmetry in the soma-to-dendrite vs. dendrite-to-soma attenuation, reversed the correlation between PIC threshold and distance of this current source from the soma. We propose the tight relation of the asymmetric signal propagation to the input resistance in the dendrites as a mechanism underlying the influence of the asymmetric signal propagation on the dendritic excitability. All these results emphasize the importance of maintaining the physiological asymmetry in dendritic signaling not only for normal function of the cells but also for biophysically realistic simulations of dendritic excitability.

Spike generation estimated from stationary spike trains in a variety of neurons in vivo

To any model of brain function, the variability of neuronal spike firing is a problem that needs to be taken into account. Whereas the synaptic integration can be described in terms of the original Hodgkin-Huxley (H-H) formulations of conductance-based electrical signaling, the transformation of the resulting membrane potential into patterns of spike output is subjected to stochasticity that may not be captured with standard single neuron H-H models. The dynamics of the spike output is dependent on the normal background synaptic noise present in vivo, but the neuronal spike firing variability in vivo is not well studied. In the present study, we made long-term whole cell patch clamp recordings of stationary spike firing states across a range of membrane potentials from a variety of subcortical neurons in the non-anesthetized, decerebrated state in vivo. Based on the data, we formulated a simple, phenomenological model of the properties of the spike generation in each neuron that accurately captured the stationary spike firing statistics across all membrane potentials. The model consists of a parametric relationship between the mean and standard deviation of the inter-spike intervals, where the parameter is linearly related to the injected current over the membrane. This enabled it to generate accurate approximations of spike firing also under inhomogeneous conditions with input that varies over time. The parameters describing the spike firing statistics for different neuron types overlapped extensively, suggesting that the spike generation had similar properties across neurons.

Characterization of motor units in behaving adult mice shows a wide primary range

The mouse is essential for genetic studies of motor function in both normal and pathological states. Thus it is important to consider whether the structure of motor output from the mouse is in fact analogous to that recorded in other animals. There is a striking difference in the basic electrical properties of mouse motoneurons compared with those in rats, cats, and humans. The firing evoked by injected currents produces a unique frequency-current (F-I) function that emphasizes recruitment of motor units at their maximum force. These F-I functions, however, were measured in anesthetized preparations that lacked two key components of normal synaptic input: high levels of synaptic noise and neuromodulatory inputs. Recent studies suggest that the alterations in the F-I function due to these two components are essential for recreating firing behavior of motor units in human subjects. In this study we provide the first data on firing patterns of motor units in the awake mouse, focusing on steady output in quiet stance. The resulting firing patterns did not match the predictions from the mouse F-I behaviors but instead revealed rate modulation across a remarkably wide range (10-60 Hz). The low end of the firing range may be due to changes in the F-I relation induced by synaptic noise and neuromodulatory inputs. The high end of the range may indicate that, unlike other species, quiet standing in the mouse involves recruitment of relatively fast-twitch motor units.

Motor unit

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

Concomitant changes in afterhyperpolarization and twitch following repetitive stimulation of fast motoneurones and motor units

The study aimed at determining changes in a course of motoneuronal afterhyperpolarization (AHP) and in contractile twitches of motor units (MUs) during activity evoked by increasing number of stimuli (from 1 to 5), at short interspike intervals (5 ms). The stimulation was applied antidromically to spinal motoneurones or to isolated axons of MUs of the medial gastrocnemius muscle within two separate series of experiments on anesthetized rats. Alterations in the amplitude and time parameters of the AHP of successive spikes were compared to changes in force and time course of successive twitches obtained by mathematical subtraction of tetanic contractions evoked by one to five stimuli. The extent of changes of the studied parameters depended on a number of applied stimuli. The maximal modulation of the AHP and twitch parameters (a prolongation and an increase in the AHP and twitch amplitudes) was typically observed after the second pulse, while higher number of pulses at the same frequency did not induce so prominent changes. One may conclude that changes observed in parameters of action potentials of motoneurons are concomitant to changes in contractile properties of MU twitches. This suggests that both modulations of the AHP and twitch parameters reflect mechanisms leading to force development at the beginning of MU activity.

