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The Spinal Control of Backward Locomotion Research Articles: Systems/Circuits The spinal control of backward locomotion https://doi.org/10.1523/JNEUROSCI.0816-20.2020 Cite as: J. Neurosci 2020; 10.1523/JNEUROSCI.0816-20.2020 Received: 8 April 2020 Revised: 16 November 2020 Accepted: 18 November 2020 This Early Release article has been peer-reviewed and accepted, but has not been through the composition and copyediting processes. The final version may differ slightly in style or formatting and will contain links to any extended data. Alerts: Sign up at www.jneurosci.org/alerts to receive customized email alerts when the fully formatted version of this article is published. Copyright © 2020 the authors 1 Title: The spinal control of backward locomotion 2 Abbreviated title: The spinal control of backward locomotion 3 4 Author names: 1Jonathan Harnie, 1Johannie Audet, 2Alexander N. Klishko, 1Adam Doelman, 5 2,#Boris I. Prilutsky and 1,#Alain Frigon 6 7 Affiliations: 1Department of Pharmacology-Physiology, Faculty of Medicine and Health 8 Sciences, Université de Sherbrooke, Sherbrooke, Quebec, Canada, J1H 5N4. 2School of 9 Biological Sciences, Georgia Institute of Technology, Atlanta, GA, USA, 30332. 10 # Equally contributing senior authors 11 Corresponding author: 12 Alain Frigon, PhD 13 Email: [email protected] 14 Number of pages: 45 pages of text + 1 table and 9 figures 15 Number of tables and figures: 1 table + 9 figures 16 Number of words: abstract (235), introduction (757), and discussion (2050) 17 Conflict of interest statement: The authors declare no competing financial interests. 18 19 Acknowledgments 20 We thank Philippe Drapeau (Université de Montréal) from the Rossignol and Drew labs for 21 developing the data collection and analysis software. The present work was supported by 22 grants from the Canadian Institutes of Health Research (PJT-156296) and the Natural 23 Sciences and Engineering Research Council of Canada (RGPIN-2016-03790) to A. Frigon, 24 by a National Institutes of health Grant R01 NS110550 to A. Frigon and B.I. Prilutsky, and by 25 a National Institute of Neurological Disorders and Stroke Grant NS100928 to B.I. Prilutsky. 26 27 Competing interests 28 The authors declare no competing interests. 1 29 Abstract 30 Animal locomotion requires changing direction, from forward to backward. Here, we tested 31 the hypothesis that sensorimotor circuits within the spinal cord generate backward 32 locomotion and adjust it to task demands. We collected kinematic and electromyography 33 data during forward and backward locomotion at different treadmill speeds before and after 34 complete spinal transection in six adult cats (3 males and 3 females). After spinal 35 transection, 5/6 cats performed backward locomotion, which required tonic somatosensory 36 input in the form of perineal stimulation. One spinal cat performed forward locomotion but not 37 backward locomotion while two others stepped backward but not forward. Spatiotemporal 38 adjustments to increasing speed were similar in intact and spinal cats during backward 39 locomotion and strategies were similar to forward locomotion, with shorter cycle and stance 40 durations and longer stride lengths. Patterns of muscle activations, including muscle 41 synergies, were similar for forward and backward locomotion in spinal cats. Indeed, we 42 identified five muscle synergies that were similar during forward and backward locomotion. 43 Lastly, spinal cats also stepped backward on a split-belt treadmill, with the left and right 44 hindlimbs stepping at different speeds. Therefore, our results show that spinal sensorimotor 45 circuits generate backward locomotion but require additional excitability compared to forward 46 locomotion. Similar strategies for speed modulation and similar patterns of muscle 47 activations and muscle synergies during forward and backward locomotion are consistent 48 with a shared spinal locomotor network, with sensory feedback from the limbs controlling the 49 direction. 50 51 Keywords: Locomotor direction, spinal cord, sensory feedback, muscle synergies, central 52 pattern generator 53 54 Significance statement 55 Animal locomotion requires changing direction, including forward, sideways and backward. 56 This paper shows that the center controlling locomotion within the spinal cord can produce a 2 57 backward pattern when instructed by sensory signals from the limbs. However, the spinal 58 locomotor network requires greater excitability to produce backward locomotion compared to 59 forward locomotion. The paper also shows that the spinal network controlling locomotion in 60 the forward direction also controls locomotion in the backward direction. 