Locomotion In Spinal Cord Research Articles

MRI Spinal Cord Reconstruction Provides Insights into Mapping and Migration Following Percutaneous Epidural Stimulation Implantation in Spinal Cord Injury

Background: Spinal cord epidural stimulation (SCES) has the potential to restore motor functions following spinal cord injury (SCI). Spinal cord mapping is a cornerstone step towards successfully configuring SCES to improve motor function, aiming to restore standing and stepping abilities in individuals with SCI. While some centers have advocated for the use of intraoperative mapping to anatomically target the spinal cord locomotor centers, this is a resource-intensive endeavor and may not be a feasible approach in all centers. Methods: Two participants underwent percutaneous SCES implantation as part of a clinical trial. Each participant underwent a temporary (1-week, two-lead) trial followed by a permanent, two-lead implantation. SCES configurations were matched between temporary and permanent mappings, and motor evoked potential in response to 2 Hz, for a duration of 250–1000 µs and with an amplitude of 1–14 mA, was measured using electromyography. T2 axial MRI images captured prior to implantation were used to retrospectively reconstruct the lumbosacral segments of the spinal cord. The effects of lead migration on mapping were further determined in one of the participants. Results: In both participants, there were recognized discrepancies in the recruitment curves of the motor evoked potentials across different muscle groups between temporary and permanent SCES mappings. These may be explained by retrospective MRI reconstruction of the spinal cord, which indicated that the percutaneous leads did not specifically target the entire L1-S2 segments in both participants. Minor lead migration appeared to have a minimal impact on spinal cord mapping outcomes in one of the participants but did dampen the motor activity of the hip and knee muscle groups. Conclusions: Temporary mapping coupled with MRI reconstruction has the potential to be considered as guidance for permanent implantation considering target activation of the spinal cord locomotor centers. Since lead migration may alter the synergistic coordination between different muscle groups and since lead migration of 1–2 contacts is expected and planned for in clinical practice, it can be better guided with proper spinal cord mapping and a diligent SCES lead trial beforehand.

Safety of mapping the motor networks in the spinal cord using penetrating microelectrodes in Yucatan minipigs.

The goal of this study was to assess the safety of mapping spinal cord locomotor networks using penetrating stimulation microelectrodes in Yucatan minipigs (YMPs) as a clinically translational animal model. Eleven YMPs were trained to walk up and down a straight line. Motion capture was performed, and electromyographic (EMG) activity of hindlimb muscles was recorded during overground walking. The YMPs underwent a laminectomy and durotomy to expose the lumbar spinal cord. Using an ultrasound-guided stereotaxic frame, microelectrodes were inserted into the spinal cord in 8 animals. Pial cuts were made to prevent tissue dimpling before microelectrode insertion. Different locations within the lumbar enlargement were electrically stimulated to map the locomotor networks. The remaining 3 YMPs served as sham controls, receiving the laminectomy, durotomy, and pial cuts but not microelectrode insertion. The Porcine Thoracic Injury Behavioral Scale (PTIBS) and hindlimb reflex assessment results were recorded for 4 weeks postoperatively. Overground gait kinematics and hindlimb EMG activity were recorded again at weeks 3 and 4 postoperatively and compared with preoperative measures. The animals were euthanized at the end of week 4, and the lumbar spinal cords were extracted and preserved for immunohistochemical analysis. All YMPs showed transient deficits in hindlimb function postoperatively. Except for 1 YMP in the experimental group, all animals regained normal ambulation and balance (PTIBS score 10) at the end of weeks 3 and 4. One animal in the experimental group showed gait and balance deficits by week 4 (PTIBS score 4). This animal was excluded from the kinematics and EMG analyses. Overground gait kinematic measures and EMG activity showed no significant (p > 0.05) differences between preoperative and postoperative values, and between the experimental and sham groups. Less than 5% of electrode tracks were visible in the tissue analysis of the animals in the experimental group. There was no statistically significant difference in damage caused by pial cuts between the experimental and sham groups. Tissue damage due to the pial cuts was more frequently observed in immunohistochemical analyses than microelectrode tracks. These findings suggest that mapping spinal locomotor networks in porcine models can be performed safely, without lasting damage to the spinal cord.