Selective mitochondrial Ca2+ uptake deficit in disease endstage vulnerable motoneurons of the SOD1G93A mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a progressive neurodegenerative disease that targets some somatic motoneuron populations, while others, e.g. those of the oculomotor system, are spared. The pathophysiological basis of this pattern of differential vulnerability, which is preserved in a transgenic mouse model of amyotrophic lateral sclerosis (SOD1(G93A)), and the mechanism of neurodegeneration in general are unknown. Hyperexcitability and calcium dysregulation have been proposed by others on the basis of data from juvenile mice that are, however, asymptomatic. No studies have been done with symptomatic mice following disease progression to the disease endstage. Here, we developed a new brainstem slice preparation for whole-cell patch-clamp recordings and single cell fura-2 calcium imaging to study motoneurons in adult wild-type and SOD1(G93A) mice up to disease endstage. We analysed disease-stage-dependent electrophysiological properties and intracellular Ca(2+) handling of vulnerable hypoglossal motoneurons in comparison to resistant oculomotor neurons. Thereby, we identified a transient hyperexcitability in presymptomatic but not in endstage vulnerable motoneurons. Additionally, we revealed a remodelling of intracellular Ca(2+) clearance within vulnerable but not resistant motoneurons at disease endstage characterised by a reduction of uniporter-dependent mitochondrial Ca(2+) uptake and enhanced Ca(2+) extrusion across the plasma membrane. Our study challenged the notion that hyperexcitability is a direct cause of neurodegeneration in SOD1(G93A) mice, but molecularly identified a Ca(2+) clearance deficit in motoneurons and an adaptive Ca(2+) handling strategy that might be targeted by future therapeutic strategies.

Reflex transmission to lumbar α-motoneurones in the mouse similar and different to those in the cat

Investigation and interpretation of defective motor circuitries in transgenic mice required further basic results from wild-type mice. Therefore, we investigated the lumbar motor reflex pattern in anaesthetised mice using intracellular motoneuronal recording and monosynaptic reflex testing. Thresholds and latencies in mice were similar to those in cats: thresholds for monosynaptic (group I) EPSPs were slightly above 1T (T=threshold for the lowest threshold fibres), around 1.5T for group II EPSPs and above 10T for group III EPSPs; group I EPSPs were maximal with a stimulus strength around 2T, group II EPSPs were maximal with 5-8T; latencies to the group I incoming volley were below 1ms for monosynaptic group I EPSPs, around 3ms for polysynaptic group II EPSPs and above 4ms for polysynaptic group III EPSPs. In contrast to reflex actions in the cat, monosynaptic gastrocnemius-soleus reflexes were facilitated by conditioning stimulation of the peroneal, sural and tibial nerves, i.e. by a variety of different, probably flexor reflex afferents. This facilitation persisted after high lumbar spinalisation indicating an independency to supraspinal influences. Nociceptive muscle afferents facilitated the peroneal monosynaptic reflex while nociceptive cutaneous afferents from the foot sole inhibited the ipsilateral but facilitated the contralateral peroneal reflex.

Simultaneous intracellular recording of a lumbar motoneuron and the force produced by its motor unit in the adult mouse in vivo

