Functional organization of long ascending propriospinal pathways linking lumbo-sacral and cervical segments in the cat

ROLE OF FORELIMB MORPHOLOGY IN MUSCLE SENSORIMOTOR FUNCTIONS DURING LOCOMOTION IN THE CAT

Previous studies established strong links between morphological characteristics of mammalian hindlimb muscles and their sensorimotor functions during locomotion. Less is known about the role of forelimb morphology in motor outputs and generation of sensory signals. Here, we measured morphological characteristics of 46 forelimb muscles from 6 cats. These characteristics included muscle attachments, physiological cross-sectional area (PCSA), fascicle length, etc. We also recorded full-body mechanics and EMG activity of forelimb muscles during level overground and treadmill locomotion in 7 and 16 adult cats of either sex, respectively. We computed forelimb muscle forces along with force- and length-dependent sensory signals mapped onto corresponding cervical spinal segments. We found that patterns of computed muscle forces and afferent activities were strongly affected by the muscle's moment arm, PCSA, and fascicle length. Morphology of the shoulder muscles suggests distinct roles of the forelimbs in lateral force production and movements. Patterns of length-dependent sensory activity of muscles with long fibers (brachioradialis, extensor carpi radialis) closely matched patterns of overall forelimb length, whereas the activity pattern of biceps brachii matched forelimb orientation. We conclude that cat forelimb muscle morphology contributes substantially to locomotor function, particularly to control lateral stability and turning, rather than propulsion.

Changes in intra- and interlimb reflexes from forelimb cutaneous afferents after staggered thoracic lateral hemisections during locomotion in cats

In quadrupeds, such as cats, cutaneous afferents from the forepaw dorsum signal external perturbations and send signals to spinal circuits to coordinate the activity in muscles of all four limbs. How these cutaneous reflex pathways from forelimb afferents are reorganized after an incomplete spinal cord injury is not clear. Using a staggered thoracic lateral hemisections paradigm, we investigated changes in intralimb and interlimb reflex pathways by electrically stimulating the left and right superficial radial nerves in seven adult cats and recording reflex responses in five forelimb and ten hindlimb muscles. After the first (right T5-T6) and second (left T10-T11) hemisections, forelimb-hindlimb coordination was altered and weakened. After the second hemisection, cats required balance assistance to perform quadrupedal locomotion. Short-, mid- and long-latency homonymous and crossed reflex responses in forelimb muscles and their phase modulation remained largely unaffected after staggered hemisections. The occurrence of homolateral and diagonal mid- and long-latency responses in hindlimb muscles evoked with left and right superficial radial nerve stimulation was significantly reduced at the first time point after the first hemisection, but partially recovered at the second time point with left superficial radial nerve stimulation. These responses were lost or reduced after the second hemisection. When present, all reflex responses, including homolateral and diagonal, maintained their phase-dependent modulation. Therefore, our results show a considerable loss in cutaneous reflex transmission from cervical to lumbar levels after incomplete spinal cord injury, albeit with preservation of phase modulation, likely affecting functional responses to external perturbations.

Identifying spinal tracts transmitting distant effects of trans-spinal magnetic stimulation

Estimating the state of tract-specific inputs to spinal motoneurons is critical to understanding movement deficits induced by neurological injury and potential pathways to recovery but remains challenging in humans. In this study, we explored the capability of trans-spinal magnetic stimulation (TSMS) to modulate distal reflex circuits in young adults. TSMS was applied over the thoracic spine to condition soleus H-reflexes involving sacral-level motoneurons. Three TSMS intensities below the motor threshold were applied at interstimulus intervals (ISIs) between 2 and 20 ms relative to peripheral nerve stimulation (PNS). Although low-intensity TSMS yielded no changes in H-reflexes across ISIs, the two higher stimulus intensities yielded two phases of H-reflex inhibition: a relatively long-lasting period at 2- to 9-ms ISIs, and a short phase at 11- to 12-ms ISIs. H-reflex inhibition at 2-ms ISI was uniquely dependent on TSMS intensity. To identify the candidate neural pathways contributing to H-reflex suppression, we constructed a tract-specific conduction time estimation model. Based upon our model, H-reflex inhibition at 11- to 12-ms ISIs is likely a manifestation of orthodromic transmission along the lateral reticulospinal tract. In contrast, the inhibition at 2-ms ISI likely reflects orthodromic transmission along sensory fibers with activation reaching the brain, before descending along motor tracts. Multiple pathways may contribute to H-reflex modulation between 4- and 9-ms ISIs, orthodromic transmission along sensorimotor tracts, and antidromic transmission of multiple motor tracts. Our findings suggest that noninvasive TSMS can influence motoneuron excitability at distal segments and that the contribution of specific tracts to motoneuron excitability may be distinguishable based on conduction velocities.NEW & NOTEWORTHY This study explored the capability of trans-spinal magnetic stimulation (TSMS) over the thoracic spine to modulate distal reflex circuits, H-reflexes involving sacral-level motoneurons, in young adults. TSMS induced two inhibition phases of H-reflex across interstimulus intervals (ISIs): a relatively long-lasting period at 2- to 9-ms ISIs, and a short phase at 11- to 12-ms ISIs. An estimated probability model constructed from tract-specific conduction velocities allowed the identification of potential spinal tracts contributing to the changes in motoneuron excitability.

Overground gait kinematics and muscle activation patterns in the Yucatan mini pig

Objective The objectives of this study were to assess gait biomechanics and the effect of overground walking speed on gait parameters, kinematics, and electromyographic (EMG) activity in the hindlimb muscles of Yucatan Minipigs (YMPs). Approach Nine neurologically-intact, adult YMPs were trained to walk overground in a straight line. Whole-body kinematics and EMG activity of hindlimb muscles were recorded and analyzed at 6 different speed ranges (0.4-0.59, 0.6-0.79, 0.8-0.99, 1.0-1.19, 1.2-1.39, and 1.4-1.6 m/s). A MATLAB program was developed to detect strides and gait events automatically from motion-captured data. The kinematics and EMG activity were analyzed for each stride based on the detected events. Main results Significant decreases in stride duration, stance and swing times and an increase in stride length were observed with increasing speed. A transition in gait pattern occurred at the 1.0m/s walking speed. Significant increases in the range of motion of the knee and ankle joints were observed at higher speeds. Also, the points of minimum and maximum joint angles occurred earlier in the gait cycle as the walking speed increased. The onset of EMG activity in the biceps femoris muscle occurred significantly earlier in the gait cycle with increasing speed. Significance YMPs are becoming frequently used as large animal models for preclinical testing and translation of novel interventions to humans. A comprehensive characterization of overground walking in neurologically-intact YMPs is provided in this study. These normative measures set the basis against which the effects of future interventions on locomotor capacity in YMPs can be compared.

Control of Mammalian Locomotion by Somatosensory Feedback

When animals walk overground, mechanical stimuli activate various receptors located in muscles, joints, and skin. Afferents from these mechanoreceptors project to neuronal networks controlling locomotion in the spinal cord and brain. The dynamic interactions between the control systems at different levels of the neuraxis ensure that locomotion adjusts to its environment and meets task demands. In this article, we describe and discuss the essential contribution of somatosensory feedback to locomotion. We start with a discussion of how biomechanical properties of the body affect somatosensory feedback. We follow with the different types of mechanoreceptors and somatosensory afferents and their activity during locomotion. We then describe central projections to locomotor networks and the modulation of somatosensory feedback during locomotion and its mechanisms. We then discuss experimental approaches and animal models used to investigate the control of locomotion by somatosensory feedback before providing an overview of the different functional roles of somatosensory feedback for locomotion. Lastly, we briefly describe the role of somatosensory feedback in the recovery of locomotion after neurological injury. We highlight the fact that somatosensory feedback is an essential component of a highly integrated system for locomotor control. © 2021 American Physiological Society. Compr Physiol 11:1-71, 2021.