Decoding the Mouse Spinal Cord Locomotor Neural Network Using Tissue Clearing, Tissue Expansion and Tiling Light Sheet Microscopy Techniques

Decoding the Mouse Spinal Cord Locomotor Neural Network Using Tissue Clearing, Tissue Expansion and Tiling Light Sheet Microscopy Techniques

Two opposite voltage-dependent currents control the unusual early development pattern of embryonic Renshaw cell electrical activity.

Renshaw cells (V1R) are excitable as soon as they reach their final location next to the spinal motoneurons and are functionally heterogeneous. Using multiple experimental approaches, in combination with biophysical modeling and dynamical systems theory, we analyzed, for the first time, the mechanisms underlying the electrophysiological properties of V1R during early embryonic development of the mouse spinal cord locomotor networks (E11.5-E16.5). We found that these interneurons are subdivided into several functional clusters from E11.5 and then display an unexpected transitory involution process during which they lose their ability to sustain tonic firing. We demonstrated that the essential factor controlling the diversity of the discharge pattern of embryonic V1R is the ratio of a persistent sodium conductance to a delayed rectifier potassium conductance. Taken together, our results reveal how a simple mechanism, based on the synergy of two voltage-dependent conductances that are ubiquitous in neurons, can produce functional diversity in embryonic V1R and control their early developmental trajectory.

Characterization of Dmrt3-Derived Neurons Suggest a Role within Locomotor Circuits.

Neuronal networks within the spinal cord, collectively known as the central pattern generator (CPG), coordinate rhythmic movements underlying locomotion. The transcription factor doublesex and mab-3-related transcription factor 3 (DMRT3) is involved in the differentiation of the dorsal interneuron 6 class of spinal cord interneurons. In horses, a non-sense mutation in the Dmrt3 gene has major effects on gaiting ability, whereas mice lacking the Dmrt3 gene display impaired locomotor activity. Although the Dmrt3 gene is necessary for normal spinal network formation and function in mice, a direct role for Dmrt3-derived neurons in locomotor-related activities has not been demonstrated. Here we present the characteristics of the Dmrt3-derived spinal cord interneurons. Using transgenic mice of both sexes, we characterized interneurons labeled by their expression of Cre driven by the endogenous Dmrt3 promoter. We used molecular, retrograde tracing and electrophysiological techniques to examine the anatomical, morphological, and electrical properties of the Dmrt3-Cre neurons. We demonstrate that inhibitory Dmrt3-Cre neurons receive extensive synaptic inputs, innervate surrounding CPG neurons, intrinsically regulate CPG neuron's electrical activity, and are rhythmically active during fictive locomotion, bursting at frequencies independent to the ventral root output. The present study provides novel insights on the character of spinal Dmrt3-derived neurons, data demonstrating that these neurons participate in locomotor coordination.SIGNIFICANCE STATEMENT In this work, we provide evidence for a role of the Dmrt3 interneurons in spinal cord locomotor circuits as well as molecular and functional insights on the cellular and microcircuit level of the Dmrt3-expressing neurons in the spinal cord. Dmrt3 neurons provide the first example of an interneuron population displaying different oscillation frequencies. This study presents novel findings on an under-reported population of spinal cord neurons, which will aid in deciphering the locomotor network and will facilitate the design and development of therapeutics for spinal cord injury and motor disorders.

Retrogradely Transportable Lentivirus Tracers for Mapping Spinal Cord Locomotor Circuits.

