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Zaback M, Tiwari E, Krupka AJ, Marchionne F, Negro F, Lemay MA, Thompson CK. Toward Assessing the Functional Connectivity of Spinal Neurons. Front Neural Circuits 2022; 16:839521. [PMID: 35310548 PMCID: PMC8927546 DOI: 10.3389/fncir.2022.839521] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2021] [Accepted: 01/28/2022] [Indexed: 12/14/2022] Open
Abstract
Spinal interneurons play a critical role in motor output. A given interneuron may receive convergent input from several different sensory modalities and descending centers and relay this information to just as many targets. Therefore, there is a critical need to quantify populations of spinal interneurons simultaneously. Here, we quantify the functional connectivity of spinal neurons through the concurrent recording of populations of lumbar interneurons and hindlimb motor units in the in vivo cat model during activation of either the ipsilateral sural nerve or contralateral tibial nerve. Two microelectrode arrays were placed into lamina VII, one at L3 and a second at L6/7, while an electrode array was placed on the surface of the exposed muscle. Stimulation of tibial and sural nerves elicited similar changes in the discharge rate of both interneurons and motor units. However, these same neurons showed highly significant differences in prevalence and magnitude of correlated activity underlying these two forms of afferent drive. Activation of the ipsilateral sural nerve resulted in highly correlated activity, particularly at the caudal array. In contrast, the contralateral tibial nerve resulted in less, but more widespread correlated activity at both arrays. These data suggest that the ipsilateral sural nerve has dense projections onto caudal lumbar spinal neurons, while contralateral tibial nerve has a sparse pattern of projections.
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Affiliation(s)
- Martin Zaback
- Department of Health and Rehabilitation Sciences, College of Public Health, Temple University, Philadelphia, PA, United States,Department of Bioengineering, College of Engineering, Temple University, Philadelphia, PA, United States
| | - Ekta Tiwari
- Department of Bioengineering, College of Engineering, Temple University, Philadelphia, PA, United States,School of Engineering, Brown University, Providence, RI, United States
| | - Alexander J. Krupka
- Department of Bioengineering, College of Engineering, Temple University, Philadelphia, PA, United States,Department of Biology, DeSales University, Center Valley, PA, United States
| | - Francesca Marchionne
- Department of Bioengineering, College of Engineering, Temple University, Philadelphia, PA, United States
| | - Francesco Negro
- Department of Clinical and Experimental Sciences, Universita degli Studi di Brescia, Brescia, Italy
| | - Michel A. Lemay
- Department of Bioengineering, College of Engineering, Temple University, Philadelphia, PA, United States
| | - Christopher K. Thompson
- Department of Health and Rehabilitation Sciences, College of Public Health, Temple University, Philadelphia, PA, United States,Shriner’s Hospital for Children, Philadelphia, PA, United States,*Correspondence: Christopher K. Thompson,
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McMahon C, Kowalski DP, Krupka AJ, Lemay MA. Single-cell and ensemble activity of lumbar intermediate and ventral horn interneurons in the spinal air-stepping cat. J Neurophysiol 2022; 127:99-115. [PMID: 34851739 PMCID: PMC8721903 DOI: 10.1152/jn.00202.2021] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2021] [Revised: 11/30/2021] [Accepted: 12/01/2021] [Indexed: 12/18/2022] Open
Abstract
We explored the relationship between population interneuronal network activation and motor output in the adult, in vivo, air-stepping, spinal cat. By simultaneously measuring the activity of large numbers of spinal interneurons, we explored ensembles of coherently firing interneurons and their relation to motor output. In addition, the networks were analyzed in relation to their spatial distribution along the lumbar enlargement for evidence of localized groups driving particular phases of the locomotor step cycle. We simultaneously recorded hindlimb EMG activity during stepping and extracellular signals from 128 channels across two polytrodes inserted within lamina V-VII of two separate lumbar segments. Results indicated that spinal interneurons participate in one of two ensembles that are highly correlated with the flexor or the extensor muscle bursts during stepping. Interestingly, less than half of the isolated single units were significantly unimodally tuned during the step cycle whereas >97% of the single units of the ensembles were significantly correlated with muscle activity. These results show the importance of population scale analysis in neural studies of behavior as there is a much greater correlation between muscle activity and ensemble firing than between muscle activity and individual neurons. Finally, we show that there is no correlation between interneurons' rostrocaudal locations within the lumbar enlargement and their preferred phase of firing or ensemble participation. These findings indicate that spinal interneurons of lamina V-VII encoding for different phases of the locomotor cycle are spread throughout the lumbar enlargement in the adult spinal cord.NEW & NOTEWORTHY We report on the ensemble organization of interneuronal activity in the spinal cord during locomotor movements and show that lumbar intermediate zone interneurons organize in two groups related to the two major phases of walking: stance and swing. Ensemble organization is also shown to better correlate with muscular output than single-cell activity, although ensemble membership does not appear to be somatotopically organized within the spinal cord.
