Selective activation of respiratory thoracic efferents in the spinal cord of the in situ rat preparation

Abstract

Previous studies of respiratory plasticity have focused primarily on phrenic motoneurons (PM) in the cervical spinal cord, while little is known about the intercostal (IC) motor system. Recent reports suggest that IC muscle activity is only modestly impaired after unilateral cervical spinal cord injury (SCI), therefore, we hypothesized that respiratory thoracic motoneurons are capable of greater plasticity than PMs. To test this hypothesis, we simultaneously recorded activity from the left phrenic nerve (PNA) and thoracic ventral roots in the perfused in situ rat. Preparations were exposed to repeated bouts of hypercapnia (9% CO2/91% O2) alternated with baseline recovery periods (5%CO2/95%O2); and/or to periods of relative hypoxia (40% O2/5% CO2/55% N2) and recovery (95%O2/5%CO2). During both exposures, the relative increase in inspiratory‐related activity was significantly greater in the thoracic efferents compared to the phrenic nerve. Further, thoracic activity showed enhanced progressive augmentation compared to the phrenic nerve during each successive perturbation. We conclude that respiratory thoracic efferents have a greater capacity for dynamic plasticity in response to both hypercapnia and hypoxia. We suggest that compensation by the IC motor system may be a significant spontaneous mechanism for respiratory recovery after SCI and could be a key target for therapeutic interventions.