Retrograde tracing is a key facet of neuroanatomical studies involving long distance projection neurons. Previous groups have utilized a variety of tools ranging from classical chemical tracers to newer methods employing viruses for gene delivery. Here, we highlight the usage of a lentivirus that permits highly efficient retrograde transport (HiRet) from synaptic terminals within the cervical and lumbar enlargements of the spinal cord. By injecting HiRet, we can clearly identify supraspinal and propriospinal circuits innervating motor neuron pools relating to forelimb and hindlimb function. We observed robust labeling of propriospinal neurons, including high fidelity details of dendritic arbors and axon terminals seldom seen with chemical tracers. In addition, we examine changes in interneuronal circuits occurring after a thoracic contusion, highlighting populations that potentially contribute to spontaneous behavioral recovery in this lesion model. Our study demonstrates that the HiRet lentivirus is a unique tool for examining neuronal circuitry within the brain and spinal cord.

Disruption of Locomotion in Response to Hindlimb Muscle Stretch at Acute and Chronic Time Points after a Spinal Cord Injury in Rats.

After spinal cord injury (SCI) muscle contractures develop in the plegic limbs of many patients. Physical therapists commonly use stretching as an approach to avoid contractures and to maintain the extensibility of soft tissues. We found previously that a daily stretching protocol has a negative effect on locomotor recovery in rats with mild thoracic SCI. The purpose of the current study was to determine the effects of stretching on locomotor function at acute and chronic time points after moderately severe contusive SCI. Female Sprague-Dawley rats with 25 g-cm T10 contusion injuries received our standard 24-min stretching protocol starting 4 days (acutely) or 10 weeks (chronically) post-injury (5 days/week for 5 or 4 weeks, respectively). Locomotor function was assessed using the BBB (Basso, Beattie, and Bresnahan) Open Field Locomotor Scale, video-based kinematics, and gait analysis. Locomotor deficits were evident in the acute animals after only 5 days of stretching and increasing the perceived intensity of stretching at week 4 resulted in greater impairment. Stretching initiated chronically resulted in dramatic decrements in locomotor function because most animals had BBB scores of 0-3 for weeks 2, 3, and 4 of stretching. Locomotor function recovered to control levels for both groups within 2 weeks once daily stretching ceased. Histological analysis revealed no apparent signs of overt and persistent damage to muscles undergoing stretching. The current study extends our observations of the stretching phenomenon to a more clinically relevant moderately severe SCI animal model. The results are in agreement with our previous findings and further demonstrate that spinal cord locomotor circuitry is especially vulnerable to the negative effects of stretching at chronic time points. While the clinical relevance of this phenomenon remains unknown, we speculate that stretching may contribute to the lack of locomotor recovery in some patients.

Molecular and cellular development of spinal cord locomotor circuitry.

The spinal cord of vertebrate animals is comprised of intrinsic circuits that are capable of sensing the environment and generating complex motor behaviors. There are two major perspectives for understanding the biology of this complicated structure. The first approaches the spinal cord from the point of view of function and is based on classic and ongoing research in electrophysiology, adult behavior, and spinal cord injury. The second view considers the spinal cord from a developmental perspective and is founded mostly on gene expression and gain-of-function and loss-of-function genetic experiments. Together these studies have uncovered functional classes of neurons and their lineage relationships. In this review, we summarize our knowledge of developmental classes, with an eye toward understanding the functional roles of each group.

A novel device for studying weight supported, quadrupedal overground locomotion in spinal cord injured rats

BackgroundProviding weight support facilitates locomotion in spinal cord injured animals. To control weight support, robotic systems have been developed for treadmill stepping and more recently for overground walking. New methodWe developed a novel device, the body weight supported ambulatory rodent trainer (i.e. BART). It has a small pneumatic cylinder that moves along a linear track above the rat. When air is supplied to the cylinder, the rats are lifted as they perform overground walking. We tested the BART device in rats that received a moderate spinal cord contusion injury and in normal rats. Locomotor training with the BART device was not performed. ResultsAll of the rats learned to walk in the BART device. In the contused rats, significantly greater paw dragging and dorsal stepping occurred in the hindlimbs compared to normal. Providing weight support significantly raised hip position and significantly reduced locomotor deficits. Hindlimb stepping was tightly coupled to forelimb stepping but only when the contused rats stepped without weight support. Three weeks after the contused rats received a complete spinal cord transection, significantly fewer hindlimb steps were performed. Comparison with existing methodsRelative to rodent robotic systems, the BART device is a simpler system for studying overground locomotion. The BART device lacks sophisticated control and sensing capability, but it can be assembled relatively easily and cheaply. ConclusionsThese findings suggest that the BART device is a useful tool for assessing quadrupedal, overground locomotion which is a more natural form of locomotion relative to treadmill locomotion.