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Affiliation(s)
- Chantal McMahon
- School of Biomedical Engineering, Science and Health Systems, Drexel University, Philadelphia, Pennsylvania
| | - David P Kowalski
- School of Biomedical Engineering, Science and Health Systems, Drexel University, Philadelphia, Pennsylvania
| | | | - Michel A Lemay
- Department of Bioengineering, Temple University, Philadelphia, Pennsylvania
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3
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Lindén H, Berg RW. Why Firing Rate Distributions Are Important for Understanding Spinal Central Pattern Generators. Front Hum Neurosci 2021; 15:719388. [PMID: 34539363 PMCID: PMC8446347 DOI: 10.3389/fnhum.2021.719388] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2021] [Accepted: 06/02/2021] [Indexed: 01/16/2023] Open
Abstract
Networks in the spinal cord, which are responsible for the generation of rhythmic movements, commonly known as central pattern generators (CPGs), have remained elusive for decades. Although it is well-known that many spinal neurons are rhythmically active, little attention has been given to the distribution of firing rates across the population. Here, we argue that firing rate distributions can provide an important clue to the organization of the CPGs. The data that can be gleaned from the sparse literature indicate a firing rate distribution, which is skewed toward zero with a long tail, akin to a normal distribution on a log-scale, i.e., a “log-normal” distribution. Importantly, such a shape is difficult to unite with the widespread assumption of modules composed of recurrently connected excitatory neurons. Spinal modules with recurrent excitation has the propensity to quickly escalate their firing rate and reach the maximum, hence equalizing the spiking activity across the population. The population distribution of firing rates hence would consist of a narrow peak near the maximum. This is incompatible with experiments, that show wide distributions and a peak close to zero. A way to resolve this puzzle is to include recurrent inhibition internally in each CPG modules. Hence, we investigate the impact of recurrent inhibition in a model and find that the firing rate distributions are closer to the experimentally observed. We therefore propose that recurrent inhibition is a crucial element in motor circuits, and suggest that future models of motor circuits should include recurrent inhibition as a mandatory element.