Role of DSCAM in the Development of Neural Control of Movement and Locomotion

Locomotion results in an alternance of flexor and extensor muscles between left and right limbs generated by motoneurons that are controlled by the spinal interneuronal circuit. This spinal locomotor circuit is modulated by sensory afferents, which relay proprioceptive and cutaneous inputs that inform the spatial position of limbs in space and potential contacts with our environment respectively, but also by supraspinal descending commands of the brain that allow us to navigate in complex environments, avoid obstacles, chase prey, or flee predators. Although signaling pathways are important in the establishment and maintenance of motor circuits, the role of DSCAM, a cell adherence molecule associated with Down syndrome, has only recently been investigated in the context of motor control and locomotion in the rodent. DSCAM is known to be involved in lamination and delamination, synaptic targeting, axonal guidance, dendritic and cell tiling, axonal fasciculation and branching, programmed cell death, and synaptogenesis, all of which can impact the establishment of motor circuits during development, but also their maintenance through adulthood. We discuss herein how DSCAM is important for proper motor coordination, especially for breathing and locomotion.

Changing coupling between the arms and legs with slow walking speeds alters regulation of somatosensory feedback

Arm swing movement is coordinated with movement of the legs during walking, where the frequency of coordination depends on walking speed. At typical speeds, arm and leg movements, respectively, are frequency locked in a 1:1 ratio but at slow speeds this changes to a 2:1 ratio. It is unknown if the changes in interlimb ratio that accompany slow walking speeds alters regulation of somatosensory feedback. To probe the neural interactions between the arms and legs, somatosensory linkages in the form of interlimb cutaneous reflexes were examined. It was hypothesized that different interlimb frequencies and walking speeds would result in changes in the modulation of cutaneous reflexes between the arms and legs. To test this hypothesis, participants walked in four combinations of walking speed (typical, slow) and interlimb coordination (1:1, and 2:1), while cutaneous reflexes and background muscle activity were evaluated with stimulation applied to the superficial peroneal nerve at the ankle and superficial radial nerve at the wrist. Results show main effects of interlimb coordination and walking speed on cutaneous reflex modulation, effects are largest in the swing phase, and a directional coupling was observed, where changes in the frequency of arm movements had a greater effect on muscle activity in the legs compared to the reverse. Task-dependent modulation was also revealed from stimulation at local and remote sources. Understanding the underlying neural mechanisms for the organization of rhythmic arm movement, and its coordination with the legs in healthy participants, can give insight into pathological walking, and will facilitate the development of effective strategies for the rehabilitation of walking.

Recovery of locomotion in cats after severe contusion of the low thoracic spinal cord

Large bilateral contusions of the T10 thoracic spinal cord were performed in 16 adult cats using a calibrated impactor. EMG and video recordings allowed weekly assessments of key locomotor parameters during treadmill training for 5 wk. Thirty-five days postcontusion, several hindlimb locomotor parameters were very similar to the prelesion ones despite some long-term deficits such as paw drag and disrupted fore-hindlimb coupling. Nine out of ten tested cats could step over obstacles placed on the treadmill. Acute electrophysiological experiments showed viable connectivity between segments rostral and caudal to the contusion. At the fifth postcontusion week, a complete spinalization was performed at T13 in 10 cats and all expressed remarkable bilateral hindlimb locomotion within 24-72 h. From our histological evaluation, we concluded that only a small percentage (~10%) of spinal cord pathways was necessary to initiate and maintain a voluntary quadrupedal locomotor pattern on a treadmill and even to negotiate obstacles. Our findings suggest that hindlimb stepping largely resulted from the activity of spinal locomotor circuits, which gradually recovered autonomy week after week. Our histological and electrophysiological evidence indicated that the persistence of specific deficits or else the maintenance of specific functions was related to the integrity of specific supraspinal and propriospinal pathways. The conclusion is that the recovery of locomotion after large spinal contusions depends on a homeostatic recalibration of a tripartite control system involving interactions between spinal circuits (central pattern generator), supraspinal influences, and sensory feedback activated through locomotor training.NEW & NOTEWORTHY The recovery of quadrupedal treadmill locomotion after a large bilateral contusion at the low thoracic T10 spinal level and the ability to negotiate obstacles were studied for 5 wk in 16 cats. Ten cats were further completely spinalized at T13 and were found to walk with the hindlimbs within 24-72 h. We conclude that the extent of locomotor recovery after large spinal contusions hinges both on remnant supraspinal pathways and on a spinal pattern generator.

Propriospinal Neurons: Essential Elements of Locomotor Control in the Intact and Possibly the Injured Spinal Cord

Propriospinal interneurons (INs) communicate information over short and long distances within the spinal cord. They act to coordinate different parts of the body by linking motor circuits that control muscles across the forelimbs, trunk, and hindlimbs. Their role in coordinating locomotor circuits near and far may be invaluable to the recovery of locomotor function lost due to injury to the spinal cord where the flow of motor commands from the brain and brainstem to spinal motor circuits is disrupted. The formation and activation of circuits established by spared propriospinal INs may promote the re-emergence of locomotion. In light of progress made in animal models of spinal cord injury (SCI) and in human patients, we discuss the role of propriospinal INs in the intact spinal cord and describe recent studies investigating the assembly and/or activation of propriospinal circuits to promote recovery of locomotion following SCI.

Effects of neuromuscular electrical stimulation and voluntary commands on the spinal reflex excitability of remote limb muscles

It is well known that contracting the upper limbs can affect spinal reflexes of the lower limb muscle, via intraneuronal networks within the central nervous system. However, it remains unknown whether neuromuscular electrical stimulation (NMES), which can generate muscle contractions without central commands from the cortex, can also play a role in such inter-limb facilitation. Therefore, the objective of this study was to compare the effects of unilateral upper limb contractions using NMES and voluntary unilateral upper limb contractions on the inter-limb spinal reflex facilitation in the lower limb muscles. Spinal reflex excitability was assessed using transcutaneous spinal cord stimulation (tSCS) to elicit responses bilaterally in multiple lower limb muscles, including ankle and thigh muscles. Five interventions were applied on the right wrist flexors for 70 s: (1) sensory-level NMES; (2) motor-level NMES; (3) voluntary contraction; (4) voluntary contraction and sensory-level NMES; (5) voluntary contraction and motor-level NMES. Results showed that spinal reflex excitability of ankle muscles was facilitated bilaterally during voluntary contraction of the upper limb unilaterally and that voluntary contraction with motor-level NMES had similar effects as just contracting voluntarily. Meanwhile, motor-level NMES facilitated contralateral thigh muscles, and sensory-level NMES had no effect. Overall, our results suggest that inter-limb facilitation effect of spinal reflex excitability in lower limb muscles depends, to a larger extent, on the presence of the central commands from the cortex during voluntary contractions. However, peripheral input generated by muscle contractions using NMES might have effects on the spinal reflex excitability of inter-limb muscles via spinal intraneuronal networks.

Central pattern generators in the brainstem and spinal cord: an overview of basic principles, similarities and differences

Central pattern generators (CPGs) are generally defined as networks of neurons capable of enabling the production of central commands, specifically controlling stereotyped, rhythmic motor behaviors. Several CPGs localized in brainstem and spinal cord areas have been shown to underlie the expression of complex behaviors such as deglutition, mastication, respiration, defecation, micturition, ejaculation, and locomotion. Their pivotal roles have clearly been demonstrated although their organization and cellular properties remain incompletely characterized. In recent years, insightful findings about CPGs have been made mainly because (1) several complementary animal models were developed; (2) these models enabled a wide variety of techniques to be used and, hence, a plethora of characteristics to be discovered; and (3) organizations, functions, and cell properties across all models and species studied thus far were generally found to be well-preserved phylogenetically. This article aims at providing an overview for non-experts of the most important findings made on CPGs in in vivo animal models, in vitro preparations from invertebrate and vertebrate species as well as in primates. Data about CPG functions, adaptation, organization, and cellular properties will be summarized with a special attention paid to the network for locomotion given its advanced level of characterization compared with some of the other CPGs. Similarities and differences between these networks will also be highlighted.