Organization of left-right coordination of neuronal activity in the mammalian spinal cord locomotor CPG: insights from computational modeling

Different gaits of locomotion in mammals are based on the appropriate coordination of neuronal activity in the spinal cord controlling movements of left and right limbs. This left-right coordination is provided in the spinal cord by the commissural interneurons (CINs) whose axons cross the midline and affect neural circuits on the contralateral side of the cord. Several types of CINs have been genetically identified, including the excitatory V3 CINs and the inhibitory (V0D) and excitatory (V0V) V0 CINs. Talpalar et al. [1] recently demonstrated that (a) ablation of both V0 CIN types leads to a left-right synchronized, “hopping” activity at all locomotor frequencies, whereas (b) selective ablation of the excitatory V0V CINs maintains alternation at low frequencies but switches to synchronized activity at high frequencies while (c) ablation of only the inhibitory V0D CINs leads to a lack of left–right alternation at low frequencies, but maintains alternation at high frequencies. The genetically identified, ipsilaterally projecting excitatory V2a interneurons are recruited with an increase in locomotor speed [2] and contribute to left-right alternation at high locomotor frequencies [3,4]. Our objective was to construct and analyze a computational model of the bilaterally interacting central pattern generators (CPGs) that could reproduce and explain these findings. In our model, the CPG on each side of the cord consisted of flexor and extensor half-centers. Each neural population, including the half-centers and CIN populations, consisted of 50-200 neurons modeled in the Hodgkin-Huxley style. The intrinsic bursting of CPG neurons was based on a persistent sodium current in these neurons. During model construction, we assumed that left-right coordination of activity depends on the balance between the three CIN pathways providing interactions between the left and right CPGs: the V3-mediated pathway that supports left-right synchronization and the V0D- and V0V- mediated pathways that provide left-right alternation. The activity of each (left and right) inhibitory V0D population was driven by the ipsilateral flexor half-center. The recruitment of V0D neurons was progressively reduced with an increase in locomotor speed, because of the reduction of burst amplitude. The left and right V0V pathways could be organized in two ways: (1) the V0V activity on each side was driven by the ipsilateral flexor half-center and its action on contralateral circuits was mediated by an inhibitory population, or (2) the V0V activity was driven by the ipsilateral extensor half-center and it excited the contralateral circuits. In any case, the V0V activation was mediated by the ipsilateral V2a neurons progressively recruited with increasing locomotor speed. The model demonstrates: (1) a left-right alternating pattern under control conditions; (2) a synchronized hopping pattern at any frequency after removing both the V0 populations; (3) a synchronized pattern at low frequencies with alternation at high frequencies after removing the V0D populations; (4) an alternating pattern at low frequencies with synchronized hopping at high frequencies after removing either V0V or V2a populations. The model closely reproduces and suggests an explanation for the experimental data of Talpalar et al. [1], Zhong et al. [2], and Crone et al. [3,4], proposes the organization of commissural interactions in the spinal cord defining the left-right alternation at different locomotor speeds, and generates predictions for future experimental investigations.