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Affiliation(s)
- Henrik Lindén
- Department of Neuroscience, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Rune W Berg
- Department of Neuroscience, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
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Klishko AN, Akyildiz A, Mehta-Desai R, Prilutsky BI. Common and distinct muscle synergies during level and slope locomotion in the cat. J Neurophysiol 2021; 126:493-515. [PMID: 34191619 DOI: 10.1152/jn.00310.2020] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Although it is well established that the motor control system is modular, the organization of muscle synergies during locomotion and their change with ground slope are not completely understood. For example, typical reciprocal flexor-extensor muscle synergies of level walking in cats break down in downslope: one-joint hip extensors are silent throughout the stride cycle, whereas hindlimb flexors demonstrate an additional stance phase-related electromyogram (EMG) burst (Smith JL, Carlson-Kuhta P, Trank TV. J Neurophysiol 79: 1702-1716, 1998). Here, we investigated muscle synergies during level, upslope (27°), and downslope (-27°) walking in adult cats to examine common and distinct features of modular organization of locomotor EMG activity. Cluster analysis of EMG burst onset-offset times of 12 hindlimb muscles revealed five flexor and extensor burst groups that were generally shared across slopes. Stance-related bursts of flexor muscles in downslope were placed in a burst group from level and upslope walking formed by the rectus femoris. Walking upslope changed swing/stance phase durations of level walking but not the cycle duration. Five muscle synergies computed using non-negative matrix factorization accounted for at least 95% of variance in EMG patterns in each slope. Five synergies were shared between level and upslope walking, whereas only three of those were shared with downslope synergies; these synergies were active during the swing phase and phase transitions. Two stance-related synergies of downslope walking were distinct; they comprised a mixture of flexors and extensors. We suggest that the modular organization of muscle activity during level and slope walking results from interactions between motion-related sensory feedback, CPG, and supraspinal inputs.NEW & NOTEWORTHY We demonstrated that the atypical EMG activities during cat downslope walking, silent one-joint hip extensors and stance-related EMG bursts in flexors, have many features shared with activities of level and upslope walking. Majority of EMG burst groups and muscle synergies were shared among these slopes, and upslope modulated the swing/stance phase duration but not cycle duration. Thus, synergistic EMG activities in all slopes might result from a shared CPG receiving somatosensory and supraspinal inputs.
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Affiliation(s)
- Alexander N Klishko
- School of Biological Sciences, Georgia Institute of Technology, Atlanta, Georgia
| | - Adil Akyildiz
- School of Biological Sciences, Georgia Institute of Technology, Atlanta, Georgia
| | - Ricky Mehta-Desai
- School of Biological Sciences, Georgia Institute of Technology, Atlanta, Georgia
| | - Boris I Prilutsky
- School of Biological Sciences, Georgia Institute of Technology, Atlanta, Georgia
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Merlet AN, Harnie J, Macovei M, Doelman A, Gaudreault N, Frigon A. Cutaneous inputs from perineal region facilitate spinal locomotor activity and modulate cutaneous reflexes from the foot in spinal cats. J Neurosci Res 2021; 99:1448-1473. [PMID: 33527519 DOI: 10.1002/jnr.24791] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2020] [Revised: 11/25/2020] [Accepted: 12/23/2020] [Indexed: 12/27/2022]
Abstract
It is well known that mechanically stimulating the perineal region potently facilitates hindlimb locomotion and weight support in mammals with a spinal transection (spinal mammals). However, how perineal stimulation mediates this excitatory effect is poorly understood. We evaluated the effect of mechanically stimulating (vibration or pinch) the perineal region on ipsilateral (9-14 ms onset) and contralateral (14-18 ms onset) short-latency cutaneous reflex responses evoked by electrically stimulating the superficial peroneal or distal tibial nerve in seven adult spinal cats where hindlimb movement was restrained. Cutaneous reflexes were evoked before, during, and after mechanical stimulation of the perineal region. We found that vibration or pinch of the perineal region effectively triggered rhythmic activity, ipsilateral and contralateral to nerve stimulation. When electrically stimulating nerves, adding perineal stimulation modulated rhythmic activity by decreasing cycle and burst durations and by increasing the amplitude of flexors and extensors. Perineal stimulation also disrupted the timing of the ipsilateral rhythm, which had been entrained by nerve stimulation. Mechanically stimulating the perineal region decreased ipsilateral and contralateral short-latency reflex responses evoked by cutaneous inputs, a phenomenon we observed in muscles crossing different joints and located in different limbs. The results suggest that the excitatory effect of perineal stimulation on locomotion and weight support is mediated by increasing the excitability of central pattern-generating circuitry and not by increasing excitatory inputs from cutaneous afferents of the foot. Our results are consistent with a state-dependent modulation of reflexes by spinal interneuronal circuits.