The spinal motoneuron has long been a good model system for studying neural function because it is a neuron of the central nervous system with the unique properties of (1) having readily identifiable targets (the muscle fibers) and therefore having a very well-known function (to control muscle contraction); (2) being the convergent target of many spinal and descending networks, hence the name of "final common pathway"; and (3) having a large soma which makes it possible to penetrate them with sharp intracellular electrodes. Furthermore, when studied in vivo, it is possible to record simultaneously the electrical activity of the motoneurons and the force developed by their muscle targets. Performing intracellular recordings of motoneurons in vivo therefore put the experimentalist in the unique position of being able to study, at the same time, all the compartments of the "motor unit" (the name given to the motoneuron, its axon, and the muscle fibers it innervates(1)): the inputs impinging on the motoneuron, the electrophysiological properties of the motoneuron, and the impact of these properties on the physiological function of the motoneurons, i.e. the force produced by its motor unit. However, this approach is very challenging because the preparation cannot be paralyzed and thus the mechanical stability for the intracellular recording is reduced. Thus, this kind of experiments has only been achieved in cats and in rats. However, the study of spinal motor systems could make a formidable leap if it was possible to perform similar experiments in normal and genetically modified mice. For technical reasons, the study of the spinal networks in mice has mostly been limited to neonatal in vitro preparations, where the motoneurons and the spinal networks are immature, the motoneurons are separated from their targets, and when studied in slices, the motoneurons are separated from most of their inputs. Until recently, only a few groups had managed to perform intracellular recordings of motoneurons in vivo(2-4 ), including our team who published a new preparation which allowed us to obtain very stable recordings of motoneurons in vivo in adult mice(5,6). However, these recordings were obtained in paralyzed animals, i.e. without the possibility to record the force output of these motoneurons. Here we present an extension of this original preparation in which we were able to obtain simultaneous recordings of the electrophysiological properties of the motoneurons and of the force developed by their motor unit. This is an important achievement, as it allows us to identify the different types of motoneurons based on their force profile, and thereby revealing their function. Coupled with genetic models disturbing spinal segmental circuitry(7-9), or reproducting human disease(10,11), we expect this technique to be an essential tool for the study of spinal motor system.

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.

Alpha, beta and gamma motoneurons: functional diversity in the motor system's final pathway

Since their discovery in the late 19th century our conception of motoneurons has steadily evolved. Motoneurons share the same general function: they drive the contraction of muscle fibers and are the final common pathway, i.e., the seat of convergence of all the central and peripheral pathways involved in motricity. However, motoneurons innervate different types of muscular targets. Ordinary muscle fibers are subdivided into three main subtypes according to their structural and mechanical properties. Intrafusal muscle fibers located within spindles can elicit either a dynamic, or a static, action on the spindle sensory endings. No less than seven categories of motoneurons have thereby been identified on the basis of their innervation pattern. This functional diversity has hinted at a similar diversity in the inputs each motoneuron receives, as well as in the electrical, or cellular, properties of the motoneurons that match the properties of their muscle targets. The notion of the diverse properties of motoneurons has been well established by the work of many prominent neuroscientists. But in today's scientific literature, it tends to fade and motoneurons are often thought of as a homogenous group, which develop from a given population of precursor cells, and which express a common set of molecules. We first present here the historical milestones that led to the recognition of the functional diversity of motoneurons. We then review how the intrinsic electrical properties of motoneurons are precisely tuned in each category of motoneurons in order to produce an output that is adapted to the contractile properties of their specific targets.

Adult mouse motor units develop almost all of their force in the subprimary range: a new all-or-none strategy for force recruitment?

Classical studies of the mammalian neuromuscular system have shown an impressive adaptation match between the intrinsic properties of motoneurons and the contractile properties of their motor units. In these studies, the rate at which motoneurons start to fire repetitively corresponds to the rate at which individual twitches start to sum, and the firing rate increases linearly with the amount of excitation ("primary range") up to the point where the motor unit develops its maximal force. This allows for the gradation of the force produced by a motor unit by rate modulation. In adult mouse motoneurons, however, we recently described a regime of firing ("subprimary range") that appears at lower excitation than what is required for the primary range, a finding that might challenge the classical conception. To investigate the force production of mouse motor units, we simultaneously recorded, for the first time, the motoneuron discharge elicited by intracellular ramps of current and the force developed by its motor unit. We showed that the motor unit developed nearly its maximal force during the subprimary range. This was found to be the case regardless of the input resistance of the motoneuron, the contraction speed, or the tetanic force of the motor unit. Our work suggests that force modulation in small mammals mainly relies on the number of motor units that are recruited rather than on rate modulation of individual motor units.

