Mechanisms of Left-Right Coordination in Mammalian Locomotor Pattern Generation Circuits: A Mathematical Modeling View

The locomotor gait in limbed animals is defined by the left-right leg coordination and locomotor speed. Coordination between left and right neural activities in the spinal cord controlling left and right legs is provided by commissural interneurons (CINs). Several CIN types have been genetically identified, including the excitatory V3 and excitatory and inhibitory V0 types. Recent studies demonstrated that genetic elimination of all V0 CINs caused switching from a normal left-right alternating activity to a left-right synchronized “hopping” pattern. Furthermore, ablation of only the inhibitory V0 CINs (V0_D subtype) resulted in a lack of left-right alternation at low locomotor frequencies and retaining this alternation at high frequencies, whereas selective ablation of the excitatory V0 neurons (V0_V subtype) maintained the left–right alternation at low frequencies and switched to a hopping pattern at high frequencies. To analyze these findings, we developed a simplified mathematical model of neural circuits consisting of four pacemaker neurons representing left and right, flexor and extensor rhythm-generating centers interacting via commissural pathways representing V3, V0_D, and V0_V CINs. The locomotor frequency was controlled by a parameter defining the excitation of neurons and commissural pathways mimicking the effects of N-methyl-D-aspartate on locomotor frequency in isolated rodent spinal cord preparations. The model demonstrated a typical left-right alternating pattern under control conditions, switching to a hopping activity at any frequency after removing both V0 connections, a synchronized pattern at low frequencies with alternation at high frequencies after removing only V0_D connections, and an alternating pattern at low frequencies with hopping at high frequencies after removing only V0_V connections. We used bifurcation theory and fast-slow decomposition methods to analyze network behavior in the above regimes and transitions between them. The model reproduced, and suggested explanation for, a series of experimental phenomena and generated predictions available for experimental testing.     

Elongation factor 2 and fragile X mental retardation protein control the dynamic translation of Arc/Arg3.1 essential for mGluR-LTD

The initiation and coordination of activity in limb muscles are the main functions of neural circuits that control locomotion. Commissural neurons connect locomotor circuits on the two sides of the spinal cord, and represent the known neural substrate for left-right coordination. Here we demonstrate that a group of ipsilateral interneurons, V2a interneurons, plays an essential role in the control of left-right alternation. In the absence of V2a interneurons, the spinal cord fails to exhibit consistent left-right alternation. Locomotor burst activity shows increased variability, but flexor-extensor coordination is unaffected. Anatomical tracing studies reveal a direct excitatory input of V2a interneurons onto commissural interneurons, including a set of molecularly defined V0 neurons that drive left-right alternation. Our findings imply that the neural substrate for left-right coordination consists of at least two components; commissural neurons and a class of ipsilateral interneurons that activate commissural pathways.     

Probing the locomotor conundrum: descending the 'V' interneuron ladder

The assembly of neuronal circuits involved in locomotor control in the mammalian spinal cord is influenced by genetic programs specifying four ventral (V) interneuron populations (V0-V3). In this issue of Neuron, Crone et al. and Zhang et al. make use of genetic tools to map connectivity patterns and to abolish the function of V2a and V3 interneurons. The absence of V2a interneurons reveals defects in left-right alternation during locomotion, whereas ablation of either V2a or V3 interneurons leads to disturbances in the precision and reliability of the motor output.     

DCC mediated axon guidance of spinal interneurons is essential for normal locomotor central pattern generator function

Coordinated limb rhythmic movements take place through organized signaling in local spinal cord neuronal networks. The establishment of these circuitries during development is dependent on the correct guidance of axons to their targets. It has previously been shown that the well-known axon guidance molecule netrin-1 is required for configuring the circuitry that provides left-right alternating coordination in fictive locomotion. The attraction of commissural axons to the midline in response to netrin-1 has been shown to involve the netrin-1 receptor DCC (deleted in Colorectal Cancer). However, the role of DCC for the establishment of CPG coordination has not yet been resolved. We show that mice carrying a null mutation of DCC displayed an uncoordinated left-right activity during fictive locomotion accompanied by a loss of interneuronal subpopulations originating from commissural progenitors. Thus, DCC plays a crucial role in the formation of spinal neuronal circuitry coordinating left-right activities. Together with the previously published results from netrin-1 deficient mice, the data presented in this study suggest a role for the most ventral originating V3 interneurons in synchronous activities over the midline. Further, it provides evidence that axon crossing in the spinal cord is more intricately controlled than in previously suggested models of DCC-netrin-1 interaction.     

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.     

CTCF regulates ataxin-7 expression through promotion of a convergently transcribed, antisense noncoding RNA

Neural networks in the spinal cord control two basic features of locomotor movements: rhythm generation and pattern generation. Rhythm generation is generally considered to be dependent on glutamatergic excitatory neurons. Pattern generation involves neural circuits controlling left-right alternation, which has been described in great detail, and flexor-extensor alternation, which remains poorly understood. Here, we use a mouse model in which glutamatergic neurotransmission has been ablated in the locomotor region of the spinal cord. The isolated in?vitro spinal cord from these mice produces locomotor-like activity-when stimulated with neuroactive substances-with prominent flexor-extensor alternation. Under these conditions, unlike in control mice, networks of inhibitory interneurons generate the rhythmic activity. In the absence of glutamatergic synaptic transmission, the flexor-extensor alternation appears to be generated by Ia inhibitory interneurons, which mediate reciprocal inhibition from muscle proprioceptors to antagonist motor neurons. Our study defines a minimal inhibitory network that is needed to produce flexor-extensor alternation during locomotion.     