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Affiliation(s)
- Angèle N Merlet
- Department of Pharmacology-Physiology, Faculty of Medicine and Health Sciences, Université de Sherbrooke, Sherbrooke, QC, Canada
| | - Jonathan Harnie
- Department of Pharmacology-Physiology, Faculty of Medicine and Health Sciences, Université de Sherbrooke, Sherbrooke, QC, Canada
| | - Madalina Macovei
- Department of Pharmacology-Physiology, Faculty of Medicine and Health Sciences, Université de Sherbrooke, Sherbrooke, QC, Canada
| | - Adam Doelman
- Department of Pharmacology-Physiology, Faculty of Medicine and Health Sciences, Université de Sherbrooke, Sherbrooke, QC, Canada
| | - Nathaly Gaudreault
- School of Rehabilitation, Faculty of Medicine and Health Sciences, Université de Sherbrooke, Sherbrooke, QC, Canada.,Centre de Recherche du CHUS, Sherbrooke, QC, Canada
| | - Alain Frigon
- Department of Pharmacology-Physiology, Faculty of Medicine and Health Sciences, Université de Sherbrooke, Sherbrooke, QC, Canada.,Centre de Recherche du CHUS, Sherbrooke, QC, Canada
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Musienko PE, Lyalka VF, Gorskii OV, Merkulyeva N, Gerasimenko YP, Deliagina TG, Zelenin PV. Comparison of operation of spinal locomotor networks activated by supraspinal commands and by epidural stimulation of the spinal cord in cats. J Physiol 2020; 598:3459-3483. [PMID: 32445488 PMCID: PMC8106146 DOI: 10.1113/jp279460] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2019] [Accepted: 05/18/2020] [Indexed: 01/01/2023] Open
Abstract
KEY POINTS Epidural electrical stimulation (ES) of the spinal cord restores/improves locomotion in patients. ES-evoked locomotor movements differ to some extent from the normal ones. Operation of the locomotor network during ES is unknown. We compared the activity of individual spinal neurons during locomotion initiated by signals from the brainstem and by ES. We demonstrated that the spinal network generating locomotion under each of the two conditions is formed by the same neurons. A part of this network operates similarly under the two conditions, suggesting that it is essential for generation of locomotion under both conditions. Another part of this network operates differently under the two conditions, suggesting that it is responsible for differences in the movement kinematics observed under the two conditions. ABSTRACT Locomotion is a vital motor function for both animals and humans. Epidural electrical stimulation (ES) of the spinal cord is used to restore/improve locomotor movements in patients. However, operation of locomotor networks during ES has never been studied. Here we compared the activity of individual spinal neurons recorded in decerebrate cats of either sex during locomotion initiated by supraspinal commands (caused by stimulation of the mesencephalic locomotor region, MLR) and by ES. We found that under both conditions, the same neurons had modulation of their activity related to the locomotor rhythm, suggesting that the network generating locomotion under the two conditions is formed by the same neurons. About 40% of these neurons had stable modulation (i.e. small dispersion of their activity phase in sequential cycles), as well as a similar phase and shape of activity burst in MLR- and ES-evoked locomotor cycles. We suggest that these neurons form a part of the locomotor network that operates similarly under the two conditions, and are critical for generation of locomotion. About 23% of the modulated neurons had stable modulation only during MLR-evoked locomotion. We suggest that these neurons are responsible for some differences in kinematics of MLR- and ES-evoked locomotor movements. Finally, 25% of the modulated neurons had unstable modulation during both MLR- and ES-evoked locomotion. One can assume that these neurons contribute to maintenance of the excitability level of locomotor networks necessary for generation of stepping, or belong to postural networks, activated simultaneously with locomotor networks by both MLR stimulation and ES.