Temporal and spatial dynamics of spinal sensorimotor processing in an intersegmental cutaneous nociceptive reflex

The cutaneus trunci muscle (CTM) reflex produces a skin "shrug" in response to pinch on a rat's back through a three-part neural circuit: 1) A-fiber and C-fiber afferents in segmental dorsal cutaneous nerves (DCNs) from lumbar to cervical levels, 2) ascending propriospinal interneurons, and 3) the CTM motoneuron pool located at the cervicothoracic junction. We recorded neurograms from a CTM nerve branch in response to electrical stimulation. The pulse trains were delivered at multiple DCNs (T₆-L₁), on both sides of the midline, at two stimulus strengths (0.5 or 5 mA, to activate Aδ fibers or Aδ and C fibers, respectively) and four stimulation frequencies (1, 2, 5, or 10 Hz) for 20 s. We quantified both the temporal dynamics (i.e., latency, sensitization, habituation, and frequency dependence) and the spatial dynamics (spinal level) of the reflex. The evoked responses were time-windowed into Early, Mid, Late, and Ongoing phases, of which the Mid phase, between the Early (Aδ fiber mediated) and Late (C fiber mediated) phases, has not been previously identified. All phases of the response varied with stimulus strength, frequency, history, and DCN level/side stimulated. In addition, we observed nociceptive characteristics like C fiber-mediated sensitization (wind-up) and habituation. Finally, the range of latencies in the ipsilateral responses were not very large rostrocaudally, suggesting a myelinated neural path within the ipsilateral spinal cord for at least the A fiber-mediated Early-phase response. Overall, these results demonstrate that the CTM reflex shares the temporal dynamics in other nociceptive reflexes and exhibits spatial (segmental and lateral) dynamics not seen in those reflexes.NEW & NOTEWORTHY We have physiologically studied an intersegmental reflex exploring detailed temporal, stimulus strength-based, stimulation history-dependent, lateral and segmental quantification of the reflex responses to cutaneous nociceptive stimulations. We found several physiological features in this reflex pathway, e.g., wind-up, latency changes, and somatotopic differences. These physiological observations allow us to understand how the anatomy of this reflex may be organized. We have also identified a new phase of this reflex, termed the "mid" response.

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.

Spinal Control of Locomotion: Individual Neurons, Their Circuits and Functions

Systematic research on the physiological and anatomical characteristics of spinal cord interneurons along with their functional output has evolved for more than one century. Despite significant progress in our understanding of these networks and their role in generating and modulating movement, it has remained a challenge to elucidate the properties of the locomotor rhythm across species. Neurophysiological experimental evidence indicates similarities in the function of interneurons mediating afferent information regarding muscle stretch and loading, being affected by motor axon collaterals and those mediating presynaptic inhibition in animals and humans when their function is assessed at rest. However, significantly different muscle activation profiles are observed during locomotion across species. This difference may potentially be driven by a modified distribution of muscle afferents at multiple segmental levels in humans, resulting in an altered interaction between different classes of spinal interneurons. Further, different classes of spinal interneurons are likely activated or silent to some extent simultaneously in all species. Regardless of these limitations, continuous efforts on the function of spinal interneuronal circuits during mammalian locomotion will assist in delineating the neural mechanisms underlying locomotor control, and help develop novel targeted rehabilitation strategies in cases of impaired bipedal gait in humans. These rehabilitation strategies will include activity-based therapies and targeted neuromodulation of spinal interneuronal circuits via repetitive stimulation delivered to the brain and/or spinal cord.

The neural control of interlimb coordination during mammalian locomotion

Neuronal networks within the spinal cord directly control rhythmic movements of the arms/forelimbs and legs/hindlimbs during locomotion in mammals. For an effective locomotion, these networks must be flexibly coordinated to allow for various gait patterns and independent use of the arms/forelimbs. This coordination can be accomplished by mechanisms intrinsic to the spinal cord, somatosensory feedback from the limbs, and various supraspinal pathways. Incomplete spinal cord injury disrupts some of the pathways and structures involved in interlimb coordination, often leading to a disruption in the coordination between the arms/forelimbs and legs/hindlimbs in animal models and in humans. However, experimental spinal lesions in animal models to uncover the mechanisms coordinating the limbs have limitations due to compensatory mechanisms and strategies, redundant systems of control, and plasticity within remaining circuits. The purpose of this review is to provide a general overview and critical discussion of experimental studies that have investigated the neural mechanisms involved in coordinating the arms/forelimbs and legs/hindlimbs during mammalian locomotion.

Anatomical and Molecular Properties of Long Descending Propriospinal Neurons in Mice

Long descending propriospinal neurons (LDPNs) are interneurons that form direct connections between cervical and lumbar spinal circuits. LDPNs are involved in interlimb coordination and are important mediators of functional recovery after spinal cord injury (SCI). Much of what we know about LDPNs comes from a range of species, however, the increased use of transgenic mouse lines to better define neuronal populations calls for a more complete characterisation of LDPNs in mice. In this study, we examined the cell body location, inhibitory neurotransmitter phenotype, developmental provenance, morphology and synaptic inputs of mouse LDPNs throughout the cervical and upper thoracic spinal cord. LDPNs were retrogradely labelled from the lumbar spinal cord to map cell body locations throughout the cervical and upper thoracic segments. Ipsilateral LDPNs were distributed throughout the dorsal, intermediate and ventral grey matter as well as the lateral spinal nucleus and lateral cervical nucleus. In contrast, contralateral LDPNs were more densely concentrated in the ventromedial grey matter. Retrograde labelling in GlyT2^GFP and GAD67^GFP mice showed the majority of inhibitory LDPNs project either ipsilaterally or adjacent to the midline. Additionally, we used several transgenic mouse lines to define the developmental provenance of LDPNs and found that V2b positive neurons form a subset of ipsilaterally projecting LDPNs. Finally, a population of Neurobiotin (NB) labelled LDPNs were assessed in detail to examine morphology and plot the spatial distribution of contacts from a variety of neurochemically distinct axon terminals. These results provide important baseline data in mice for future work on their role in locomotion and recovery from SCI.

Characterization of Motor and Somatosensory Evoked Potentials in the Yucatan Micropig Using Transcranial and Epidural Stimulation

Yucatan micropigs have brain and spinal cord dimensions similar to humans and are useful for certain spinal cord injury (SCI) translational studies. Micropigs are readily trained in behavioral tasks, allowing consistent testing of locomotor loss and recovery. However, there has been little description of their motor and sensory pathway neurophysiology. We established methods to assess motor and sensory cortical evoked potentials in the anesthetized, uninjured state. We also evaluated epidurally evoked motor and sensory stimuli from the T6 and T9 levels, spanning the intended contusion injury epicenter. Response detection frequency, mean latency and amplitude values, and variability of evoked potentials were determined. Somatosensory evoked potentials were reliable and best detected during stimulation of peripheral nerve and epidural stimulation by referencing the lateral cortex to midline Fz. The most reliable hindlimb motor evoked potential (MEP) occurred in tibialis anterior. We found MEPs in forelimb muscles in response to thoracic epidural stimulation likely generated from propriospinal pathways. Cranially stimulated MEPs were easier to evoke in the upper limbs than in the hindlimbs. Autopsy studies revealed substantial variations in cortical morphology between animals. This electrophysiological study establishes that neurophysiological measures can be reliably obtained in micropigs in a time frame compatible with other experimental procedures, such as SCI and transplantation. It underscores the need to better understand the motor control pathways, including the corticospinal tract, to determine which therapeutics are suitable for testing in the pig model.

Characterization of dendritic morphology and neurotransmitter phenotype of thoracic descending propriospinal neurons after complete spinal cord transection and GDNF treatment

After spinal cord injury (SCI), poor regeneration of damaged axons of the central nervous system (CNS) causes limited functional recovery. This limited spontaneous functional recovery has been attributed, to a large extent, to the plasticity of propriospinal neurons, especially the descending propriospinal neurons (dPSNs). Compared with the supraspinal counterparts, dPSNs have displayed significantly greater regenerative capacity, which can be further enhanced by glial cell line-derived neurotrophic factor (GDNF). In the present study, we applied a G-mutated rabies virus (G-Rabies) co-expressing green fluorescence protein (GFP) to reveal Golgi-like dendritic morphology of dPSNs. We also investigated the neurotransmitters expressed by dPSNs after labeling with a retrograde tracer Fluoro-Gold (FG). dPSNs were examined in animals with sham injuries or complete spinal transections with or without GDNF treatment. Bilateral injections of G-Rabies and FG were made into the 2nd lumbar (L2) spinal cord at 3 days prior to a spinal cord transection performed at the 11th thoracic level (T11). The lesion gap was filled with Gelfoam containing either saline or GDNF in the injury groups. Four days post-injury, the rats were sacrificed for analysis. For those animals receiving G-rabies injection, the GFP signal in the T7-9 spinal cord was visualized via 2-photon microscopy. Dendritic morphology from stack images was traced and analyzed using a Neurolucida software. We found that dPSNs in sham injured animals had a predominantly dorsal-ventral distribution of dendrites. Transection injury resulted in alterations in the dendritic distribution with dorsal-ventral retraction and lateral-medial extension. Treatment with GDNF significantly increased the terminal dendritic length of dPSNs. The density of spine-like structures was increased after injury, and treatment with GDNF enhanced this effect. For the group receiving FG injections, immunohistochemistry for glutamate, choline acetyltransferase (ChAT), glycine, and GABA was performed in the T7-9 spinal cord. We show that the majority of FG retrogradely-labeled dPSNs were located in the Rexed Lamina VII. Over 90% of FG-labeled neurons were glutamatergic, with the other three neurotransmitters contributing less than 10% of the total. To our knowledge this is the first report describing the morphologic characteristics of dPSNs and their neurotransmitter expressions, as well as the dendritic response of dPSNs after transection injury and GDNF treatment.

