Origin of excitatory drive to a spinal locomotor network (original) (raw)
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Dynamic control of spinal locomotion circuits
2006 IEEE International Symposium on Circuits and Systems, 2006
We show that an ongoing locomotor pattern can that the physiology of the lamprey's spinal system generalizes to be modulated by application of discrete electrical stimuli to limbed vertebrates, as well [8]. During forward swimming, the the spinal cord at speci£c phases of the locomotor cycle. Data activity of muscles on the left side of the body is 1800 out-of-phase is presented from a series of experiments on in vitro lamprey with the activity of muscles on the right side in any given spinal spinal cords, which were used as an animal model for severe segment, and the duty cycle for each side is about 40%. The same spinal cord injury. For any given stimulus, the effects on activation pattern is observed in lamprey spinal cords in vitro, after frequency, length, and symmetry of locomotor output show a the cord is excised from the body and chemically activated [9]. This strong dependence on the phase at which stimulation is applied. so-called "£ctive swimming" can be recorded in the ventral roots, The most signi£cant changes are seen when stimulation occurs and reliably represents the expected motor output. during motor bursting: stimuli applied to the ipsilateral spinal Inducing a specifc change in the CPG output affects the motoneuhemicord increase the burst length, while stimuli applied to the ron output, which in turn alters the motor output, thereby affecting contralateral spinal hemicord decrease the burst length. Simula-behavior. For example, to generate rightward turns, the lamprey tions using experimentally-measured phase-dependent responses increases the duration of bursting on the right side for a single indicate that by monitoring the state of the neural system, it cycle, then increases the duration of bursting on the left side in the should be possible to apply stimuli at the appropriate times to subsequent cycle [10], [11]. A number of models have been proposed modulate the lamprey "gait" on a cycle-by-cycle basis. Eventually, in the literature to explain both forward swimming [8], [12]-[16] and this approach could lead to development of a neuroprosthetic turning movements [10], [17], but these models primarily focus on device for restoring locomotion after paralysis. the segmental rhythm-generating network and do not include details of the many ascending, descending, and intersegmental £bers [10],
Development and neuromodulation of spinal locomotor networks in the metamorphosing frog
Journal of Physiology-Paris, 2006
Metamorphosis in the anuran frog, Xenopus laevis, involves profound structural and functional transformations in most of the organism's physiological systems as it encounters a complete alteration in body plan, habitat, mode of respiration and diet. The metamorphic process also involves a transition in locomotory strategy from axial-based undulatory swimming using alternating contractions of left and right trunk muscles, to bilaterally-synchronous kicking of the newly developed hindlimbs in the young adult. At critical stages during this behavioural switch, functional larval and adult locomotor systems co-exist in the same animal, implying a progressive and dynamic reconfiguration of underlying spinal circuitry and neuronal properties as limbs are added and the tail regresses. To elucidate the neurobiological basis of this developmental
Mechanosensory neurons control the timing of spinal microcircuit selection during locomotion
eLife, 2017
Despite numerous physiological studies about reflexes in the spinal cord, the contribution of mechanosensory feedback to active locomotion and the nature of underlying spinal circuits remains elusive. Here we investigate how mechanosensory feedback shapes active locomotion in a genetic model organism exhibiting simple locomotion-the zebrafish larva. We show that mechanosensory feedback enhances the recruitment of motor pools during active locomotion. Furthermore, we demonstrate that inputs from mechanosensory neurons increase locomotor speed by prolonging fast swimming at the expense of slow swimming during stereotyped acoustic escape responses. This effect could be mediated by distinct mechanosensory neurons. In the spinal cord, we show that connections compatible with monosynaptic inputs from mechanosensory Rohon-Beard neurons onto ipsilateral V2a interneurons selectively recruited at high speed can contribute to the observed enhancement of speed. Altogether, our study reveals the basic principles and a circuit diagram enabling speed modulation by mechanosensory feedback in the vertebrate spinal cord.
European Journal of Neuroscience, 1995
Our aim was to test the hypothesis that the frequency of neuronal rhythm-generating networks is partly controlled by the size of the active premotor interneuron population. We have tested possible mechanisms for frequency changes in a population model of the Xenopus laevis embryo spinal rhythm-generating networks for swimming. After initiation by a brief sensory excitation, the frequency of swimming activity decreases to a steady level determined by the properties of the 24 interneurons and their connections. The initial frequency decrease was dependent on the time-course of initiating sensory synaptic excitation. When some premotor excitatory interneurons were given weaker synaptic connections to reflect the variability in the spinal cord, they could drop out and stop firing during the initial frequency decrease while swimming activity continued. If the synaptic input of such weak excitatory interneurons was graded finely, they could drop out consecutively. This led to further decreases in the level of tonic excitation and in network frequency which depended on the number, type and distribution of excitatory interneurons that stopped firing. Silent weak excitatory interneurons could be recruited by a second sensory excitation and cause an increase in tonic depolarization and frequency which outlasted the sensory input. Such recruitment could occur on both sides after local sensory stimulation to only one region or one side of the model. We conclude that these computer simulations support the hypothesis that premotor interneuron drop-out and recruitment is one mechanism which can control frequency in a locomotor central pattern generator.