Ephrin-B2 expression in the proprioceptive sensory system

Recent studies have shown that ephrin-B2 on sensory afferent fibers from the dorsal root ganglia (DRG) controls transmission of pain sensation to the spinal cord. We examined ephrin-B2 expression in mouse DRG and spinal cord using an ephrin-B2/ß-galactosidase chimeric allele. We found that ephrin-B2 is expressed exclusively in proprioceptive neurons and fibers in neonates, while expression in lamina III and IV of the adult spinal cord was observed in addition to that in the deeper laminae. We confirmed that ephrin-B2 protein causes co-clustering of EphB2 and glutamate receptors in spinal cord neurons. Our data are consistent with a role for ephrin-B2 in transmission of positional information to the CNS, and thus suggest a role in synaptic plasticity of spinal cord locomotor circuits that are known to be sensitive to proprioceptive sensory input after spinal cord injury.

Hindbrain V2a Neurons in the Excitation of Spinal Locomotor Circuits during Zebrafish Swimming

During locomotion in vertebrates, reticulospinal neurons in the hindbrain play critical roles in providing descending excitation to the spinal cord locomotor systems. However, despite the fact that many genes that are used to classify the neuronal identities of neurons in the hindbrain have been identified, the molecular identity of the reticulospinal neurons that are critically involved in locomotor drive is not well understood. Chx10-expressing neurons (V2a neurons) are ipsilaterally projecting glutamatergic neurons in the spinal cord and the hindbrain. Many of the V2a neurons in the hindbrain are known to project to the spinal cord in zebrafish, making hindbrain V2a neurons a prime candidate in descending locomotor drive. We investigated the roles of hindbrain V2a neurons using optogenetic and electrophysiological approaches. The forced activation of hindbrain V2a neurons using channelrhodopsin efficiently evoked swimming, whereas the forced inactivation of them using Archearhodopsin3 or Halorhodpsin reliably stopped ongoing swimming. Electrophysiological recordings of two populations of hindbrain reticulospinal V2a neurons showed that they were active during swimming. One population of neurons, small V2a neurons in the caudal hindbrain, fired with low rhythmicity, whereas the other population of neurons, large reticulospinal V2a neurons, called MiV1 neurons, fired more rhythmically. These results indicated that hindbrain reticulospinal V2a neurons play critical roles in providing excitation to the spinal locomotor circuits during swimming by providing both tonic and phasic inputs to the circuits.

Mutation of zebrafish dihydrolipoamide branched-chain transacylase E2 results in motor dysfunction and models maple syrup urine disease

SUMMARYAnalysis of zebrafish mutants that demonstrate abnormal locomotive behavior can elucidate the molecular requirements for neural network function and provide new models of human disease. Here, we show that zebrafish quetschkommode (que) mutant larvae exhibit a progressive locomotor defect that culminates in unusual nose-to-tail compressions and an inability to swim. Correspondingly, extracellular peripheral nerve recordings show that que mutants demonstrate abnormal locomotor output to the axial muscles used for swimming. Using positional cloning and candidate gene analysis, we reveal that a point mutation disrupts the gene encoding dihydrolipoamide branched-chain transacylase E2 (Dbt), a component of a mitochondrial enzyme complex, to generate the que phenotype. In humans, mutation of the DBT gene causes maple syrup urine disease (MSUD), a disorder of branched-chain amino acid metabolism that can result in mental retardation, severe dystonia, profound neurological damage and death. que mutants harbor abnormal amino acid levels, similar to MSUD patients and consistent with an error in branched-chain amino acid metabolism. que mutants also contain markedly reduced levels of the neurotransmitter glutamate within the brain and spinal cord, which probably contributes to their abnormal spinal cord locomotor output and aberrant motility behavior, a trait that probably represents severe dystonia in larval zebrafish. Taken together, these data illustrate how defects in branched-chain amino acid metabolism can disrupt nervous system development and/or function, and establish zebrafish que mutants as a model to better understand MSUD.