Fictive locomotion in the adult decerebrate and spinal mouse in vivo

Recently, transgenic mice have been created with mutations affecting the components of the mammalian spinal central pattern generator (CPG) for locomotion; however, it has currently only been possible to evoke fictive locomotion in mice, using neonatal in vitro preparations. Here, we demonstrate that it is possible to evoke fictive locomotion in the adult decerebrate mouse in vivo using l-3,4-dihydroxyphenylalanine methyl ester hydrochloride (l-DOPA) and 5-hydroxytryptophan (5HTP) following injection of the monoaminoxiadase inhibitor Nialamide. We investigate the effects of afferent stimulation and spinalization as well as demonstrate the possibility of simultaneous intracellular recording of rhythmically active motoneurones. Our results demonstrate that several features of the mouse locomotor CPG are similar to those that have been observed in rat, cat, rabbit and monkey suggesting a fairly conserved organisation and allowing for future results in transgenic mice to be extrapolated to existing knowledge of CPG components and circuitry obtained in larger species.

An in vitro spinal cord slice preparation for recording from lumbar motoneurons of the adult mouse

The development of central nervous system slice preparations for electrophysiological studies has led to an explosion of knowledge of neuronal properties in health and disease. Studies of spinal motoneurons in these preparations, however, have been largely limited to the early postnatal period, as adult motoneurons are vulnerable to the insults sustained by the preparation. We therefore sought to develop an adult spinal cord slice preparation that permits recording from lumbar motoneurons. To accomplish this, we empirically optimized the composition of solutions used during preparation in order to limit energy failure, reduce harmful ionic fluxes, mitigate oxidative stress, and prevent excitotoxic cell death. In addition to other additives, this involved the use of ethyl pyruvate, which serves as an effective nutrient and antioxidant. We also optimized and incorporated a host of previously published modifications used for other in vitro preparations, such as the use of polyethylene glycol. We provide an in-depth description of the preparation protocol and discuss the rationale underlying each modification. By using this protocol, we obtained stable whole cell patch-clamp recordings from identified fluorescent protein-labeled motoneurons in adult slices; here, we describe the firing properties of these adult motoneurons. We propose that this preparation will allow further studies of how motoneurons integrate activity to produce adult motor behaviors and how pathological processes such as amyotrophic lateral sclerosis affect these neurons.

A decerebrate adult mouse model for examining the sensorimotor control of locomotion

As wild-type and genetically modified mice are progressively becoming the predominant models for studying locomotor physiology, the technical ability to record sensory and motor components from adult mice, in vivo, are expected to contribute to a better understanding of sensorimotor spinal cord networks. Here, specific technical and surgical details are presented on how to produce an adult decerebrate mouse preparation that can reliably produce sustained bouts of stepping, in vivo, in the absence of anesthetic drugs. Data are presented demonstrating the ability of this preparation to produce stepping during treadmill locomotion, adaptability in its responses to changes in the treadmill speed, and left-right alternation. Furthermore, intracellular recordings from motoneurons and interneurons in the spinal cord are presented from preparations where muscle activity was blocked. Intraaxonal recordings are also presented demonstrating that individual afferents can be recorded using this preparation. These data demonstrate that the adult decerebrate mouse is a tractable preparation for the study of sensorimotor systems.

Characterization of the axon initial segment (AIS) of motor neurons and identification of a para-AIS and a juxtapara-AIS, organized by protein 4.1B

Background

The axon initial segment (AIS) plays a crucial role: it is the site where neurons initiate their electrical outputs. Its composition in terms of voltage-gated sodium (Nav) and voltage-gated potassium (Kv) channels, as well as its length and localization determine the neuron's spiking properties. Some neurons are able to modulate their AIS length or distance from the soma in order to adapt their excitability properties to their activity level. It is therefore crucial to characterize all these parameters and determine where the myelin sheath begins in order to assess a neuron's excitability properties and ability to display such plasticity mechanisms. If the myelin sheath starts immediately after the AIS, another question then arises as to how would the axon be organized at its first myelin attachment site; since AISs are different from nodes of Ranvier, would this particular axonal region resemble a hemi-node of Ranvier?