Modular organization of motor behavior in the frog's spinal cord
Trends in Neurosciences, 1995
The complex issue of translating the planning of arm movements into muscle forces is discussed in relation to the recent discovery of structures in the spinal cord. These structures contain circuitry that, when activated, produce precisely balanced contractions in groups of muscles. These synergistic contractions generate forces that direct the limb toward an equilibrium point in space. Remarkably, the force outputs, produced by activating different spinal-cord structures, sum vectorially.This vectorial combination of motor outputs might be a mechanism for producing a vast repertoire of motor behaviors in a simple manner.
Modeling of the Spinal Neuronal Circuitry Underlying Locomotion in a Lower Vertebratea
Annals of the New York Academy of Sciences, 1998
The neural circuitry generating lamprey undulatory swimming is among the most accessible and best known of the vertebrate neuronal locomotor systems. It therefore serves as an experimental model for such systems. Modeling and computer simulation of this system was initiated at a point when a significant part of the network had been identified, although much detail was still lacking. The model has been further developed over 10 years in close interaction with experiments. The local burst generating circuitry is formed by ipsilateral excitatory neurons and crossed reciprocal inhibitory neurons. Early models also incorporated an off-switch lateral interneuron (L), the connectivity of which suggested it could contribute to burst termination at moderate to high bursting frequencies. Later examination of this model suggested, however, that the L interneuron was not of primary importance for burst termination, and this was later verified experimentally. Further, early models explained the effects of 5-HT on bursting frequency, spike frequency, and burst duration as being due to its modulatory action on the spike frequency adaptation of lamprey premotor interneurons. In current network models, accumulated adaptation is in addition the main burst terminating factor. Drive-related modulation of adaptation is explored as a mechanism for control of burst duration. This produces an adequate burst frequency range and a constant burst proportion within each cycle. It further allows for hemisegmental bursting, which has been observed experimentally. The local burst generator forms the basis of a network model of the distributed pattern generator that extends along the spinal cord. Phase constancy and flexibility of intersegmental coordination has been studied in such a simulated network. Current modeling work focuses on neuromodulator circuitry and action, network responses to input transients, how to model the intact versus an isolated piece of spinal cord, as well as on improving an earlier neuromechanical model of lamprey swimming.
Locomotor circuits in the mammalian spinal cord
Annual Review of Neuroscience, 2006
Intrinsic spinal networks, known as central pattern generators (CPGs), control the timing and pattern of the muscle activity underlying locomotion in mammals. This review discusses new advances in understanding the mammalian CPGs with a focus on experiments that address the overall network structure as well as the identification of CPG neurons. I address the identification of excitatory CPG neurons and their role in rhythm generation, the organization of flexor-extensor networks, and the diverse role of commissural interneurons in coordinating left-right movements. Molecular and genetic approaches that have the potential to elucidate the function of populations of CPG interneurons are also discussed.
Development and functional organization of spinal locomotor circuits
Current opinion in neurobiology, 2011
The coordination and timing of muscle activities during rhythmic movements, like walking and swimming, are generated by intrinsic spinal motor circuits. Such locomotor networks are operational early in development and are found in all vertebrates. This review outlines and compares recent advances that have revealed the developmental and functional organization of these fundamental spinal motor networks in limbed and non-limbed animals. The comparison will highlight common principles and divergence in the organization of the spinal locomotor network structure in these different species as well as point to unresolved issues regarding the assembly and functioning of these networks.
Mechanisms of Spontaneous Activity in the Developing Spinal Cord and Their Relevance to Locomotion
Annals of the New York Academy of Sciences, 1998
The isolated lumbosacral cord of the chick embryo generates spontaneous episodes of rhythmic activity. Muscle nerve recordings show that the discharge of sartorius (flexor) and femorotibialis (extensor) motoneurons alternates even though the motoneurons are depolarized simultaneously during each cycle. The alternation occurs because sartorius motoneuron firing is shunted or voltage-clamped by its synaptic drive at the time of peak femorotibialis discharge. Ablation experiments have identified a region dorsomedial to the lateral motor column that may be required for the alternation of sartorius and femorotibialis motoneurons. This region overlaps the location of interneurons activated by ventral root stimulation. Wholecell recordings from interneurons receiving short latency ventral root input indicate that they fire at an appropriate time to contribute to the cyclical pause in firing of sartorius motoneurons. Spontaneous activity was modeled by the interaction of three variables: network activity and two activity-dependent forms of network depression. A "slow" depression which regulates the occurrence of episodes and a "fast" depression that controls cycling during an episode. The model successfully predicts several aspects of spinal network behavior including spontaneous rhythmic activity and the recovery of network activity following blockade of excitatory synaptic transmission.
A Neural Basis for Motor Primitives in the Spinal Cord
The Journal of Neuroscience, 2010
Motor primitives and modularity may be important in biological movement control. However, their neural basis is not understood. To investigate this, we recorded 302 neurons, making multielectrode recordings in the spinal cord gray of spinalized frogs, at 400, 800, and 1200 μm depth, at the L2/L3 segment border. Simultaneous muscle activity recordings were used with independent components analysis to infer premotor drive patterns. Neurons were divided into groups based on motor pattern modulation and sensory responses, depth recorded, and behavior. The 187 motor pattern modulated neurons recorded comprised 14 cutaneous neurons and 28 proprioceptive neurons at 400 μm in the dorsal horn, 131 intermediate zone interneurons from ∼800 μm depth without sensory responses, and 14 motoneuron-like neurons at ∼1200 μm. We examined all such neurons during spinal behaviors. Mutual information measures showed that cutaneous neurons and intermediate zone neurons were related better to premotor driv...