Gradient based spinal cord axogenesis and locomotor connectome of the hatchling Xenopus tadpole

Understanding the mechanisms underlying the self-assembly and organization of functional neuronal networks is a crucial problem confronting both experimental and theoretical neuroscience alike. Early in development, functional neuronal networks self-assemble with astonishing rapidity. It is, therefore, imperative to investigate and understand how far simple basic mechanisms can allow primary functioning neuronal circuits to develop. To address this ‘structure-function’ issue, we model anatomy and electrophysiology of young hatchling Xenopus tadpole’s spinal cord [1-3]. Our bottom-up approach to modeling of neuronal connectivity is based on developmental process of axon growth – we develop a gradient-based mathematical model for axon growth. It is known that in the developing vertebrate spinal cord, neurons arise from progenitor cells in the neural tube and thereafter the axons grow under influence of chemical morphogenes released from the dorsal roof plate (‘BMP’), ventral floor plate (‘shh’) and hindbrain regions (‘Wnt’). Distribution of these guidance molecules along the spinal cord set up a gradient field which steer the axons in appropriate locations and thus ensure formation of proper connections. We grow axons of spinal neurons and generate synaptic connections similar to biological developmental process based on the data from Professor Alan Roberts Lab at University of Bristol [4]. Using the gradient-based model we were able to grow axons for all seven types of spinal neurons which are believed to be involved in swimming and struggling behavior of tadpole. These spinal neurons include sensory neurons which are responsible for receiving stimulus from environment; interneurons which process the sensory information and pass it onto the motor which translate these signals into appropriate locomotive activities. We successfully modeled both morphologically different types of spinal neurons– firstly commissural neurons grow axons ventrally on the same side of the spinal cord at first and turn longitudinally on the other (contralateral) side of the spinal cord, whereas non-commissural neurons grow their axons on the same (ipsilateral) side of spinal cord. The model incorporates experimental data for somata distribution in the spinal cord, the outgrowth angles, axon lengths, etc. The computer modeling of the axon growth of spinal neurons has enabled both us and our Biology partners to construct and test various hypotheses particularly the roles of the different guidance cues in the axon growth. We also implemented an optimization technique to determine the model parameters for a particular type of neuron based on minimizing a cost function which comprises of the difference between the axon trajectory distribution of the model axons and the corresponding experimental data and also difference between the tortuisities of the model and experimental axons. The complete reconstruction of the spinal cord includes about 2000 neurons. In the reconstruction, both the neurons and the dendrites are randomly distributed along the spinal cord on the ‘left’ and ‘right’ side of the body according to the experimental measurements. Thus, the reconstruction of the connectivity is biologically realistic. To model spiking activity we use Hodgkin-Huxley type conductance model. Parameters of the model are chosen according to the available neurophysiological measurements. Activity dynamics of neural network comprising both the reconstruction model and conductance based spiking neurons demonstrates swimming patterns, i.e., anti-phase oscillations on opposite sides of the body and metachronal wave in longitudinal direction in a wide range of model parameters.

Studies of locomotor network neuroprotection by the selective poly(ADP‐ribose) polymerase‐1 inhibitor PJ‐34 against excitotoxic injury to the rat spinal cord in vitro

Delayed neuronal destruction after acute spinal injury is attributed to excitotoxicity mediated by hyperactivation of poly(ADP-ribose) polymerase-1 (PARP-1) that induces 'parthanatos', namely a non-apoptotic cell death mechanism. With an in vitro model of excitotoxicity, we have previously observed parthanatos of rat spinal cord locomotor networks to be decreased by a broad spectrum PARP-1 inhibitor. The present study investigated whether the selective PARP-1 inhibitor N-(6-oxo-5,6-dihydrophenanthridin-2-yl)-(N,N-dimethylamino)acetamide.HCl (PJ-34) not only protected networks from kainate-evoked excitotoxicity, but also prevented loss of locomotor patterns recorded as fictive locomotion from lumbar (L) ventral roots (VRs) 24 h later. PJ-34 (60 μm) blocked PARP-1 activation and preserved dorsal, central and ventral gray matter with maintained reflex activity even after a large dose of kainate. Fictive locomotion could not, however, be restored by either electrical stimulation or bath-applied neurochemicals (N-methyl-D-aspartate plus 5-hydroxytryptamine). A low kainate concentration induced less histological damage that was widely prevented by PJ-34. Nonetheless, fictive locomotion was observed in just over 50% of preparations whose histological profile did not differ (except for the dorsal horn) from those lacking such a rhythm. Our data show that inhibition of PARP-1 could amply preserve spinal network histology after excitotoxicity, with return of locomotor patterns only when the excitotoxic stimulus was moderate. These results demonstrated divergence between histological and functional outcome, implying a narrow borderline between loss of fictive locomotion and neuronal preservation. Our data suggest that either damage of a few unidentified neurons or functional network inhibition was critical for ensuring locomotor cycles.