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Affiliation(s)
- Pavel E Musienko
- Institute of Translational Biomedicine, St. Petersburg State University, St. Petersburg, 199034, Russia
- Pavlov Institute of Physiology, St. Petersburg, 199034, Russia
| | - Vladimir F Lyalka
- Department of Neuroscience, Karolinska Institute, Stockholm, SE-17177, Sweden
| | - Oleg V Gorskii
- Pavlov Institute of Physiology, St. Petersburg, 199034, Russia
| | - Natalia Merkulyeva
- Institute of Translational Biomedicine, St. Petersburg State University, St. Petersburg, 199034, Russia
- Pavlov Institute of Physiology, St. Petersburg, 199034, Russia
| | | | - Tatiana G Deliagina
- Department of Neuroscience, Karolinska Institute, Stockholm, SE-17177, Sweden
| | - Pavel V Zelenin
- Department of Neuroscience, Karolinska Institute, Stockholm, SE-17177, Sweden
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Thompson CK, Negro F, Johnson MD, Holmes MR, McPherson LM, Powers RK, Farina D, Heckman CJ. Robust and accurate decoding of motoneuron behaviour and prediction of the resulting force output. J Physiol 2018; 596:2643-2659. [PMID: 29726002 DOI: 10.1113/jp276153] [Citation(s) in RCA: 78] [Impact Index Per Article: 13.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2018] [Accepted: 04/30/2018] [Indexed: 01/16/2023] Open
Abstract
KEY POINTS The spinal alpha motoneuron is the only cell in the human CNS whose discharge can be routinely recorded in humans. We have reengineered motor unit collection and decomposition approaches, originally developed in humans, to measure the neural drive to muscle and estimate muscle force generation in the in vivo cat model. Experimental, computational, and predictive approaches are used to demonstrate the validity of this approach across a wide range of modes to activate the motor pool. The utility of this approach is shown through the ability to track individual motor units across trials, allowing for better predictions of muscle force than the electromyography signal, and providing insights in to the stereotypical discharge characteristics in response to synaptic activation of the motor pool. This approach now allows for a direct link between the intracellular data of single motoneurons, the discharge properties of motoneuron populations, and muscle force generation in the same preparation. ABSTRACT The discharge of a spinal alpha motoneuron and the resulting contraction of its muscle fibres represents the functional quantum of the motor system. Recent advances in the recording and decomposition of the electromyographic signal allow for the identification of several tens of concurrently active motor units. These detailed population data provide the potential to achieve deep insights into the synaptic organization of motor commands. Yet most of our understanding of the synaptic input to motoneurons is derived from intracellular recordings in animal preparations. Thus, it is necessary to extend the new electrode and decomposition methods to recording of motor unit populations in these same preparations. To achieve this goal, we use high-density electrode arrays and decomposition techniques, analogous to those developed for humans, to record and decompose the activity of tens of concurrently active motor units in a hindlimb muscle in the in vivo cat. Our results showed that the decomposition method in this animal preparation was highly accurate, with conventional two-source validation providing rates of agreement equal to or superior to those found in humans. Multidimensional reconstruction of the motor unit action potential provides the ability to accurately track the same motor unit across multiple contractions. Additionally, correlational analyses demonstrate that the composite spike train provides better estimates of whole muscle force than conventional estimates obtained from the electromyographic signal. Lastly, stark differences are observed between the modes of activation, in particular tendon vibration produced quantal interspike intervals at integer multiples of the vibration period.
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Affiliation(s)
| | - Francesco Negro
- Department of Clinical and Experimental Sciences, Università degli Studi di Brescia, Chicago, IL, USA
| | | | - Matthew R Holmes
- Department of Physiology, Northwestern University, Chicago, IL, USA
| | | | - Randall K Powers
- Department of Physiology and Biophysics, University of Washington, Seattle, WA, USA
| | - Dario Farina
- Department of Bioengineering, Imperial College London, London, UK
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Synergy temporal sequences and topography in the spinal cord: evidence for a traveling wave in frog locomotion. Brain Struct Funct 2015; 221:3869-3890. [PMID: 26501407 DOI: 10.1007/s00429-015-1133-5] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2014] [Accepted: 10/08/2015] [Indexed: 12/27/2022]
Abstract
Locomotion is produced by a central pattern generator. Its spinal cord organization is generally considered to be distributed, with more rhythmogenic rostral lumbar segments. While this produces a rostrocaudally traveling wave in undulating species, this is not thought to occur in limbed vertebrates, with the exception of the interneuronal traveling wave demonstrated in fictive cat scratching (Cuellar et al. J Neurosci 29:798-810, 2009). Here, we reexamine this hypothesis in the frog, using the seven muscle synergies A to G previously identified with intraspinal NMDA (Saltiel et al. J Neurophysiol 85:605-619, 2001). We find that locomotion consists of a sequence of synergy activations (A-B-G-A-F-E-G). The same sequence is observed when focal NMDA iontophoresis in the spinal cord elicits a caudal extension-lateral force-flexion cycle (flexion onset without the C synergy). Examining the early NMDA-evoked motor output at 110 sites reveals a rostrocaudal topographic organization of synergy encoding by the lumbar cord. Each synergy is preferentially activated from distinct regions, which may be multiple, and partially overlap between different synergies. Comparing the sequence of synergy activation in locomotion with their spinal cord topography suggests that the locomotor output is achieved by a rostrocaudally traveling wave of activation in the swing-stance cycle. A two-layer circuitry model, based on this topography and a traveling wave reproduces this output and explores its possible modifications under different afferent inputs. Our results and simulations suggest that a rostrocaudally traveling wave of excitation takes advantage of the topography of interneuronal regions encoding synergies, to activate them in the proper sequence for locomotion.