Results

We have characterized the AIS of mouse somatic motor neurons. In addition to constant determinants of excitability properties, we found heterogeneities, in terms of AIS localization and Nav composition. We also identified in all α motor neurons a hemi-node-type organization, with a contactin-associated protein (Caspr)⁺ paranode-type, as well as a Caspr2⁺ and Kv1⁺ juxtaparanode-type compartment, referred to as a para-AIS and a juxtapara (JXP)-AIS, adjacent to the AIS, where the myelin sheath begins. We found that Kv1 channels appear in the AIS, para-AIS and JXP-AIS concomitantly with myelination and are progressively excluded from the para-AIS. Their expression in the AIS and JXP-AIS is independent from transient axonal glycoprotein-1 (TAG-1)/Caspr2, in contrast to juxtaparanodes, and independent from PSD-93. Data from mice lacking the cytoskeletal linker protein 4.1B show that this protein is necessary to form the Caspr⁺ para-AIS barrier, ensuring the compartmentalization of Kv1 channels and the segregation of the AIS, para-AIS and JXP-AIS.

Conclusions

α Motor neurons have heterogeneous AISs, which underlie different spiking properties. However, they all have a para-AIS and a JXP-AIS contiguous to their AIS, where the myelin sheath begins, which might limit some AIS plasticity. Protein 4.1B plays a key role in ensuring the proper molecular compartmentalization of this hemi-node-type region.

Excitatory projections from the amygdala to neurons in the nucleus pontis oralis in the rat: an intracellular study

There is a consensus that active (REM) sleep (AS) is controlled by cholinergic projections from the laterodorsal and pedunculopontine tegmental nuclei (LDT/PPT) to neurons in the nucleus pontis oralis (NPO) that generate AS (i.e. AS-Generator neurons). The present study was designed to provide evidence that other projections to the NPO, such as those from the amygdala, are also capable of inducing AS. Accordingly, the responses of neurons, recorded intracellularly in the NPO, were examined following stimulation of the ipsilateral central nucleus of the amygdala (CNA) in urethane-anesthetized rats. Single pulse stimulation in the CNA produced an early, fast depolarizing potential (EPSP) in neurons within the NPO. The mean latency to the onset of these excitatory postsynaptic potentials (EPSPs) was 3.6±0.2 ms. A late, small-amplitude inhibitory synaptic potential (IPSP) was present following EPSPs in a portion of the NPO neurons. Following stimulation of the CNA with a train of 8-10 pulses, NPO neurons exhibited a sustained depolarization (5-10 mV) of their resting membrane potential. When single subthreshold intracellular depolarizing current pulses were delivered to NPO neurons, CNA-induced EPSPs were sufficient to promote the discharge of these cells. Stimulation of the CNA with a short train of stimuli induced potent temporal facilitation of EPSPs in NPO neurons. Two forms of synaptic plasticity were revealed by the patterns of response of NPO neurons following stimulation of the CNA: paired-pulse facilitation (PPF) and post-tetanic potentiation (PTP). Six of recorded NPO neurons were identified morphologically with neurobiotin. They were medium to large, multipolar cells with diameters >20 μM, which resemble AS-on cells in the NPO. The present results demonstrate that amygdalar projections are capable of exerting a powerful excitatory postsynaptic drive that activates NPO neurons. Therefore, we suggest that the amygdala is capable of inducing AS via direct projections to AS-Generator neurons in the NPO.

Mixed mode oscillations in mouse spinal motoneurons arise from a low excitability state

We explain the mechanism that elicits the mixed mode oscillations (MMOs) and the subprimary firing range that we recently discovered in mouse spinal motoneurons. In this firing regime, high-frequency subthreshold oscillations appear a few millivolts below the spike voltage threshold and precede the firing of a full blown spike. By combining intracellular recordings in vivo (including dynamic clamp experiments) in mouse spinal motoneurons and modeling, we show that the subthreshold oscillations are due to the spike currents and that MMOs appear each time the membrane is in a low excitability state. Slow kinetic processes largely contribute to this low excitability. The clockwise hysteresis in the I-F relationship, frequently observed in mouse motoneurons, is mainly due to a substantial slow inactivation of the sodium current. As a consequence, less sodium current is available for spiking. This explains why a large subprimary range with numerous oscillations is present in motoneurons displaying a clockwise hysteresis. In motoneurons whose I-F curve exhibits a counterclockwise hysteresis, it is likely that the slow inactivation operates on a shorter time scale and is substantially reduced by the de-inactivating effect of the afterhyperpolarization (AHP) current, thus resulting in a more excitable state. This accounts for the short subprimary firing range with only a few MMOs seen in these motoneurons. Our study reveals a new role for the AHP current that sets the membrane excitability level by counteracting the slow inactivation of the sodium current and allows or precludes the appearance of MMOs.