Exercise dependent increase in axon regeneration into peripheral nerve grafts by propriospinal but not sensory neurons after spinal cord injury is associated with modulation of regeneration-associated genes

Insufficient regeneration of central nervous system (CNS) axons contributes to persisting neurological dysfunction after spinal cord injury (SCI). Peripheral nerve grafts (PNGs) support regeneration by thousands of injured intraspinal axons and help them bypass some of the extracellular barriers that form after SCI. However this number represents but a small portion of the total number of axons that are injured. Here we tested if rhythmic sensory stimulation during cycling exercise would boost the intrinsic regenerative state of neurons to enhance axon regeneration into PNGs after a lower thoracic (T12) spinal transection of adult rats. Using True Blue retrograde tracing, we show that 4 weeks of cycling improves regeneration into a PNG from lumbar interneurons but not by primary sensory neurons. The majority of neurons that regenerate their axon are within 5 mm of the lesion and their number increased 70% with exercise. Importantly propriospinal neurons in more distant regions (5-20 mm from the lesion) that routinely exhibit very limited regeneration responded to exercise by increasing the number of regenerating neurons by 900%. There was no exercise-associated increase in regeneration from sensory neurons. Analyses using fluorescent in situ hybridization showed that this increase in regenerative response is associated with changes in levels of mRNAs encoding the regeneration associated genes (RAGs) GAP43, β-actin and Neuritin. While propriospinal neurons showed increased mRNA levels in response to SCI alone and then to grafting and exercise, sensory neurons did not respond to SCI, but there was a response to the presence of a PNG. Thus, exercise is a non-invasive approach to modulate gene expression in injured neurons leading to an increase in regeneration. This sets the stage for future studies to test whether exercise will promote axon outgrowth beyond the PNG and reconnection with spinal cord neurons, thereby demonstrating a potential clinical application of this combined therapeutic intervention.

Rhythmic arm swing enhances patterned locomotor-like muscle activity in passively moved lower extremities

The use of driven gait orthosis (DGO) has drawn attention in gait rehabilitation for patients after central nervous system (CNS) lesions. By imposing a passive locomotor-like kinematic pattern, the neural mechanisms responsible for locomotion can be activated as in a normal gait. To further enhance this activity, discussions on possible intervention are necessary. Given the possible functional linkages between the upper and lower limbs, we investigated in healthy subjects the degree of modification in the lower limb muscles during DGO-induced passive gait by the addition of swing movement in the upper extremity. The results clearly showed that muscle activity in the ankle dorsiflexor TA muscle was significantly enhanced when the passive locomotor-like movement was accompanied by arm swing movement. The modifications in the TA activity were not a general increase through the stride cycles, but were observed under particular phases as in normal gaits. Voluntary effort to swing the arms may have certain effects on the modification of the muscle activity. The results provide clinical implications regarding the usefulness of voluntary arm swing movement as a possible intervention in passive gait training using DGO, since ordinary gait training using DGO does not induce spontaneous arm swing movement despite its known influence on the lower limb movement.

Preservation of common rhythmic locomotor control despite weakened supraspinal regulation after stroke

The basic pattern of arm and leg movement during rhythmic locomotor tasks is supported by common central neural control from spinal and supraspinal centers in neurologically intact participants. The purpose of this study was to test the hypothesis that following a cerebrovascular accident, shared systems from interlimb cutaneous networks facilitating arm and leg coordination persist across locomotor tasks. Twelve stroke participants (>6 months post CVA) performed arm and leg (A&L) cycling using a stationary ergometer and walking on a motorized treadmill. In both tasks cutaneous reflexes were evoked via surface stimulation of the nerves innervating the dorsum of the hand (superficial radial; SR) and foot (superficial peroneal; SP) of the less affected limbs. Electromyographic (EMG) activity from the tibialis anterior, soleus, flexor carpi radialis, and posterior deltoid were recorded bilaterally with surface electrodes. Full-wave rectified and filtered EMG data were separated into eight equal parts or phases and aligned to begin with maximum knee extension for both walking and A&L cycling. At each phase of movement, background EMG data were quantified as the peak normalized response for each participant and cutaneous reflexes were quantified as the average cumulative reflex over 150 ms following stimulation. In general, background EMG was similar between walking and A&L cycling, seen especially in the distal leg muscles. Cutaneous reflexes were evident and modified in the less and more affected limbs during walking and A&L cycling and similar modulation patterns were observed suggesting activity in related control networks between tasks. After a stroke common neural patterning from conserved subcortical regulation is seen supporting the notion of a common core in locomotor tasks involving arm and leg movement. This has translational implications for rehabilitation where A&L cycling could be usefully applied to improve walking function.

Schwann cell transplantation and descending propriospinal regeneration after spinal cord injury

After spinal cord injury (SCI), poor ability of damaged axons of the central nervous system (CNS) to regenerate causes very limited functional recovery. Schwann cells (SCs) have been widely explored as promising donors for transplantation to promote axonal regeneration in the CNS including the spinal cord. Compared with other CNS axonal pathways, injured propriospinal tracts display the strongest regenerative response to SC transplantation. Even without providing additional neurotrophic factors, propriospinal axons can grow into the SC environment which is rarely seen in supraspinal tracts. Propriospinal tract has been found to respond to several important neurotrophic factors secreted by SCs. Therefore, the SC is considered to be one of the most promising candidates for cell-based therapies for SCI. Since many reviews have already appeared on topics of SC transplantation in SCI repair, this review will focus particularly on the rationale of SC transplantation in mediating descending propriospinal axonal regeneration as well as optimizing such regeneration by using different combinatorial strategies. This article is part of a Special Issue entitled SI: Spinal cord injury.

Characterization of long descending premotor propriospinal neurons in the spinal cord

The motor function of the spinal cord requires the computation of the local neuronal circuits within the same segments as well as the long-range coordination of different spinal levels. Implicated players in this process are the propriospinal neurons (PPNs) that project their axons across different levels of the spinal cord. However, their cellular, molecular, and functional properties remain unknown. Here we use a recombinant rabies virus-based method to label a specific type of long-projecting premotor PPNs in the mouse upper spinal cord that are monosynaptically connected to the motor neurons in the lumbar spinal cord. With a whole spinal cord imaging method, we find that these neurons are distributed along the entire length of the upper spinal cord with more in the lower thoracic levels. Among them, a subset of thoracic PPNs receive substantial numbers of sensory inputs, suggesting a function in coordinating the activity of trunk and hindlimb muscles. Although many PPNs in the cervical and thoracic spinal cord receive the synaptic inputs from corticospinal tract or serotonergic axons, limited bouton numbers suggested that these supraspinal inputs might not be major regulators of the PPNs in intact animals. Molecularly, these PPNs appear to be distinct from other known premotor interneurons, but some are derived from Chx10+ lineages. This study provides an anatomical basis for further exploring different functions of PPNs.