61 62 INTRODUCTION 63 Locomotion in different directions is essential when adjusting to the environment and for 64 goal-oriented behaviors. Although dynamic sensorimotor interactions between supraspinal 65 inputs, spinal circuits and somatosensory feedback generate locomotion (Rossignol et al., 66 2006), their relative contributions in different directions is not well understood. A specialized 67 network within the spinal cord termed the locomotor central pattern generator (CPG) 68 produces the basic pattern for forward locomotion [reviewed in (Grillner, 1981;Kiehn, 69 2011;Kiehn, 2016;McCrea and Rybak, 2008;Grillner and El Manira, 2015)]. In cats with a 70 complete spinal transection (i.e. spinal cats), hindlimb locomotion recovers and adjusts to 71 different treadmill speeds and to different speeds for the left and right hindlimbs on a split- 72 belt treadmill (Forssberg et al., 1980;Frigon et al., 2013;Frigon et al., 2017). This indicates 73 that sensory feedback from the limbs adjusts the output of the spinal locomotor CPG for 74 different forward speeds and directions. 75 What about locomotion in the backward direction? The neural control of backward 76 locomotion has been investigated in intact or decerebrate cats (Buford and Smith, 77 1993;Buford and Smith, 1990;Buford et al., 1990;Perell et al., 1993;Pratt et al., 1996;Trank 78 and Smith, 1996;Merkulyeva et al., 2018;Musienko et al., 2012;Deliagina et al., 79 2019;Musienko et al., 2007;Zelenin et al., 2011), and humans (Choi and Bastian, 80 2007;Duysens et al., 1996;Grasso et al., 1998;Hoogkamer et al., 2014;Ivanenko et al., 81 2008;Mahaki et al., 2017;Thorstensson, 1986;Winter et al., 1989). Studies in cats and 82 humans have shown that backward locomotor kinematics are mainly a reversal of forward 83 locomotion (Grasso et al., 1998;Winter et al., 1989;Buford et al., 1990;Viviani et al., 84 2011;Thorstensson, 1986). While some have proposed that distinct functional networks 3 85 generate forward and backward locomotion (Choi and Bastian, 2007;Mahaki et al., 2017), 86 others have argued that they share common neural circuits (Buford and Smith, 1990;Winter 87 et al., 1989;Grillner, 1985;Grillner, 1981;Musienko et al., 2007;Duysens et al., 88 1996;Hoogkamer et al., 2014;Lamb and Yang, 2000;Matsushima and Grillner, 89 1992;Cappellini et al., 2018). However, whether spinal locomotor networks without inputs 90 from the brain can generate backward locomotion has received little to no attention. Human 91 infants, with immature descending supraspinal pathways, can generate treadmill locomotion 92 in forward and backward directions when supported (Lamb and Yang, 2000). In spinal 93 animals, two studies reported that spinal kittens and adult cats could not perform backward 94 locomotion on a treadmill (Robinson and Goldberger, 1986;Buford and Smith, 1990). 95 However, two other studies showed hindlimb muscle activity and stance phases during 96 backward locomotion on a treadmill in spinal animals that received pharmacological 97 treatment and/or electrical epidural stimulation of the spinal cord (Barbeau et al., 98 1987;Courtine et al., 2009). The first study showed backward locomotion in a spinal cat 99 treated with clonidine, an α-2 noradrenergic agonist, and the other in a spinal rat treated with 100 quipazine, a serotonergic agonist, that stepped in an upright position with epidural electrical 101 spinal cord stimulation. Although these studies showed that spinal animals generated 102 backward stepping movements on a treadmill, which was not their main goal, several 103 questions remain unexplored. 104 In the present study, we ask four basic questions that address different fundamental 105 aspects of the spinal control of backward locomotion. 1) Can spinal cats produce backward 106 locomotion without increasing spinal cord excitability pharmacologically? We confirm and 107 extend findings of Barbeau et al. (1987) in a larger sample of spinal cats that they can 108 generate backward locomotion. However, we found backward locomotion required perineal 109 stimulation, which increases spinal excitability through an undefined mechanism (Rossignol 110 et al., 2006;Alluin et al., 2015;Harnie et al., 2019). 2) Can backward locomotion in spinal cats 111 adjust to different treadmill speeds and to different speeds for the left and right hindlimbs on 112 a split-belt treadmill? We test the hypothesis that sensory feedback from the limbs adjust the 4 113 output of the spinal CPG for backward locomotion to meet task demands. 3) Are 114 spatiotemporal parameters, such as cycle and phase durations, stride and step lengths and 115 paw placements, during backward locomotion similar in intact and spinal cats? Here, we test 116 the hypothesis that sensorimotor circuits within the spinal cord adjust spatiotemporal 117 parameters for backward locomotion. 4) Are spatiotemporal adjustments and
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