Functional organization of vestibulospinal inputs on thoracic motoneurons responsible for trunk postural control in Xenopus. A. Olechowski-Bessaguet, R. Grandemange, L. Cardoit, E. Courty, F. M. Lambert*, D. Le Ray* (*co-last authors)
The Journal of Physiology. doi: 10.1113/JP278599
The article is available at the following address:
https://physoc.onlinelibrary.wiley.com/doi/full/10.1113/JP278599
A step forward in the comprehension of balance: distinct and complementary, direct and indirect vestibulospinal pathways responsible for the control of body posture at rest. Based on anatomical, electrophysiological and functional approaches in the juvenile Xenopus laevis, we provide novel data on the organization of the vestibulospinal circuits that regulate the activity of the body postural control two main effectors in vertebrates (back and hindlimb muscles) in response to both linear or angular acceleration.
Abstract: In vertebrates, trunk postural stabilization is known to rely mainly on direct vestibulospinal inputs on spinal axial motoneurons. However, a substantial role of central spinal commands ascending from lumbar segments is not excluded during active locomotion. In the adult Xenopus a lumbar drive dramatically overwhelms the descending inputs onto thoracic postural motoneurons during swimming. Given that vestibulospinal fibers also project onto the lumbar segments that shelter the locomotor generators, we investigated whether such a lumbo-thoracic pathway may relay vestibular information and consequently, also be involved in the control of posture at rest. We show that thoracic postural motoneurons exhibit particular dendritic spatial organization allowing them to gather information from both sides of the cord. In response to passive head motion, these motoneurons display both early and delayed discharges, the latter occurring in phase with ipsilateral hindlimb extensor bursts. We demonstrate that both vestibulospinal and lumbar ascending fibers converge onto postural motoneurons, and that thoracic motoneurons monosynaptically respond to the electrical stimulation of either pathway. Finally, we show that vestibulospinal fibers project to and activate lumbar interneurons with thoracic projections. Altogether, our results complete the scheme of the vestibulospinal control of posture by illustrating the existence of a novel, indirect pathway, which implicates lumbar interneurons relaying vestibular inputs to thoracic motoneurons, and participate in global body postural stabilization in the absence of active locomotion.
A-B. Summary of postural responses during forward (A1) and backward (A2) linear translations, and during rightward (B1) and leftward (B2) angular rotations. Dark colors: thoracic response alone; light colors: coupled thoracic and extensor motor responses.
C. Schematic organization of direct and lumbar-relayed LVST and TAN pathways to thoracic dorsalis MNs. Bs=brainstem; Lumbar Sc=lumbar spinal cord; iIN=ipsilateral interneuron; cIN=contralateral interneuron