Modular functional organisation of the axial locomotor system in salamanders

Morphological Correlates of Locomotion in the Aquatic and the Terrestrial Phases of Pleurodeles waltl Newts from Southwestern Iberia

Animals capable of moving in different environments might face conflicting selection on morphology, thus posing trade-offs on the relationships between morphology and locomotor performance in each of these environments. Moreover, given the distinct ecological roles of the sexes, these relationships can be sexually dimorphic. In this article, I studied the relationships between morphological traits and locomotor performance in male and female semiaquatic Pleurodeles waltl newts in their aquatic and their terrestrial stages. Morphology was sexually dimorphic: males have proportionally longer limbs and tails, as well as a better body condition (only in the aquatic phase), whereas females were larger and had greater body mass in both phases. Nonetheless, these morphological differences did not translate into sexual divergence in locomotor performance in either stage. This finding suggests other functions for the morphological traits measured, among which only SVL showed a positive relationship with locomotor performance in both stages, whereas the effect of SMI was negative only in the terrestrial stage, and that of tail length was positive only in the aquatic stage. In any case, the morphological correlates of terrestrial and aquatic locomotion did not conflict, which suggests no trade-off between both locomotory modes in the newts studied.

Reproducing Five Motor Behaviors in a Salamander Robot With Virtual Muscles and a Distributed CPG Controller Regulated by Drive Signals and Proprioceptive Feedback

Diverse locomotor behaviors emerge from the interactions between the spinal central pattern generator (CPG), descending brain signals and sensory feedback. Salamander motor behaviors include swimming, struggling, forward underwater stepping, and forward and backward terrestrial stepping. Electromyographic and kinematic recordings of the trunk show that each of these five behaviors is characterized by specific patterns of muscle activation and body curvature. Electrophysiological recordings in isolated spinal cords show even more diverse patterns of activity. Using numerical modeling and robotics, we explored the mechanisms through which descending brain signals and proprioceptive feedback could take advantage of the flexibility of the spinal CPG to generate different motor patterns. Adapting a previous CPG model based on abstract oscillators, we propose a model that reproduces the features of spinal cord recordings: the diversity of motor patterns, the correlation between phase lags and cycle frequencies, and the spontaneous switches between slow and fast rhythms. The five salamander behaviors were reproduced by connecting the CPG model to a mechanical simulation of the salamander with virtual muscles and local proprioceptive feedback. The main results were validated on a robot. A distributed controller was used to obtain the fast control loops necessary for implementing the virtual muscles. The distributed control is demonstrated in an experiment where the robot splits into multiple functional parts. The five salamander behaviors were emulated by regulating the CPG with two descending drives. Reproducing the kinematics of backward stepping and struggling however required stronger muscle contractions. The passive oscillations observed in the salamander's tail during forward underwater stepping could be reproduced using a third descending drive of zero to the tail oscillators. This reduced the drag on the body in our hydrodynamic simulation. We explored the effect of local proprioceptive feedback during swimming and forward terrestrial stepping. We found that feedback could replace or reduce the need for different drives in both cases. It also reduced the variability of intersegmental phase lags toward values appropriate for locomotion. Our work suggests that different motor behaviors do not require different CPG circuits: a single circuit can produce various behaviors when modulated by descending drive and sensory feedback.

Locomotion and dynamic posture: neuro-evolutionary basis of bipedal gait

Body displacement during locomotion is a major challenge for motor control, requiring complex synergistic postural regulation and the integrated functioning of all body musculature, including that of the four limbs, trunk and neck. Despite the obvious pivotal role played by the trunk during locomotion, most studies devoted to understanding the neural basis of locomotor control have only addressed the operation of the neural circuits driving leg movements, and relatively little is known of the networks that control trunk muscles in limbed vertebrates. This review addresses this issue, both in animals and humans. We first review studies addressing the central role played by central pattern generator (CPG) circuit interactions within the spinal cord in coordinating trunk and hind limb muscle activities in a variety of vertebrates, and present evidence that vestibulo-spinal reflexes are differentially involved in trunk and hind limb control. We finally highlight the role of the various components that participate in maintaining dynamic equilibrium during stepping, including connective tissues. We propose that many aspects of the organization of the motor systems involved in trunk-hind limb movement control in vertebrates have been highly conserved throughout evolution.