Ascending and descending propriospinal pathways between lumbar and cervical segments in the rat: evidence for a substantial ascending excitatory pathway

Precise mechanisms are required to coordinate the locomotor activity of fore- and hind-limbs in quadrupeds and similar mechanisms persist to coordinate movement of arms and legs in humans. Propriospinal neurons (PSNs) are major components of the networks that coordinate these mechanisms. The b subunit of cholera toxin (CTb) was injected unilaterally into either L1 or L3 segments in order to label ascending and descending propriospinal pathways. Labelled cells were examined with light or confocal microscopy. Cells projecting to lumbar segments were evenly distributed, bilaterally throughout all cervical segments. However many more cells were labelled from L1 injections than L3 injections. Roughly 15% of cells in both sides of the C2 segment was found to be immunoreactive for calretinin and a small number (4%) was immunoreactive for calbindin. Axons projecting from L1 to cervical segments formed predominant ipsilateral projections to the cervical intermediate grey matter and ventral horn. Very large numbers of terminals were concentrated within the ventrolateral motor (VLM) nuclei of C7-8 segments but there was sparse innervation of the contralateral nucleus. The vast majority (85%) of these axon terminals in the ipsilateral VML was immunoreactive for the vesicular glutamate transporter 2 (VGLUT2) and the remaining 15% was immunoreactive for the vesicular GABA transporter (VGAT); many of these contained GABA and/or glycine. Inhibitory and excitatory terminals were also found in the contralateral VLM. Most of the terminals in the VLM made contacts with motoneurons. The major finding of this study is the existence of a substantial excitatory propriospinal pathway that projects specifically to the VLM. Motoneurons in the VLM supply muscles of the axilla therefore this pathway is likely to have a profound influence on the activity of the shoulder joint. This pathway may synchronise lumbar and cervical pattern generators and hence the coordination of locomotor activity in the fore- and hind limbs.

Effect of afferent feedback and central motor commands on soleus H-reflex suppression during arm cycling

Suppression of soleus H-reflex amplitude in stationary legs is seen during rhythmic arm cycling. We examined the influence of various arm-cycling parameters on this interlimb reflex modulation to determine the origin of the effect. We previously showed the suppression to be graded with the frequency of arm cycling but not largely influenced by changes in peripheral input associated with crank length. Here, we more explicitly explored the contribution of afferent feedback related to arm movement on the soleus H-reflex suppression. We explored the influence of load and rate of muscle stretch by manipulating crank-load and arm-muscle vibration during arm cycling. Furthermore, internally driven ("Active") and externally driven ("Passive") arm cycling was compared. Soleus H-reflexes were evoked with tibial nerve stimulation during stationary control and rhythmic arm-cycling conditions, including: 1) six different loads; 2) with and without vibration to arm muscles; and 3) Active and Passive conditions. No significant differences were seen in the level of suppression between the different crank loads or between conditions with and without arm-muscle vibration. Furthermore, in contrast to the clear effect seen during active cycling, passive arm cycling did not significantly suppress the soleus H-reflex amplitude. Current results, in conjunction with previous findings, suggest that the afferent feedback examined in these studies is not the primary source responsible for soleus H-reflex suppression. Instead, it appears that central motor commands (supraspinal or spinal in origin) associated with frequency of arm cycling are relatively more dominant sources.

Plasticity in ascending long propriospinal and descending supraspinal pathways in chronic cervical spinal cord injured rats

The high clinical relevance of models of incomplete cervical spinal cord injury (SCI) creates a need to address the spontaneous neuroplasticity that underlies changes in functional activity that occur over time after SCI. There is accumulating evidence supporting long projecting propriospinal neurons as suitable targets for therapeutic intervention after SCI, but focus has remained primarily oriented toward study of descending pathways. Long ascending axons from propriospinal neurons at lower thoracic and lumbar levels that form inter-enlargement pathways are involved in forelimb-hindlimb coordination during locomotion and are capable of modulating cervical motor output. We used non-invasive magnetic stimulation to assess how a unilateral cervical (C5) spinal contusion might affect transmission in intact, long ascending propriospinal pathways, and influence spinal cord plasticity. Our results show that transmission is facilitated in this pathway on the ipsilesional side as early as 1 week post-SCI. We also probed for descending magnetic motor evoked potentials (MMEPs) and found them absent or greatly reduced on the ipsilesional side as expected. The frequency-dependent depression (FDD) of the H-reflex recorded from the forelimb triceps brachii was bilaterally decreased although H_max/M_max was increased only on the ipsilesional side. Behaviorally, stepping recovered, but there were deficits in forelimb–hindlimb coordination as detected by BBB and CatWalk measures. Importantly, epicenter sparing correlated to the amplitude of the MMEPs and locomotor recovery but it was not significantly associated with the inter-enlargement or segmental H-reflex. In summary, our results indicate that complex plasticity occurs after a C5 hemicontusion injury, leading to differential changes in ascending vs. descending pathways, ipsi- vs. contralesional sides even though the lesion was unilateral as well as cervical vs. lumbar local spinal networks.

Chondroitinase ABC promotes recovery of adaptive limb movements and enhances axonal growth caudal to a spinal hemisection

A number of studies have shown that chondroitinase ABC (Ch'ase ABC) digestion of inhibitory chondroitin sulfate glycosaminoglycans significantly enhances axonal growth and recovery in rodents following spinal cord injury (SCI). Further, our group has shown improved recovery following SCI in the larger cat model. The purpose of the current study was to determine whether intraspinal delivery of Ch'ase ABC, following T10 hemisections in adult cats, enhances adaptive movement features during a skilled locomotor task and/or promotes plasticity of spinal and supraspinal circuitry. Here, we show that Ch'ase ABC enhanced crossing of a peg walkway post-SCI and significantly improved ipsilateral hindlimb trajectories and integration into a functional forelimb-hindlimb coordination pattern. Recovery of these complex movements was associated with significant increases in neurofilament immunoreactivity immediately below the SCI in the ipsilateral white (p = 0.033) and contralateral gray matter (p = 0.003). Further, the rubrospinal tract is critical in the normal cat during skilled movements that require accurate paw placements and trajectories like those seen during peg walkway crossing. Rubrospinal connections were assessed following Fluoro-Gold injections, caudal to the hemisection. Significantly more retrogradely labeled right (axotomized) red nucleus (RN) neurons were seen in Ch'ase ABC-treated (23%) compared with control-treated cats (8%; p = 0.032) indicating that a larger number of RN neurons in Ch'ase ABC-treated cats had axons below the lesion level. Thus, following SCI, Ch'ase ABC may facilitate axonal growth at the spinal level, enhance adaptive features of locomotion, and affect plasticity of rubrospinal circuitry known to support adaptive behaviors in the normal cat.

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'.

Anterograde labeling of ventrolateral funiculus pathways with spinal enlargement connections in the adult rat spinal cord

The ventrolateral funiculus in the spinal cord has been identified as containing important ascending and descending pathways related to locomotion and interlimb coordination. The purpose of this descriptive study was to investigate the patterns of axon termination of long ascending and descending ventrolateral pathways within the cervical and lumbar enlargements of the adult rat spinal cord. To accomplish this, we made discrete unilateral injections of the tracer biotinylated dextran-amine (BDA) into the ventrolateral white matter at T9. Although some BDA-labeled axons with varicosities were found bilaterally at all cervical levels, particularly dense BDA labeling was observed in laminae VIII and IX ipsilaterally at the C6 and C8 levels. In the same animals, dense terminal labeling was found in the lumbar enlargement in medial lamina VII and ventromedial laminae VIII and IX contralaterally. This labeling was most apparent in the more rostral lumbar segments. These observations continue the characterization of inter-enlargement (long propriospinal) pathways, illustrating a substantial and largely reciprocal inter-enlargement network with large numbers of both ascending and descending ventrolateral commissural neurons. These pathways are anatomically well-suited to the task of interlimb coordination and to participate in the remarkable recovery of locomotor function seen in the rat following thoracic spinal cord injuries that spare as little as 20% of the total white matter cross sectional area.

Whole-Body Responses: Neural Control and Implications for Rehabilitation and Fall Prevention

Humans are one of the unique species that utilize bipedal gait to ambulate in our environment. Despite this fact, coordination of the arms with the legs and the rest of body is essential for many daily activities. As such, whole-body responses have emerged as the preferred strategy following perturbations to balance during both standing and walking. Complex neural circuitry may allow for this coordination through the use of propriospinal pathways linking lumbar and cervical pattern generators in the spinal cord, with supraspinal centers altering this control depending on the context of the situation. Based on these findings, we argue that whole-body reactions may be exploited for rehabilitation purposes. Preliminary results have indicated training programs designed to elicit whole-body responses are effective in reducing falls and improving functional mobility in older adults with and without neurological impairment.

