15
views
0
recommends
+1 Recommend
0 collections
    0
    shares
      • Record: found
      • Abstract: found
      • Article: found
      Is Open Access

      Response: Commentary: Elimination of Left-Right Reciprocal Coupling in the Adult Lamprey Spinal Cord Abolishes the Generation of Locomotor Activity

      article-commentary
      1 , 2 , *
      Frontiers in Neural Circuits
      Frontiers Media S.A.
      locomotion, central pattern generators, oscillators, coordination, coupling

      Read this article at

      Bookmark
          There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.

          Abstract

          In the CNS, central pattern generators (CPGs) produce the basic temporal pattern of electrical motor activity that produces rhythmic behaviors such as locomotion. A major issue in motor control is whether CPGs are organized into distinct modules that can function autonomously or if the different CPG modules are interdependent (Figure 1 in Messina et al., 2017). Regarding the previous commentary (Cangiano and Grillner, 2018), there are at least two relevant questions concerning the locomotor rhythm generating capabilities of CPG modules in right and left hemi-spinal cords in adult lampreys: (a) Does the burst activity generated by hemi-spinal cords represent well-coordinated locomotor activity? (b) Does generation of burst activity by hemi-spinal cords represent a normal physiological capability of these spinal circuits, or is this capability manifested under certain, possibly artificial, experiment conditions? In the text below, all page numbers and figures refer to those in our previous study (Messina et al., 2017). For our previous study, a longitudinal midline (ML) lesion was performed along the rostral spinal cord of adult lampreys, and rostral muscle activity (adjacent to ML spinal lesion) and caudal muscle activity (below ML spinal lesion) were recorded in response to sensory stimulation of the anterior head (Figure 3A). The rationale was that initiation of spinal motor activity in whole animals in response to sensory stimulation and descending brainstem activation would provide the most physiological situation possible in which to test the rhythm generating capabilities of hemi-spinal cords (also see Jackson et al., 2005). First, under these conditions, sensory stimulation elicited coordinated rostral and caudal locomotor muscle burst activity (Figure 3B1), but with somewhat higher than normal burst proportions (BP) and intersegmental rostrocaudal phase lags (ΦINT) (Figure 3C). Second, following a subsequent spinal transection at the caudal end of the ML lesion in the same animals (Figure 3A), thereby isolating rostral right and left hemi-spinal cords from intact caudal cord, sensory stimulation usually elicited tonic, unpatterned rostral muscle activity (Figure 3B2). However, for ~30% of the trials, stimulation could elicit relatively high frequency rostral ipsilateral “burstlet” activity (Figures 5–7, 9). Importantly, there are at least six features of “burstlet” activity suggesting that it does not represent well-coordinated locomotor activity (p. 14). Our muscle “burstlet” activity (Messina et al., 2017) and the in vitro “high-frequency rhythm” that is assumed to represent locomotor activity (Cangiano and Grillner, 2003, 2005) have some significant differences, and it is uncertain if they are equivalent. In the previous commentary (Cangiano and Grillner, 2018), there were several incomplete, misleading, or incorrect statements. First, the commentary stated that stimulation of the in vitro lamprey hemi-spinal cord initiates a “bout of locomotor burst activity” consisting of “well-coordinated bursting.” However, the term “well-coordinated locomotor activity” embodies more than just burst frequency, but also should include data for BP and ΦINT (Wallén and Williams, 1984; Boyd and McClellan, 2002). Because BP and ΦINT are obvious and easy parameters to measure, it is surprising that these parameters were not reported in the previous studies (Cangiano and Grillner, 2003, 2005). For the “burstlet” activity in our study, plots of BP vs. cycle time (CT) usually were abnormal (Figures 6A2–C2), and phase lags usually were absent (Figure 9; p. 9, lower right column). Second, the previous commentary stated that prior to a more caudal spinal transection in our study, the muscle burst activity produced by rostral hemi-spinal cords was “driven from the intact [caudal] part of the spinal cord.” However, in our experiments under these conditions, average ΦINT values were positive (Figures 3B1, 3C3) and did not change significantly with CT (unpublished), suggesting that intact caudal spinal circuits were not driving (activating) bursts in the rostral hemi-spinal cords. Also, experimental evidence does not support this proposed activation mechanism in the lamprey spinal cord. Lastly, BP and ΦINT values for rostral muscle burst activity (adjacent to spinal ML lesion) were only moderately higher than those during normal swimming (Figures 3C2, 8; p. 9), suggesting that the CPGs in rostral hemi-spinal cords were functional when connected to intact caudal spinal locomotor circuitry. Third, the previous commentary stated that in our study, the presence of rostral muscle burst activity prior to a more caudal spinal transection (Figures 3A, 3B1) was not a convincing test of the integrity of the CPGs in rostral hemi-spinal cords. They suggested that the mini-scalpel blade we used to make ML spinal lesions might have damaged the spinal cord, and they showed a misleading image of the “dorsal view” of this blade (i.e., wider, non-cutting edge). (a) The spinal cords of animals in our study were ~200–300 μm thick (~1 mm wide), only the very tip of this blade was used for cutting, and right and left halves of the spinal cord often separated slightly during ML lesioning. (b) For ~35% of the experiments in Cangiano and Grillner (2003), ML spinal lesions were made with a 0.4 mm dia. needle, which is more blunt and wider than the tip of the blade we used, yet in their previous study there were no reported experimental differences using the needle compared to using an ophthalmic blade. Fourth, the previous commentary stated that the frequencies of muscle “burstlet” activity for our experiments (Messina et al., 2017) were “somewhat higher” than those obtained for their in vitro experiments (Cangiano and Grillner, 2003, 2005). However, for our experiments, the average (~25 Hz) and maximum (>60 Hz) “burstlet” frequencies were about 4–5 times higher than those observed for their in vitro experiments (~6 and ~12–15 Hz, respectively). (Note: the somewhat higher burst frequencies in Cangiano et al., 2012 presumably occurred because recordings were made within minutes after performing ML lesions, when the excitability of hemi-spinal cords probably was very high.) Also, the average and maximum “burstlet” frequencies in our study were ~6 and ~10 times higher than the average and maximum muscle burst frequencies, respectively, during swimming for normal whole animals (McClellan et al., 2016). The relatively high “burstlet” frequencies appear to be due to changes in the properties of hemi-spinal cords, such as an increase in excitability because of the lack of left-right coupling (p. 13) and possibly lesion-induced cellular and synaptic plasticity (Hoffman and Parker, 2010). Fifth, the previous commentary stated that the “isolated condition [in vitro spinal cord] is a much cleaner situation” than in our whole-animal experiments. However, we are not convinced that synchronous, non-specific, high-frequency (33 Hz) stimulation of the surface of hyper-excitable in vitro hemi-spinal cords (Cangiano and Grillner, 2003, 2005) is cleaner or more physiological compared to sensory stimulation and descending brainstem activation of hemi-spinal cords in whole animals (Messina et al., 2017; also see Jackson et al., 2005). Sixth, the previous commentary incorrectly stated that in our paper we claimed “that the burst generation is crucially dependent on reciprocal inhibition.” On the contrary, we stated that “reciprocal inhibition mainly regulates left-right phasing of [lamprey] locomotor activity and is not critical for rhythmogenesis” (p. 14; also see Hagevik and McClellan, 1994). In addition, we emphasized that right-left coupling involves both reciprocal inhibition as well as reciprocal excitation (p. 14, 15; Figure 1). Thus, blocking left-right reciprocal inhibition is not a definitive test for rhythmogenesis of hemi-spinal cords because right and left motor networks would still be coupled by reciprocal excitation. Seventh, the previous commentary stated that the higher than normal BP values for “burstlet” activity in our study (Figures 6A2–C2) are expected, thereby implying that the “burstlet” activity represents lamprey locomotor activity. Indeed, the higher than normal BPs are expected (p. 13), but this represents a small part of only 1 of the 6 reasons we provided to suggest that our “burstlet” activity does not represent well-coordinated swimming activity (p. 14). For example, for the majority of our experiments, BP values for “burstlet” activity changed significantly with CT (Figures 6A2–C2), which is not characteristic of swimming motor activity in the lamprey. In conclusion, our experiments employed the most physiological conditions possible for testing the rhythm generating capabilities of hemi-spinal cords in adult lampreys. Our results strongly suggest that isolated right and left spinal cord modules are not autonomous and, by themselves, do not generate coordinated ipsilateral locomotor burst activity. Author contributions The author confirms being the sole contributor of this work and approved it for publication. Conflict of interest statement The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

