Modulation of phrenic respiratory networks by spinal cholinergic interneurons in the intact and lesioned spinal cord

Abstract

Breathing is an essential rhythmic motor behaviour that must continue throughout life and adjust to continuously changing metabolic demands. The respiratory rhythm is produced by central pattern generator circuits in the brainstem and relayed to respiratory muscles by motoneurons (MNs), including phrenic MNs in the cervical spinal cord that innervate the diaphragm. Rhythmic contractions of the diaphragm facilitate pulmonary ventilation, enabling oxygen and carbon dioxide exchange.

While the main synaptic drive to phrenic MNs derives from bulbospinal pathways, cervical spinal interneurons (INs) are suggested to modulate phrenic MN output and contribute to compensatory neuroplasticity post-spinal cord injury (SCI). However, the mechanisms by which specific INs shape phrenic MN activity in normal conditions and post-SCI remain poorly understood.

This thesis investigates the anatomical connectivity and functional roles of cholinergic V0c INs within the phrenic respiratory network. Moreover, using the unilateral C2 hemisection (C2Hx) injury model, it explores neuroplastic changes that may occur in V0c INs post-SCI.

We found that V0c INs are anatomically and functionally connected to phrenic MNs, via C-bouton synapses. Blocking M2 receptors, which mediate synaptic transmission at the C-bouton, within the cervical spinal cord reduced the amplitude of respiratory-related activity recorded from C3/C4 ventral roots of in-vitro brainstem-spinal cord preparations, suggesting modulation of phrenic MN output by endogenous cholinergic signalling, presumably V0c-derived. Preliminary data from in-vivo terminal diaphragm electromyography in a mouse model lacking V0c INs suggest they play a role in augmenting phrenic MN output and diaphragm contraction in response to hypercapnia. Finally, we found that C-bouton number is unchanged at 7 days and 4 weeks post-C2Hx, indicating that this system is spared by the lesion.

This thesis represents the first in-depth assessment of the role of V0c INs in modulating breathing in physiological conditions and begins to explore their potential contribution to functional recovery post-SCI.