Shaping Appropriate Locomotive Motor Output Through Interlimb Neural Pathway Within Spinal Cord in Humans

Direct evidence supporting the contribution of upper limb motion on the generation of locomotive motor output in humans is still limited. Here, we aimed to examine the effect of upper limb motion on locomotor-like muscle activities in the lower limb in persons with spinal cord injury (SCI). By imposing passive locomotion-like leg movements, all cervical incomplete ( n = 7) and thoracic complete SCI subjects ( n = 5) exhibited locomotor-like muscle activity in their paralyzed soleus muscles. Upper limb movements in thoracic complete SCI subjects did not affect the electromyographic (EMG) pattern of the muscle activities. This is quite natural since neural connections in the spinal cord between regions controlling upper and lower limbs were completely lost in these subjects. On the other hand, in cervical incomplete SCI subjects, in whom such neural connections were at least partially preserved, the locomotor-like muscle activity was significantly affected by passively imposed upper limb movements. Specifically, the upper limb movements generally increased the soleus EMG activity during the backward swing phase, which corresponds to the stance phase in normal gait. Although some subjects showed a reduction of the EMG magnitude when arm motion was imposed, this was still consistent with locomotor-like motor output because the reduction of the EMG occurred during the forward swing phase corresponding to the swing phase. The present results indicate that the neural signal induced by the upper limb movements contributes not merely to enhance but also to shape the lower limb locomotive motor output, possibly through interlimb neural pathways. Such neural interaction between upper and lower limb motions could be an underlying neural mechanism of human bipedal locomotion.

Inter-enlargement pathways in the ventrolateral funiculus of the adult rat spinal cord

The ventrolateral funiculus (VLF) in the spinal cord contains important ascending and descending pathways related to locomotion and interlimb coordination. The primary purpose of this descriptive study was to investigate the distribution of inter-enlargement pathways in the adult rat spinal cord with an emphasis on the VLF. We made discrete unilateral injections of Fluoro-Gold (FG) into the right VLF at thoracic segment (T) 9, and either unilateral or bilateral injections of Fluoro-Ruby (FR) into the intermediate gray matter at the cervical (C) 5-6, C7-8, or lumbar (L) 2 segmental levels. Inter-enlargement neurons with ascending axons in the right VLF were found bilaterally in laminae VII and VIII throughout the rostral lumbar spinal cord (L1-L3) and predominantly contralaterally in the caudal lumbosacral (L4-S1) spinal cord. Following left unilateral FR injections at C5-6 or C7-8 and right unilateral VLF injections of FG at T9, very few double-labeled neurons could be found anywhere in the lumbar spinal cord. Similar injections of FR at L2 revealed an almost symmetrical bilateral distribution of double-labeled neurons throughout the cervical spinal cord (C1-8). These results describe ascending and descending pathways within the spinal cord that interconnect the two enlargements and involve both commissural and ipsilateral interneurons. The majority of inter-enlargement neurons had axons within the VLF at T9. These observations support the hypothesis that the VLF contains long ascending and descending axons with propriospinal inter-enlargement, commissural and ipsilateral connections that are anatomically well-suited to mediate interlimb coordination.

Magnetically evoked inter-enlargement response: an assessment of ascending propriospinal fibers following spinal cord injury

The aim of the present study was to characterize a novel electrophysiological assessment, the magnetically evoked interenlargement response (MIER), by defining the anatomical location of the fast conducting (large myelinated) ascending axons that mediate the response and the relationship between the response and locomotor function following experimental spinal cord injury. Electromyographic (EMG) responses were recorded from the triceps muscles following magnetic stimulation of one hip. Short-latency (approximately 6 ms) EMGs were recorded from triceps muscles in normal controls and following different laceration injuries (dorsal, lateral or dorsal and lateral hemisections) or a 150-kilodyne (kd) contusion injury at the T9 level. The amplitude of the triceps MIER was significantly correlated to the area of spared white matter in the lateral funiculus and to hindlimb function during open field locomotion (r2 = 0.55). Following a complete lateral hemisection, MIERs were present in the triceps bilaterally following stimulation of either hip. Responses could also be recorded from the masseter muscles indicating that the influence of this pathway extends beyond the spinal cord. Anatomical evidence of a bilaterally distributed propriospinal pathway was found when biotinylated dextran amine (BDA) was injected into the lateral white matter on one side of the spinal cord at T9. BDA-labeled axons with varicosities were found bilaterally in the intermediate and ventral gray matter of the caudal region of the cervical enlargement. These observations suggest that MIERs may be useful to quantitatively assess neurotransmission and functional recovery over time after experimental spinal cord injury.

Coordination of fore and hind leg stepping in cats on a transversely-split treadmill

To gain insight into the mechanism of coordination of stepping in the fore and hind legs of quadrupeds, we examined the kinematics of leg movements and the motor patterns in fore and hind leg flexor muscles in decerebrate walking cats when the two pairs of legs stepped on separate treadmills running at different speeds. When the front treadmill was slowed progressively from 0.6 to 0.3 m/s with the rear treadmill running at 0.6 m/s, the rate of stepping in both the fore and hind legs decreased and a 1:1 stepping ratio was maintained. The decrease in the rate of stepping in the hind legs was due primarily to an increase in the duration of the swing phase. Slowing the speed of the rear treadmill while keeping the front treadmill speed at 0.6 m/s decreased the rate of stepping of the hind legs, but had relatively little influence on the average rate of stepping in the forelegs. In this situation stepping in the fore and hind legs was uncoupled and the time of stepping in one hind leg relative to the ipsilateral foreleg progressively shifted during a walking sequence. Analysis of the timing of electromyographic (EMG) recordings from flexor muscles of the hip and elbow joints yielded insight into the neuronal mechanisms underlying the asymmetry in slowing either the front or rear treadmill. We propose that ipsilateral pattern generating networks are asymmetrically coupled via descending inhibitory pathways and an ascending excitatory pathway. We discuss how the characteristics of these linkages are functionally appropriate for establishing the normal timing of stepping in the hind and forelegs during slow walking.

Propriospinal neurons contribute to bulbospinal transmission of the locomotor command signal in the neonatal rat spinal cord

This study examines whether propriospinal transmission contributes to descending propagation of the brainstem locomotor command signal in the in vitro neonatal rat spinal cord. Using double bath partitions, synaptic transmission was suppressed in the cervicothoracic region while monitoring locomotor-like activity on lumbar ventral roots evoked by either chemical or electrical stimulation of the brainstem. Locomotor-like activity induced by electrical stimulation was more stable (cycle period coefficient of variation (CV) 11.7 +/- 6.1%) than the rhythm induced by chemical stimulation (CV 31.3 +/- 6.4%). Ca(2+)-free bath solution, elevated Mg(2+) ion concentration, excitatory amino acid receptor antagonists (AP5 and/or CNQX), and the muscarinic receptor antagonist, atropine, were used in attempts to block synaptic transmission. Each of these manipulations, except muscarinic receptor blockade, was capable of blocking locomotor-like activity induced by brainstem stimulation. However, locomotor-like activity induced by higher intensity electrical stimulation of the brainstem (1.2-5 times threshold) was relatively refractory to synaptic suppression using AP5 and CNQX, and Ca(2+)-free solution was more effective if combined with high Mg(2+) (15 mm) or EGTA. Enhancement of neuronal excitation in the cervicothoracic region, using Mg(2+)-free bath solution, facilitated brainstem activation of locomotor-like activity in the lumbar cord, consistent with a propriospinal mechanism of locomotor signal propagation. Blockade of brainstem-induced locomotor-like activity was related to the number of cervicothoracic segments exposed to synaptic suppression, being most effective if five or more segments were included. These results provide direct evidence that propriospinal pathways contribute to bulbospinal activation of the locomotor network in the in vitro neonatal rat brainstem-spinal cord preparation, and suggest that a propriospinal system is recruited in parallel with long direct projections that activate the locomotor network.