          Related collections

          Most cited references11

          • Record: found
          • Abstract: found
          • Article: not found

          Fictive locomotion in the lamprey spinal cord in vitro compared with swimming in the intact and spinal animal.

          A comparison has been made of the patterns of muscle activity during swimming in the intact and spinal lamprey, and the patterns of ventral root activity in the in vitro preparation of the lamprey spinal cord. Electromyographic (e.m.g.) activity was recorded with intramuscular bipolar electrodes from three segmental levels in intact lampreys swimming in a swim-mill at a range of swimming speeds. The patterns of activity obtained were similar to those seen in elasmobranch and teleost fish. After high spinal transection, lampreys could be induced to swim continuously for a period of several minutes in the swim-mill by a light initial mechanical stimulation of the tail or dorsal fin. The patterns of e.m.g. activity obtained from spinal animals at a range of swimming speeds were similar to those obtained in the intact state. Portions of spinal cord were isolated encompassing those segments from which e.m.g. recordings had been made and ventral root recordings were made in vitro of the rhythmic activity induced by bath application of D-glutamate. In all experiments the mean duration of the bursts of activity at any segmental level was directly proportional to the mean cycle duration, and the constant of proportionality (about 0.36) was similar for all three types of preparation. In all preparations the mean time delay for the activation of segments in the rostral-caudal direction was proportional to the cycle duration and to the number of segments between recording positions. The proportionality constant, or phase lag per segment, was approximately equal to 0.01 in all three types of preparation.
            Bookmark
            • Record: found
            • Abstract: found
            • Article: not found

            Fast and slow locomotor burst generation in the hemispinal cord of the lamprey.

