How Does the Central Nervous System for Posture and Locomotion Cope With Damage-Induced Neural Asymmetry?

Functional organization of vestibulospinal inputs responsible for tail postural control in larval Xenopus

In all vertebrates, maintaining trunk posture primarily depends on descending commands originating from brainstem vestibulospinal nuclei. Despite being broadly outlined across species, the detailed anatomical and operational structure of these vestibulospinal networks remains poorly understood. Xenopus frogs have previously served as an excellent model for exploring such anatomical and functional aspects in relation to the animal's behavioral requirements. In this study, we examined the reflex motor reactions induced by vestibular stimulation in pre-metamorphic tadpoles. Our findings indicate that natural vestibular stimulation in the horizontal plane yields greater efficacy compared to stimulation in other planes, a phenomenon replicated in a frequency-dependent manner through specific galvanic stimulation (GVS) of the horizontal semicircular canals. With the exception of a very rostral cluster of neurons that receive vestibular inputs and project to the spinal cord, the overall anatomical segregation of vestibulospinal nuclei in the brainstem mirrors that observed in juvenile frogs. However, our results suggest closer similarities to mammalian organization than previously acknowledged. Moreover, we demonstrated that vestibulospinal cells project not only to spinal motoneurons in rostral segments but also to more distal segments that undergo regression during metamorphosis. Lastly, we illustrated how vestibular-induced spinal reflexes change during larval development, transitioning from tail swim-based activity to rostral trunk bursting responses, likely anticipating postural control in post-metamorphic frogs.

A sensory signal related to left-right symmetry modulates intra- and interlimb cutaneous reflexes during locomotion in intact cats

Introduction

During locomotion, cutaneous reflexes play an essential role in rapidly responding to an external perturbation, for example, to prevent a fall when the foot contacts an obstacle. In cats and humans, cutaneous reflexes involve all four limbs and are task- and phase modulated to generate functionally appropriate whole-body responses.

Methods

To assess task-dependent modulation of cutaneous interlimb reflexes, we electrically stimulated the superficial radial or superficial peroneal nerves in adult cats and recorded muscle activity in the four limbs during tied-belt (equal left-right speeds) and split-belt (different left-right speeds) locomotion.

Results

We show that the pattern of intra- and interlimb cutaneous reflexes in fore- and hindlimbs muscles and their phase-dependent modulation were conserved during tied-belt and split-belt locomotion. Short-latency cutaneous reflex responses to muscles of the stimulated limb were more likely to be evoked and phase-modulated when compared to muscles in the other limbs. In some muscles, the degree of reflex modulation was significantly reduced during split-belt locomotion compared to tied-belt conditions. Split-belt locomotion increased the step-by-step variability of left-right symmetry, particularly spatially.

Discussion

These results suggest that sensory signals related to left-right symmetry reduce cutaneous reflex modulation, potentially to avoid destabilizing an unstable pattern.

Perinatal development of central vestibular neurons in mice

Central circuitry of the vestibular nuclei integrates sensory inputs in the adaptive control of motor behaviors such as posture, locomotion, and gaze stabilization. Thus far, such circuits have been mostly examined at mature stages, whereas their emergence and early development have remained poorly described. Here, we focused on the perinatal period of murine development, from embryonic day E14.5 to post-natal day P5, to investigate the ontogeny of two functionally distinct vestibular neuronal groups, neurons projecting to the spinal cord via the lateral vestibulospinal tract (LVST) and commissural neurons of the medial vestibular nucleus that cross the midline to the contralateral nucleus. Using transgenic mice and retrograde labeling, we found that network-constitutive GABAergic and glycinergic neurons are already established in the two vestibular groups at embryonic stages. Although incapable of repetitive firing at E14.5, neurons of both groups can generate spike trains from E15.5 onward and diverge into previously established A or B subtypes according to the absence (A) or presence (B) of a two-stage spike after hyperpolarization. Investigation of several voltage-dependent membrane properties indicated that solely LVST neurons undergo significant maturational changes in their electrophysiological characteristics during perinatal development. The proportions of A vs B subtypes also evolve in both groups, with type A neurons remaining predominant at all stages, and type B commissural neurons appearing only post-natally. Together, our results indicate that vestibular neurons acquire their distinct morpho-functional identities after E14.5 and that the early maturation of membrane properties does not emerge uniformly in the different functional subpopulations of vestibulo-motor pathways.

Effects of Occlusal Splints on Spinal Posture in Patients with Temporomandibular Disorders: A Systematic Review

There is still a gap in the scientific knowledge on the linkage between craniofacial structure and spinal postural control in temporomandibular disorder (TMD) patients. This systematic review aimed to assess the role of occlusal splints on spinal posture of TMD patients. PubMed, Web of Science, and Scopus were systematically searched from inception until 5 January 2022 to identify observational studies with a longitudinal study design presenting: patients with diagnosis of TMD according to the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD); occlusal splint therapy as intervention; postural assessment as outcome. Out of 133 records identified, 104 were suitable for data screening, and only 7 articles were included satisfying the eligibility criteria. We found that occlusal splints might have a positive effect on posture in TMD patients, albeit there is little evidence of appropriate investigation for postural assessment. This systematic review suggested that the occlusal splint might be considered a non-invasive therapeutic approach for patients with TMD. However, the low number of studies with high-quality methodology in these patients showed an urgent need for further research using combined force platform stabilometry and kinematic evaluation of the spine to investigate the impact of occlusal splints on posture.