Prolonged electrical stimulation over hip flexors increases locomotor output in human SCI

Objectives The objective of this study was to determine whether enhanced feedback from thigh afferents improves locomotor output in human spinal cord injury (SCI). Methods The effects of afferent feedback originating from the upper thigh muscles on locomotion was examined using electrical stimulation in 10 subjects with incomplete SCI and three neurologically intact controls during robotic-assisted treadmill walking. Electrical stimulation consisted of 20 pulses at 30 Hz, applied bilaterally to the skin of the medial thigh, approximately over the sartorius muscle. The stimulation was applied at four different phases of the gait cycle. Torque responses of hip and knee joints and electromyograms of both legs were recorded during baseline with no stimulation, stimulation, and post-stimulation. Results During stimulation, enhanced hip and knee extension and flexion torque responses were observed during the stance and swing phases, respectively, for all four different stimulation conditions. Larger hip extension torque was observed when the stimulation was applied during the stance phase and the transition from stance to swing. Conclusions Enhanced afferent feedback produced by electrical stimulation may increase the excitability of the spinal cord locomotor circuits in human SCI. Significance Results from this study emphasize the contribution of sensory information from thigh muscles, particularly the sartorius muscle afferents, to locomotor control in human SCI during treadmill walking.

Spontaneous rhythmogenic capabilities of sympathetic neuronal assemblies in the rat spinal cord slice

Neuronal networks generating rhythmic activity as an emergent property are common throughout the nervous system. Some are responsible for rhythmic behaviours, as is the case for the spinal cord locomotor networks; however, for others the function is more subtle and usually involves information processing and/or transfer. An example of the latter is sympathetic nerve activity, which is synchronized into rhythmic bursts in vivo. This arrangement is postulated to offer improved control of target organ responses compared to tonic nerve activity. Traditionally, oscillogenic circuits in the brainstem are credited with generating these rhythms, despite evidence for the persistence of some frequencies in spinalized preparations. Here, we show that rhythmic population activity can be recorded from the intermediolateral cell column (IML) of thoracic spinal cord slices. Recorded in slices from 10- to 12-day-old rats, this activity was manifest as 8–22 Hz oscillations in the field potential and was spatially restricted to the IML. Oscillations often occurred spontaneously, but could also be induced by application of 5-HT, α-methyl 5-HT or MK212. These agents also significantly increased the strength of spontaneous oscillations. Rhythmic activity was abolished by TTX and attenuated by application of gap junction blockers or by antagonists of GABAA receptors. Together these data indicate that this rhythm is an emergent feature of a population of spinal neurons coupled by gap junctions. This work questions the assumption that sympathetic rhythms are dependent on supraspinal pacemaker circuits, by highlighting a surprisingly strong rhythmogenic capability of the reduced sympathetic networks of the spinal cord slice.

The Effect of IGF-I Overexpression in Skeletal Muscle Following Spinal Cord Injury and Locomotor Training

The Effect of IGF-I Overexpression in Skeletal Muscle Following Spinal Cord Injury and Locomotor Training

Descending interactions with spinal cord networks: a time to build?

Descending interactions with spinal cord networks: a time to build?