Differential vulnerability of propriospinal tract neurons to spinal cord contusion injury

The propriospinal system is important in mediating reflex control and in coordination during locomotion. Propriospinal neurons (PNs) present varied patterns of projections with ascending and/or descending fibers. Following spinal cord contusion injury (SCI) in the rat, certain supraspinal pathways, such as the corticospinal tract, appear to be completely abolished, whereas others, such as the rubrospinal and vestibuospinal tracts, are only partially damaged. The amount of damage to propriospinal axons following different severities of SCI is not fully known. In the present study retrograde and anterograde tracing techniques were used to assess the projection patterns of propriospinal neurons in order to determine how this system is affected following SCI. Our findings reveal that PNs have differential vulnerabilities to SCI. While short thoracic propriospinal axons are severely damaged after injury, 5-7% of long descending propriospinal tract (LDPT) projections survive following 50 and 12.5-mm weight drop contusion lesions, respectively, albeit with a reduced intensity of retrograde label. Even though the axons of short thoracic propriospinal cells are damaged, their cell bodies of origin remain intact 2 weeks after injury, indicating that they have not undergone postaxotomy retrograde cell death at this time point. Thus, short PNs may constitute a very attractive population of cells to study regenerative approaches, whereas LDPT neurons with spared axons could be targeted with therapeutic interventions, seeking to enhance recovery of function following incomplete lesions to the spinal cord.

Commissural propriospinal connections between the lateral aspects of laminae III-IV in the lumbar spinal cord of rats

It has been established that there is a strong functional link between sensory neural circuits on the two sides of the spinal cord. In one of our recent studies we provided a morphological confirmation of this functional phenomenon, presenting evidence for the presence of a direct commissural connection between the lateral aspects of the dorsal horn on the two sides of the lumbar spinal cord. By using a combination of neural tracing and immunocytochemical detection of neural markers like vesicular glutamate transporters, glutamic acid decarboxylase, glycine transporter, and met-enkephalin (which are characteristic of various subsets of excitatory and inhibitory neurons), we investigated here the distribution, synaptic relations, and neurochemical characteristics of the commissural axon terminals. We found that the cells of origin of commissural fibers in the lateral aspect of the dorsal horn were confined to laminae III-IV and projected to the corresponding area of the contralateral gray matter. Most of the commissural axon terminals established synaptic contacts with dendrites. Axospinous or axosomatic synaptic contacts were found in limited numbers. We demonstrated that interactions among commissural neurons also exist. More than three-fourths of the labeled axon terminals were immunostained for glutamic acid decarboxylase and/or glycine transporter, but none of them showed positive immunoreaction for met-enkephalin and vesicular glutamate transporters. The results indicate that there is a substantial reciprocal commissural synaptic interaction between the lateral aspects of laminae III-IV on the two sides of the lumbar spinal cord and that this pathway may transmit both inhibitory and excitatory signals to their postsynaptic targets.

Interlimb reflex activity after spinal cord injury in man: strengthening response patterns are consistent with ongoing synaptic plasticity

OBJECTIVE

Previous reports from our laboratory have described short-latency contractions in muscles of the distal upper limb following stimulation of lower limb nerves or skin in persons with injury to the cervical spinal cord. It takes 6 or more months for interlimb reflexes (ILR) to appear following acute spinal cord injury (SCI), suggesting they might be due to new synaptic interconnections between lower limb sensory afferents and motoneurons in the cervical enlargement. In this study, we asked if once formed, the strength of these synaptic connections increased over time, a finding that would be consistent with the above hypothesis.

METHODS

We studied persons with sub-acute and/or chronic cervical SCI. ILR were elicited by brief trains of electrical pulses applied to the skin overlying the tibial nerve at the back of the knee. Responses were quantified based on their presence or absence in different upper limb muscles. We also generated peri-stimulus time histograms for single motor unit response latency, probability, and peak duration. Comparisons of these parameters were made in subjects at sub-acute versus chronic stages post-injury.

RESULTS

In persons with sub-acute SCI, the probability of seeing ILR in a given muscle of the forearm or hand was low at first, but increased substantially over the next 1-2 years. Motor unit responses at this sub-acute stage had a prolonged and variable latency, with a lower absolute response probability, compared to findings from subjects with chronic (i.e. stable) SCI.

CONCLUSIONS

Our findings demonstrate that interlimb reflex activity, once established after SCI, shows signs of strengthening synaptic contacts between afferent and efferent components, consistent with ongoing synaptic plasticity.

SIGNIFICANCE

Neurons within the adult human spinal cord caudal to a lesion site are not static, but appear to be capable of developing novel-yet highly efficacious-synaptic contacts following trauma-induced partial denervation. In this case, such contacts between ascending afferents and cervical motoneurons do not appear to provide any functional benefit to the subject. In fact their presence may limit the regenerative effort of supraspinal pathways which originally innervated these motoneurons, should effort in animal models to promote regeneration across the lesion epicenter be successfully translated to humans with chronic SCI.

Regulation of arm and leg movement during human locomotion

Walking can be a very automated process, and it is likely that central pattern generators (CPGs) play a role in the coordination of the limbs. Recent evidence suggests that both the arms and legs are regulated by CPGs and that sensory feedback also regulates the CPG activity and assists in mediating interlimb coordination. Although the strength of coupling between the legs is stronger than that between the arms, arm and leg movements are similarly regulated by CPG activity and sensory feedback (e.g., reflex control) during locomotion.

Neural coupling between upper and lower limbs during recumbent stepping

During gait rehabilitation, therapists or robotic devices often supply physical assistance to a patient's lower limbs to aid stepping. The expensive equipment and intensive manual labor required for these therapies limit their availability to patients. One alternative solution is to design devices where patients could use their upper limbs to provide physical assistance to their lower limbs (i.e., self-assistance). To explore potential neural effects of coupling upper and lower limbs, we investigated neuromuscular recruitment during self-driven and externally driven lower limb motion. Healthy subjects exercised on a recumbent stepper using different combinations of upper and lower limb exertions. The recumbent stepper mechanically coupled the upper and lower limbs, allowing users to drive the stepping motion with upper and/or lower limbs. We instructed subjects to step with 1) active upper and lower limbs at an easy resistance level (active arms and legs); 2) active upper limbs and relaxed lower limbs at easy, medium, and hard resistance levels (self-driven); and 3) relaxed upper and lower limbs while another person drove the stepping motion (externally driven). We recorded surface electromyography (EMG) from six lower limb muscles. Self-driven EMG amplitudes were always higher than externally driven EMG amplitudes ( P < 0.05). As resistance and upper limb exertion increased, self-driven EMG amplitudes also increased. EMG bursts during self-driven and active arms and legs stepping occurred at similar times. These results indicate that active upper limb movement increases neuromuscular activation of the lower limbs during cyclic stepping motions. Neurologically impaired humans that actively engage their upper limbs during gait rehabilitation may increase neuromuscular activation and enhance activity-dependent plasticity.

Possible contributions of CPG activity to the control of rhythmic human arm movement

There is extensive modulation of cutaneous and H-reflexes during rhythmic leg movement in humans. Mechanisms controlling reflex modulation (e.g., phase- and task-dependent modulation, and reflex reversal) during leg movements have been ascribed to the activity of spinal central pattern generating (CPG) networks and peripheral feedback. Our working hypothesis has been that neural mechanisms (i.e., CPGs) controlling rhythmic movement are conserved between the human lumbar and cervical spinal cord. Thus reflex modulation during rhythmic arm movement should be similar to that for rhythmic leg movement. This hypothesis has been tested by studying the regulation of reflexes in arm muscles during rhythmic arm cycling and treadmill walking. This paper reviews recent studies that have revealed that reflexes in arm muscles show modulation within the movement cycle (e.g., phase-dependency and reflex reversal) and between static and rhythmic motor tasks (e.g., task-dependency). It is concluded that reflexes are modulated similarly during rhythmic movement of the upper and lower limbs, suggesting similar motor control mechanisms. One notable exception to this pattern is a failure of contralateral arm movement to modulate reflex amplitude, which contrasts directly with observations from the leg. Overall, the data support the hypothesis that CPG activity contributes to the neural control of rhythmic arm movement.Key words: central pattern generator, locomotion, motor control, neural control.

