A novel preparation of the entire isolated CNS unveils the modulatory influences of supra-pontine structures on brainstem and spinal networks

Abstract

Several spinal motor output and essential rhythmic behaviors are controlled by supraspinal structures. Although of great advantage in deciphering their functional organization, existing preparations of the isolated brainstem and spinal networks only focus on local circuitry. The goal of our study has been to better characterize the contribution of higher centers to the neuronal networks involved in respiration and locomotion. Thus, a novel in vitro preparation from the isolated CNS of neonatal rodents was introduced to simultaneously record a stable respiratory rhythm from both cervical and lumbar ventral roots (VRs). Selective electrical pulses supplied to the pons and medulla evoked distinct VR responses in rostrocaudal direction with a staggered onset, while stimulation of ventrolateral medulla resulted in higher events from homolateral VRs. Moreover, electrical stimulation of a lumbar dorsal root (DR) elicited responses even from cervical VRs, albeit small and delayed, confirming functional ascending pathways. Furthermore, prototypical fictive locomotion was induced by trains of pulses applied to either the ventrolateral medulla or a DR. By progressively removing higher centers, duration of respiratory burst was reduced after a precollicular decerebration, which also affected the area of lumbar DR and VR potentials elicited by DR stimulation while frequency of respiration increased after a following pontobulbar transection. Keeping legs attached to the CNS allows for expressing the respiratory rhythm during peripheral stimulation of limbs. The study demonstrates that supra-pontine centers regulate the spontaneous respiratory rhythm, as well as electrically-evoked reflexes and spinal network activity. Thus, the current approach contributes to clarifying the modulatory influence of the brain on the brainstem and spinal micro-circuits.