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Abstract
In reduced preparations, hindlimb movements can be generated by a minimal network of neurons in the limb innervating spinal segments. The network of neurons that generates real movements is less well delineated. In an ex vivo carapace-spinal cord preparation from adult turtles (Trachemys scripta elegans), we show that ventral horn interneurons in mid-thoracic spinal segments are functionally integrated in the hindlimb scratch network. First, mid-thoracic interneurons receive intense synaptic input during scratching and behave like neurons in the hindlimb enlargement. Second, some mid-thoracic interneurons activated during scratching project descending axons toward the hindlimb enlargement. Third, elimination of mid-thoracic segments leads to a weakening of scratch rhythmicity. We conclude that densely innervated interneurons in mid-thoracic segments contribute to hindlimb scratching and may be part of a distributed motor network that secures motor coherence.
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Cuellar C, Trejo A, Linares P, Delgado-Lezama R, Jiménez-Estrada I, Abyazova L, Baltina T, Manjarrez E. Spinal neurons bursting in phase with fictive scratching are not related to spontaneous cord dorsum potentials. Neuroscience 2014; 266:66-79. [DOI: 10.1016/j.neuroscience.2014.02.003] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2013] [Revised: 01/17/2014] [Accepted: 02/03/2014] [Indexed: 01/14/2023]
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Auyong N, Ollivier-Lanvin K, Lemay MA. Population spatiotemporal dynamics of spinal intermediate zone interneurons during air-stepping in adult spinal cats. J Neurophysiol 2011; 106:1943-53. [PMID: 21775722 DOI: 10.1152/jn.00258.2011] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The lumbar spinal cord circuitry can autonomously generate locomotion, but it remains to be determined which types of neurons constitute the locomotor generator and how their population activity is organized spatially in the mammalian spinal cord. In this study, we investigated the spatiotemporal dynamics of the spinal interneuronal population activity in the intermediate zone of the adult mammalian cord. Segmental interneuronal population activity was examined via multiunit activity (MUA) during air-stepping initiated by perineal stimulation in subchronic spinal cats. In contrast to single-unit activity, MUA provides a continuous measure of neuronal activity within a ∼100-μm volume around the recording electrode. MUA was recorded during air-stepping, along with hindlimb muscle activity, from segments L3 to L7 with two multichannel electrode arrays placed into the left and right hemicord intermediate zones (lamina V-VII). The phasic modulation and spatial organization of MUA dynamics were examined in relation to the locomotor cycle. Our results show that segmental population activity is modulated with respect to the ipsilateral step cycle during air-stepping, with maximal activity occurring near the ipsilateral swing to stance transition period. The phase difference between the population activity within the left and right hemicords was also found to correlate to the left-right alternation of the step cycle. Furthermore, examination of MUA throughout the rostrocaudal extent showed no differences in population dynamics between segmental levels, suggesting that the spinal interneurons targeted in this study may operate as part of a distributed "clock" mechanism rather than a rostrocaudal oscillation as seen with motoneuronal activity.
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Affiliation(s)
- Nicholas Auyong
- Department of Neurobiology and Anatomy, Drexel University College of Medicine, Philadelphia, Pennsylvania, USA
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