Exciting times in the tadpole spinal cord

Given the ‘relative’ simplicity of the spinal cord compared to other areas of the CNS spinal locomotor networks should have been among the easiest vertebrate networks to characterise. Brown provided the first evidence for a spinal central pattern-generating network almost a century ago (Brown, 1911). He also provided insight into their basic organisation that has formed the foundation for all subsequent analyses of spinal networks. However, the characterisation of these networks has not been straightforward: given the difficulties of understanding much simpler networks this should probably not have come as a surprise. Understanding any network at the very least requires information on the network organisation (the neurons that the network contains and their direct synaptic interactions), as well as the functional properties of network cells and synapses. While there are still significant gaps in the understanding of mammalian locomotor networks, there is some insight into locomotor networks in lower vertebrates, the lamprey and tadpole. The organisation of the spinal cord locomotor network in the lamprey is claimed to be fully characterised (e.g. Grillner et al. 2005 for a recent example): these claims are widely accepted, but they ignore uncertainty over the identification of network neurons and the significant gaps in our knowledge of network connectivity (see Parker, 2006). While it has traditionally been less celebrated, insight into the tadpole locomotor network has increased dramatically over the last several years. Li, Soffe and Roberts in Bristol have developed a tadpole preparation that has allowed paired recordings to be made routinely from identified classes of spinal cord neurons. As a result insight into the organization and functional properties of the tadpole locomotor network has increased markedly. While not widely regarded as such, it has arguably become the best-understood spinal cord locomotor network in terms of network organization and functional properties. The study by Li et al. (2009) in this issue of The Journal of Physiology has continued their characterisation of the locomotor network by examining electrical coupling between excitatory network interneurons (dINs) in the hindbrain and spinal cord, and the significance of this coupling during swimming. They describe a very high (but not absolute) degree of exclusivity in electrical coupling between the dINs through the use of pairwise recordings from identified cell types. It should not need emphasizing (but still does) that while it is difficult and time-consuming, detailed electrophysiological analyses of this sort are essential to any genuine characterization of networks. The data also highlight the impossibility of extrapolating within or between cell populations. The authors suggest that the electrical coupling between dINs contributes to the high degree of reliability of spiking in these neurons during swimming, and move on to try and link the effects of this coupling to the network output. This link is generally difficult to make, and where claims have been made they inevitably rest on assumptions and extrapolations rather than direct causal links. Here, the authors have tried to make this link by blocking the electrical connections between dINs using the gap junction blocker 18-β-glycyrrhetinic acid (18-β-GA). They found that 18-β-GA decreased the reliability of dIN spiking and the synchronization across the dIN population during swimming. In addition, it drastically shortened the duration of swimming episodes. They conclude that electrical coupling between dINs underlies the reliability and synchronization of spiking within the dIN population and the ability of these neurons to sustain swimming episodes. It thus provides a potential mechanism for sustaining rhythmic activity in response to a brief initial stimulus. These claims inevitably carry certain assumptions: there is the possibility of an effect of 18-β-GA on electrical connections between MNs, and the possibility of non-specific effects of gap junction blockers, a feature that complicates the use of these drugs in network analyses. The authors went some way to addressing these issues. Firstly, from their previous analyses electrical synapses are found between motor neurons (Perrins & Roberts, 1995) but there is no evidence for inputs to the dINs or other premotor interneurons, and the sustained activity in hindbrain dINs is not affected by removing the spinal cord (and thus motor neurons; Li et al. 2006), both features suggesting that the effects of 18-β-GA on network activity were not due to effects on electrical connections made by motor neurons. Secondly, they chose 18-β-GA from four gap junction blockers based on analyses of non-specific cellular and synaptic effects of these drugs. 18-β-GA was chosen because of its lack of non-specific effects on the properties they studied (although other effects cannot be ruled out). These features strengthen the link between the coupling between dINs and the changes in the network output, and thus make a reasonable link between cellular properties and network behaviour. This study thus continues the impressive series of analyses in this system.