Whither motoneurons?

In the preceding series of articles, the history of vertebrate motoneuron and motor unit neurobiological studies has been discussed. In this article, we select a few examples of recent advances in neuroscience and discuss their application or potential application to the study of motoneurons and the control of movement. We conclude, like Sherrington, that in order to understand normal, traumatized, and diseased human behavior, it is critical to continue to study motoneuron biology using all available and emerging tools. This article is part of a Special Issue entitled Historical Review.

Altered postnatal maturation of electrical properties in spinal motoneurons in a mouse model of amyotrophic lateral sclerosis

Spinal motoneurons are highly vulnerable in amyotrophic lateral sclerosis (ALS).Previous research using a standard animal model, the mutant superoxide dismutase-1 (SOD1)mouse, has revealed deficits in many cellular properties throughout its lifespan. The electrical properties underlying motoneuron excitability are some of the earliest to change; starting at 1 week postnatal, persistent inward currents (PICs) mediated by Na+ are upregulated and electrical conductance, a measure of cell size, increases. However, during this period these properties and many others undergo large developmental changes which have not been fully analysed.Therefore, we undertook a systematic analysis of electrical properties in more than 100 normal and mutant SOD1 motoneurons from 0 to 12 days postnatal, the neonatal to juvenile period.We compared normal mice with the most severe SOD1 model, the G93A high-expressor line. We found that the Na+ PIC and the conductance increased during development. However, mutant SOD1 motoneurons showed much greater increases than normal motoneurons; the mean Na+PIC in SOD1 motoneurons was double that of wild-type motoneurons. Additionally, in mutant SOD1 motoneurons the PIC mediated by Ca2+ increased, spike width decreased and the time course of the after-spike after-hyperpolarization shortened. These changes were advances of the normal effects of maturation. Thus, our results show that the development of normal and mutant SOD1 motoneurons follows generally similar patterns, but that the rate of development is accelerated in the mutant SOD1 motoneurons. Statistical analysis of all measured properties indicates that approximately 55% of changes attributed to the G93A SOD1 mutation can be attributed to an increased rate of maturation.

The role of propriospinal interneurons in recovery from spinal cord injury

Over one hundred years ago, Sir Charles Sherrington described a population of spinal cord interneurons (INs) that connect multiple spinal cord segments and participate in complex or 'long' motor reflexes. These neurons were subsequently termed propriospinal neurons (PNs) and are known to play a crucial role in motor control and sensory processing. Recent work has shown that PNs may also be an important substrate for recovery from spinal cord injury (SCI) as they contribute to plastic reorganisation of spinal circuits. The location, inter-segmental projection pattern and sheer number of PNs mean that after SCI, a significant number of them are capable of 'bridging' an incomplete spinal cord lesion. When these properties are combined with the capacity of PNs to activate and coordinate locomotor central pattern generators (CPGs), it is clear they are ideally placed to assist locomotor recovery. Here we summarise the anatomy, organisation and function of PNs in the uninjured spinal cord, briefly outline the pathophysiology of SCI, describe how PNs contribute to recovery of motor function, and finally, we discuss the mechanisms that underlie PN plasticity. We propose there are two major challenges for PN research. The first is to learn more about ways we can promote PN plasticity and manipulate the 'hostile' micro-environment that limits regeneration in the damaged spinal cord. The second is to study the cellular/intrinsic properties of PNs to better understand their function in both the normal and injured spinal cord. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.