Effect of Rhythmic Arm Movement on Reflexes in the Legs: Modulation of Soleus H-Reflexes and Somatosensory Conditioning

During locomotor tasks such as walking, running, and swimming, the arms move rhythmically with the legs. It has been suggested that connections between the cervical and lumbosacral spinal cord may mediate some of this interlimb coordination. However, it is unclear how these interlimb pathways modulate reflex excitability during movement. We hypothesized that rhythmic arm movement would alter the gain of reflex pathways in the stationary leg. Soleus H-reflexes recorded during arm cycling were compared with those recorded at similar positions with the arms stationary. Nerve stimulation was delivered with the right arm at approximately 70° shoulder flexion or 10° shoulder extension. H-reflexes were evoked alone (unconditioned) or with sural or common peroneal nerve (CP) conditioning to decrease or increase soleus IA presynaptic inhibition, respectively. Both conditioning stimuli were also delivered with no H-reflex stimulation. H-reflex amplitudes were compared at similar M-wave amplitudes and activation levels of the soleus. Arm cycling significantly reduced ( P < 0.05) unconditioned soleus H-reflexes at shoulder flexion by 21.7% and at shoulder extension by 8.8% compared with static controls. The results demonstrate a task-dependent modulation of soleus H-reflexes between arm cycling and stationary trials. Sural nerve stimulation facilitated H-reflexes at shoulder extension but not at shoulder flexion during static and cycling trials. CP nerve stimulation significantly reduced H-reflex amplitude in all conditions. Reflexes in soleus when sural and CP nerve stimulation were delivered alone, were not different between cycling and static trials; thus the task-dependent change in H reflex amplitude was not due to changes in motoneuron excitability. Therefore modulation occurred at a pre-motoneuronal level, probably by presynaptic inhibition of the IA afferent volley. Results indicate that neural networks coupling the cervical and lumbosacral spinal cord in humans are activated during rhythmic arm movement. It is proposed that activation of these networks may assist in reflex linkages between the arms and legs during locomotor tasks.

Premotor nitric oxide synthase immunoreactive pathway connecting lumbar segments with the ventral motor nucleus of the cervical enlargement in the dog

In this study we investigate the occurrence and origin of punctate nitric oxide synthase immunoreactivity in the neuropil of the ventral motor nucleus in C7-Th1 segments of the dog spine, which are supposed to be the terminal field of an ascending premotor propriospinal nitric oxide synthase-immunoreactive pathway. As the first step, nitric oxide synthase immunohistochemistry was used to distinguish nitric oxide synthase-immunoreactive staining of the ventral motor nucleus. Dense, punctate nitric oxide synthase immunoreactivity was found on control sections in the neuropil of the ventral motor nucleus. After hemisection at Th10-11, axotomy-induced retrograde changes consisting in a strong upregulation of nitric oxide synthase-containing neurons were found mostly unilaterally in lamina VIII, the medial part of lamina VII and in the pericentral region in all segments of the lumbosacral enlargement. Concurrently, a strong depletion of the punctate nitric oxide synthase immunopositivity in the neuropil of the ventral motor nucleus ipsilaterally with the hemisection was detected, thus revealing that an uncrossed ascending premotor propriospinal pathway containing a fairly high number of nitric oxide synthase-immunoreactive fibers terminates in the ventral motor nucleus. Application of the retrograde fluorescent tracer Fluorogold injected into the ventral motor nucleus and analysis of alternate sections processed for nitric oxide synthase immunocytochemistry revealed the presence of Fluorogold-labeled and nitric oxide synthase-immunoreactive axons in the ventrolateral funiculus and in the lateral and medial portions of the ventral column throughout the thoracic and upper lumbar segments. A noticeable number of Fluorogold-labeled and nitric oxide synthase-immunoreactive somata detected on consecutive sections were found in the lumbosacral enlargement, mainly in laminae VIII-IX, the medial part of lamina VII and in the pericentral region (lamina X), ipsilaterally with the injection of Fluorogold into the ventral motor nucleus. In summary, the present study provides evidence for a hitherto unknown ascending premotor propriospinal nitric oxide synthase-immunoreactive pathway connecting the lumbosacral enlargement with the motoneurons of the ventral motor nucleus in the dog.

Synaptic inputs from low threshold afferents of trunk muscles to motoneurons innervating the longissimus lumborum muscle in the spinal cat

(1) We studied the reflex actions of group I and II afferents to longissimus lumborum (Long) motoneurons in the L1-L5 spinal segments from the epaxial muscle, m. Long, and the hypaxial muscle, m. obliquus externus abdominus (OEA). (2) Postsynaptic potentials (PSPs) recorded from 140 Long motoneurons in 30 spinal cats were analyzed. Under the present experimental conditions, the stimulation of Long and OEA nerves at an intensity below 1.5 times threshold (T) activated only group I muscle afferents, while stimulations at 2-5T activated group II muscle afferents as well. (3) The incidence of PSPs was related to the proximity of the spinal segments of the nerves stimulated to the spinal segment of the motoneurons; the shorter the distance the larger the PSPs and higher incidence of PSPs. The Long motoneurons received group I afferent input mainly from the same and adjacent segments, and received group II afferent inputs from a wider range of segments. (4) A short (i.e., less than 1.0 ms) latency of excitatory PSPs (EPSPs) evoked by ipsilateral group I afferents of Long at the same or adjacent segment indicated a monosynaptic connection. In general, the central latencies became longer as the distance between spinal segments of stimulated nerves and motoneurons increased. Major PSP components were produced by polysynaptic neuronal pathways. The spatial facilitation between PSPs evoked by afferents of different nerves (i.e., ipsilateral Long (iLong) and contralateral Long (cLong) of the same segment; iLongs of different segments; and iLong and iOEA of the same segment) indicated that they shared common interneurons. (5) Although iLong and iOEA muscle afferents produced predominantly EPSPs, and contralateral muscle afferents elicited predominantly IPSPs in Long motoneurons at each spinal segment, the patterns of convergence from Long and OEA muscle afferents of different spinal segments and of different sides differed considerably among motoneurons. (6) These findings demonstrated various input patterns of individual motoneurons within the same motoneuron pool, which might reflect the complexity of neuronal control of the back muscles for various trunk movements, including lateral and dorsal bending, rotating, and fixation of the trunk.

Simultaneous movements of upper and lower limbs are coordinated by motor representations that are shared by both limbs: a PET study

The purpose of this study was to examine the cerebral control of simultaneous movements of the upper and lower limbs. We examined two hypotheses on how the brain coordinates movement: (i) by the involvement of motor representations shared by both limbs; or (ii) by the engagement of specific neural populations. We used positron emission tomography to measure the relative cerebral blood flow in healthy subjects performing isolated cyclic flexion-extension movements of the wrist and ankle (i.e. movements of wrist or ankle alone), and simultaneous movements of the wrist and ankle (a rest condition was also included). The simultaneous movements were performed in the same directions (iso-directional) and in opposite directions (antidirectional). There was no difference in the brain activity between these two patterns of coordination. In several motor-related areas (e.g. the contralateral ventral premotor area, the dorsal premotor area, the supplementary motor area, the parietal operculum and the posterior parietal cortex), the representation of the isolated wrist movement overlapped with the representation of the isolated ankle movement. Importantly, the simultaneous movements activated the same set of motor-related regions that were active during the isolated movements. In the contralateral ventral premotor cortex, dorsal premotor cortex and parietal operculum, there was less activity during the simultaneous movements than for the sum of the activity for the two isolated movements (interaction analysis). Indeed, in the ventral premotor cortex and parietal operculum, the activity was practically identical regardless whether only the wrist, only the ankle, or both the wrist and the ankle were moved. Taken together, these findings suggest that interlimb coordination is mediated by motor representations shared by both limbs, rather than being mediated by specific additional neural populations.

Lamina VII and VIII neurons of the S2 segment bilaterally projecting to the C6 segment of the spinal cord in the cat

Intracellular and extracellular recordings of antidromic action potentials were applied to investigate neurons of the S2 segment projecting to the C6 segment of the cat spinal cord. The cell bodies were located in laminae VII and VIII of the gray matter while axons ascended in lateral funiculi. Thirty-two out of the total 45 neurons were found to project to the C6 segment bilaterally, seven ipsilaterally and six contralaterally. The axonal conduction velocities were in the 42-96 m/s range and in some neurons were significantly lower in distal parts of axons, supposing that some neurons may give off collateral branches to various segments of the spinal cord. It is discussed if the investigated neurons form a part of the propriospinal system or if their cervical projections are only collaterals of long tracts ascending to supraspinal levels. The organisation of the presented connections between spinal enlargements indicates their contribution in complex mechanisms of co-ordination of movements of the limbs.