            A fundamental question in vertebrate locomotion is whether distinct spinal networks exist that are capable of generating rhythmic output for each group of muscle synergists. In many vertebrates including the lamprey, it has been claimed that burst activity depends on reciprocal inhibition between antagonists. This question was addressed in the isolated lamprey spinal cord in which the left and right sides of each myotome display rhythmic alternating activity. We sectioned the spinal cord along the midline and tested whether rhythmic motor activity could be induced in the hemicord with bath-applied D-glutamate or N-methyl-D-aspartate (NMDA) as in the intact spinal cord or by brief trains of electrical stimuli. Fast rhythmic bursting (2-12 Hz), coordinated across ventral roots, was observed with all three methods. Furthermore, to diminish gradually the crossed glycinergic inhibition, a progressive surgical lesioning of axons crossing the midline was implemented. This resulted in a gradual increase in burst frequency, linking firmly the fast hemicord rhythm [6.6 +/- 1.7 (SD) Hz] to fictive swimming in the intact cord (2.4 +/- 0.7 Hz). Ipsilateral glycinergic inhibition was not required for the hemicord burst pattern generation, suggesting that an interaction between excitatory glutamatergic neurons suffices to produce the unilateral burst pattern. In NMDA, burst activity at a much lower rate (0.1-0.4 Hz) was also encountered, which required the voltage-dependent properties of NMDA receptors in contrast to the fast rhythm. Swimming is thus produced by pairs of unilateral burst generating networks with reciprocal inhibitory connections that not only ensure left/right alternation but also downregulate frequency.
              Bookmark
              • Record: found
              • Abstract: found
              • Article: not found

              Mechanisms of rhythm generation in a spinal locomotor network deprived of crossed connections: the lamprey hemicord.

              The spinal network coordinating locomotion in the lamprey serves as a model system, in which it has been possible to elucidate connectivity and cellular mechanisms using the isolated spinal cord. Locomotor burst activity alternates between the left and right side of a segment through reciprocal inhibition. We have recently shown that the burst generation itself in a hemisegment does not require inhibitory mechanisms. The focus of this study is the intrinsic operation of this hemisegmental burst-generating component of the locomotor network. Brief electrical stimulation (0.3 s) of the hemicord evokes long-lasting bouts (>2 min) of bursts (2-15 Hz) in the mid to high-frequency range of locomotion. Bout release is an all-or-none phenomenon requiring a threshold intensity of stimulation and glutamatergic transmission within a population of excitatory interneurons, with axons extending over several segments. The progressive activity-dependent decrease in burst frequency that takes place during a bout is followed by a slow recovery process lasting >20 min. Intracellular recordings of single motoneurons, excitatory interneurons, and inhibitory interneurons show that locomotor bouts, in general, are accompanied by a marked depolarization. Membrane potential oscillations and spikes occur in phase with the ventral root (VR) bursts. Active motoneurons and interneurons fire one spike per VR burst, as also confirmed by axonal recordings. Thus, the reciprocal inhibition between opposite hemisegments in the intact cord not only ensures left-right alternation and lowers the locomotor frequency but also promotes a shift from single to multiple action potentials per cycle in network neurons.
                Bookmark

                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
                02 August 2018
                2018
                : 12
                : 62
                Affiliations
                [1] 1Division of Biological Sciences, University of Missouri , Columbia, MO, United States
                [2] 2Interdisciplinary Neuroscience Program, University of Missouri , Columbia, MO, United States
                Author notes

                Edited by: Jorn Hounsgaard, University of Copenhagen, Denmark

                Reviewed by: Wen-Chang Li, University of St Andrews, United Kingdom; Erik Svensson, Uppsala University, Sweden

                *Correspondence: Andrew D. McClellan McclellanA@ 123456missouri.edu
                Article
                10.3389/fncir.2018.00062
                6082958
                d66e9079-c700-4a3a-9cc9-bfcdc3c96bca
                Copyright © 2018 McClellan.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) 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
                : 24 May 2018
                : 11 July 2018
                Page count
                Figures: 0, Tables: 0, Equations: 0, References: 11, Pages: 3, Words: 1949
                Categories
                Neuroscience
                General Commentary

                Neurosciences
                locomotion,central pattern generators,oscillators,coordination,coupling
                Neurosciences
                locomotion, central pattern generators, oscillators, coordination, coupling

                Comments

                Comment on this article