Influence of Brain Stem on Axial and Hindlimb Spinal Locomotor Rhythm Generating Circuits of the Neonatal Mouse

The trunk plays a pivotal role in limbed locomotion. Yet, little is known about how the brain stem controls trunk activity during walking. In this study, we assessed the spatiotemporal activity patterns of axial and hindlimb motoneurons (MNs) during drug-induced fictive locomotor-like activity (LLA) in an isolated brain stem-spinal cord preparation of the neonatal mouse. We also evaluated the extent to which these activity patterns are affected by removal of brain stem. Recordings were made in the segments T7, L2, and L5 using calcium imaging from individual axial MNs in the medial motor column (MMC) and hindlimb MNs in lateral motor column (LMC). The MN activities were analyzed during both the rhythmic and the tonic components of LLA, the tonic component being used as a readout of generalized increase in excitability in spinal locomotor networks. The most salient effect of brain stem removal was an increase in locomotor rhythm frequency and a concomitant reduction in burst durations in both MMC and LMC MNs. The lack of effect on the tonic component of LLA indicated specificity of action during the rhythmic component. Cooling-induced silencing of the brain stem reproduced the increase in rhythm frequency and accompanying decrease in burst durations in L2 MMC and LMC, suggesting a dependency on brain stem neuron activity. The work supports the idea that the brain stem locomotor circuits are operational already at birth and further suggests an important role in modulating trunk activity. The brain stem may influence the axial and hindlimb spinal locomotor rhythm generating circuits by extending their range of operation. This may represent a critical step of locomotor development when learning how to walk in different conditions and environments is a major endeavor.

Flexibility of the axial central pattern generator network for locomotion in the salamander

In tetrapods, limb and axial movements are coordinated during locomotion. It is well established that inter- and intralimb coordination show considerable variations during ongoing locomotion. Much less is known about the flexibility of the axial musculoskeletal system during locomotion and the neural mechanisms involved. Here we examined this issue in the salamander Pleurodeles waltlii, which is capable of locomotion in both aquatic and terrestrial environments. Kinematics of the trunk and electromyograms from the mid-trunk epaxial myotomes were recorded during four locomotor behaviors in freely moving animals. A similar approach was used during rhythmic struggling movements since this would give some insight into the flexibility of the axial motor system. Our results show that each of the forms of locomotion and the struggling behavior is characterized by a distinct combination of mid-trunk motor patterns and cycle durations. Using in vitro electrophysiological recordings in isolated spinal cords, we observed that the spinal networks activated with bath-applied N-methyl-d-aspartate could generate these axial motor patterns. In these isolated spinal cord preparations, the limb motor nerve activities were coordinated with each mid-trunk motor pattern. Furthermore, isolated mid-trunk spinal cords and hemicords could generate the mid-trunk motor patterns. This indicates that each side of the cord comprises a network able to generate coordinated axial motor activity. The roles of descending and sensory inputs in the behavior-related changes in axial motor coordination are discussed.

Axial systems and their actuation: new twists on the ancient body of craniates

Craniate animals--vertebrates and their jawless sister taxa--have evolved a body axis with powerful muscles, a distributed nervous system to control those muscles, and an endoskeleton that starts at the head and ends at the caudal fin. The body axis undulates, bends, twists, or holds firm, depending on the behavior. In this introduction to the special issue on axial systems and their actuation, we provide an overview of the latest research on how the body axis functions, develops, and evolves. Based on this research, we hypothesize that the body axis of craniates has three primary, post-cranial modules: precaudal, caudal, and tail. The term "module" means a portion of the body axis that functions, develops, and evolves in relative independence from other modules; "relative independence" means that structures and processes within a module are more tightly correlated in function, development, and behavior than the same processes are among modules.