Glutamate drives the touch response through a rostral loop in the spinal cord of zebrafish embryos

Characterizing connectivity in the spinal cord of zebrafish embryos is not only prerequisite to understanding the development of locomotion, but is also necessary for maximizing the potential of genetic studies of circuit formation in this model system. During their first day of development, zebrafish embryos show two simple motor behaviors. First, they coil their trunks spontaneously, and a few hours later they start responding to touch with contralateral coils. These behaviors are contemporaneous until spontaneous coils become infrequent by 30 h. Glutamatergic neurons are distributed throughout the embryonic spinal cord, but their contribution to these early motor behaviors in immature zebrafish is still unclear. We demonstrate that the kinetics of spontaneous coiling and touch‐evoked responses show distinct developmental time courses and that the touch response is dependent on AMPA‐type glutamate receptor activation. Transection experiments suggest that the circuits required for touch‐evoked responses are confined to the spinal cord and that only the most rostral part of the spinal cord is sufficient for triggering the full response. This rostral sensory connection is presumably established via CoPA interneurons, as they project to the rostral spinal cord. Electrophysiological analysis demonstrates that these neurons receive short latency AMPA‐type glutamatergic inputs in response to ipsilateral tactile stimuli. We conclude that touch responses in early embryonic zebrafish arise only after glutamatergic synapses connect sensory neurons and interneurons to the contralateral motor network via a rostral loop. This helps define an elementary circuit that is modified by the addition of sensory inputs, resulting in behavioral transformation. © 2009 Wiley Periodicals, Inc. Develop Neurobiol 2009     

The neuronal targets for GABAergic reticulospinal inhibition that stops swimming in hatchling frog tadpoles

In most animals locomotion can be started and stopped by specific sensory cues. We are using a simple vertebrate, the hatchling Xenopus tadpole, to study a neuronal pathway that turns off locomotion. In the tadpole, swimming stops when the head contacts solid objects or the water's surface meniscus. The primary sensory neurons are in the trigeminal ganglion and directly excite inhibitory reticulospinal neurons in the hindbrain. These project axons into the spinal cord and release GABA to inhibit spinal neurons and stop swimming. We ask whether there is specificity in the types of spinal neuron inhibited. We used single-neuron recording to determine which classes of spinal neurons receive inhibition when the head skin is pressed. Ventral motoneurons and premotor interneurons involved in generating the swimming rhythm receive reliable GABAergic inhibition. More dorsal inhibitory premotor interneurons are inhibited less reliably and some are excited. Dorsal sensory pathway interneurons that start swimming following a touch to the trunk skin do not appear to receive such inhibition. There is therefore specificity in the formation of descending inhibitory connections so that more ventral neurons producing swimming are most strongly inhibited.     

Functional identification of interneurons responsible for left-right coordination of hindlimbs in mammals

Local neuronal networks that are responsible for walking are poorly characterized in mammals. Using an innovative approach to identify interneuron inputs onto motorneuron populations in a neonatal rodent spinal cord preparation, we have investigated the network responsible for left-right coordination of the hindlimbs. We demonstrate how commissural interneurons (CINs), whose axons traverse the midline to innervate contralateral neurons, are organized such that distinct flexor and extensor centers in the rostral lumbar spinal cord define activity in both flexor and extensor caudal motor pools. In addition, the nature of some connections are reconfigured on switching from rest to locomotion via a mechanism that might be associated with synaptic plasticity in the spinal cord. These results from identified pattern-generating interneurons demonstrate how interneuron populations create an effective network to underlie behavior in mammals.     

Neuroanatomical and functional analysis of neural tube formation in notochordless Xenopus embryos; laterality of the ventral spinal cord is lost.

Notochordless Xenopus embryos were produced by u.v. irradiation of the uncleaved fertilized egg. The spinal cords were examined using intermediate filament staining for glial cells, retrograde HRP staining for neuronal morphology and an anti-glycinergic antibody to reveal commissural cells and axons. The floorplate cells of the normal cord appear to be absent and their position along the ventral midline of the cord is occupied by motor neurones, Kolmer-Agduhr cells, radial glial cells and a ventrally placed marginal zone containing the longitudinal axons. Motor neurone number is reduced to 15% of control values, and the sensory extramedullary cell number is increased twentyfold. Commissural axons are still able to cross the ventral cord but do so at abnormal angles and some commissural axons continue to grow circumferentially up the contralateral side of the cord rather than turning to grow longitudinally. Extracellular electrophysiological recordings from motor axons reveal that the normal alternation of locomotor activity on the left and right side of the embryo is lost in notochordless animals. These results suggest that the notochord and/or the normal floor plate structure are important for the development of the laterality of spinal cord connections and may influence motor neurone proliferation or differentiation.     

The neuroanatomy of an amphibian embryo spinal cord

Horseradish peroxidase has been used to stain spinal cord neurons in late embryos of the clawed toad (Xenopus laevis). It has shown clearly the soma, dendrites and axonal projections of spinal sensory, motor and interneurons. On the basis of light microscopy we describe nine differentiated spinal cord neuron classes. These include the Rohon-Beard cells and extramedullary cells which are both primary sensory neurons, one class of motoneurons that innervate the segmental myotomes, two classes of interneurons with decussating axons, three classes of interneurons with ipsilateral axons and a previously undescribed class of ciliated ependymal cells with axons projecting ipsilaterally to the brain. We believe that all differentiated neuron classes are described and that this anatomical account is the most complete for any vertebrate spinal cord.