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      The sacral networks and neural pathways used to elicit lumbar motor rhythm in the rodent spinal cord

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          Abstract

          Identification of neural networks and pathways involved in activation and modulation of spinal central pattern generators (CPGs) in the absence of the descending control from the brain is important for further understanding of neural control of movement and for developing innovative therapeutic approaches to improve the mobility of spinal cord injury patients. Activation of the hindlimb innervating segments by sacrocaudal (SC) afferent input and by specific application of neurochemicals to the sacral networks is feasible in the isolated spinal cord preparation of the newborn rat. Here we review our recent studies of sacral relay neurons with lumbar projections and evaluate their role in linking the sacral and thoracolumbar (TL) networks during different motor behaviors. Our major findings show that: (1) heterogeneous groups of dorsal, intermediate and ventral sacral-neurons with ventral and lateral ascending funicular projections mediate the activation of the locomotor CPGs through sacral sensory input; and (2) rhythmic excitation of lumbar flexor motoneurons, produced by bath application of alpha-1 adrenoceptor agonists to the sacral segments is mediated exclusively by ventral clusters of sacral-neurons with lumbar projections through the ventral funiculus.

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          Genetic identification of spinal interneurons that coordinate left-right locomotor activity necessary for walking movements.

          The sequential stepping of left and right limbs is a fundamental motor behavior that underlies walking movements. This relatively simple locomotor behavior is generated by the rhythmic activity of motor neurons under the control of spinal neural networks known as central pattern generators (CPGs) that comprise multiple interneuron cell types. Little, however, is known about the identity and contribution of defined interneuronal populations to mammalian locomotor behaviors. We show a discrete subset of commissural spinal interneurons, whose fate is controlled by the activity of the homeobox gene Dbx1, has a critical role in controlling the left-right alternation of motor neurons innervating hindlimb muscles. Dbx1 mutant mice lacking these ventral interneurons exhibit an increased incidence of cobursting between left and right flexor/extensor motor neurons during drug-induced locomotion. Together, these findings identify Dbx1-dependent interneurons as key components of the spinal locomotor circuits that control stepping movements in mammals.
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            Distribution of networks generating and coordinating locomotor activity in the neonatal rat spinal cord in vitro: a lesion study.

            The isolated spinal cord of the newborn rat contains networks that are able to create a patterned motor output resembling normal locomotor movements. In this study, we sought to localize the regions of primary importance for rhythm and pattern generation using specific mechanical lesions. We used ventral root recordings to monitor neuronal activity and tested the ability of various isolated parts of the caudal thoraciclumbar cord to generate rhythmic bursting in a combination of 5-HT and NMDA. In addition, pathways mediating left/right and rostrocaudal burst alternation were localized. We found that the isolated ventral third of the spinal cord can generate normally coordinated rhythmic activity, whereas lateral fragments resulting from sagittal sections showed little or no rhythmogenic capability compared with intact control preparations. The ability to generate fast and regular rhythmic activity decreased in the caudal direction, but the rhythm-generating network was found to be distributed over the entire lumbar region and to extend into the caudal thoracic region. The pathways mediating left/ right alternation exist primarily in the ventral commissure. As with the rhythmogenic ability, these pathways were distributed along the lumbar enlargement. Both lateral and ventral funiculi were sufficient to coordinate activity in the rostral and caudal regions. We conclude that the networks organizing locomotor-related activity in the spinal cord of the newborn rat are distributed.
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              On the central generation of locomotion in the low spinal cat.

              A central network of neurones in the spinal cord has been shown to produce a rhythmic motor output similar to locomotion after suppression of all afferent inflow. The experiments were performed mainly in acute spinal cats (th. 12), which had received DOPA i.v. and the monoamine oxidase inhibitor Nialamide. In some preparations all dorsal roots supplying the spinal cord were transected, in others phasic afferent activity was suppressed by curarization. The activity was recorded as neurograms from nerve filaments or as electromyograms. It is concluded that: 1. alternating activity between flexors and extensors of foot, ankel, knee, and hip of one limb can still occur 2. the duration of the flexor discharges vary less with the cycle duration than the extensor discharges 3. different flexor muscles may retain individual patterns 4. the activity at different joints can be dissociated 5. there is at least one network for each limb. 6. the coordination between the two hindlimbs can be alternating as in walking or be more closely spaced as in galloping 7. alternating activity in the ankle remains even when only segments L6, L7 and S1 are intact.
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                Author and article information

                Contributors
                Journal
                Front Neural Circuits
                Front Neural Circuits
                Front. Neural Circuits
                Frontiers in Neural Circuits
                Frontiers Media S.A.
                1662-5110
                03 December 2014
                2014
                : 8
                : 143
                Affiliations
                [1]Department of Medical Neurobiology, Institute for Medical Research—Israel-Canada, IMRIC, The Hebrew University Medical School Jerusalem, Israel
                Author notes

                Edited by: Brian R. Noga, University of Miami, USA

                Reviewed by: Uner Tan, Cukurova University, Turkey; Patrick John Whelan, University of Calgary, Canada; Kristine C. Cowley, University of Manitoba, Canada

                *Correspondence: Aharon Lev-Tov, Department of Medical Neurobiology, Institute for Medical Research – Israel-Canada, IMRIC, The Hebrew University Medical School, P.O. Box 12272, Jerusalem 9112102, Israel e-mail: aharonl@ 123456ekmd.huji.ac.il

                This article was submitted to the journal Frontiers in Neural Circuits.

                Article
                10.3389/fncir.2014.00143
                4253665
                25520624
                70a27fe0-3a8f-47ce-8945-a7837609b6cd
                Copyright © 2014 Cherniak, Etlin, Strauss, Anglister and Lev-Tov.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution and reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                History
                : 16 June 2014
                : 11 November 2014
                Page count
                Figures: 4, Tables: 0, Equations: 0, References: 65, Pages: 8, Words: 6605
                Categories
                Neuroscience
                Review Article

                Neurosciences
                sacrocaudal afferents,calcium imaging,spinal interneurons,ascending pathways,central pattern generators,locomotor,alpha1 adrenoceptors

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