Respiratory motor plasticity and cervical spinal cord injury

Traumatic injury to the cervical spinal cord is often accompanied by respiratory dysfunction. This results from direct damage to respiratory-related neurons and interruption of bulbospinal respiratory pathways. Injuries above the fourth cervical segment (C4) in particular may directly impact control of the diaphragm muscle, necessitating mechanical ventilation for survival. Thus, finding ways to enhance recovery of breathing function following spinal cord injury (SCI) is an important clinical priority. This dissertation presents original research exploring spontaneous respiratory recovery in rats following experimental cervical SCI and treatments to augment this recovery.;We first investigated the spontaneous "crossed-phrenic phenomenon" (CPP) and its contributions to respiratory recovery in the initial days to several weeks following unilateral cervical SCI (i.e. hemisection). Commonly studied as a model of evoked neuroplasticity in the respiratory system, the role of the CPP in spontaneous recovery from SCI is less understood. In this study we developed a method to quantify the contribution of the spontaneous CPP to tidal volume recovery by cutting the phrenic nerve in spontaneously breathing, anesthetized rats. Our results suggest that the CPP may play an important role in normal breathing function in the weeks following hemisection. However, the relative contribution of the CPP is similar during both normal breathing and during respiratory challenge with hypercapnia, an unexpected result.;This led to a follow-up study examining the role of intercostal (IC) muscles in spontaneous respiratory recovery following cervical hemisection. It is established that IC muscles play an active role in normal breathing and can be recruited during periods of high respiratory demand (e.g. vigorous exercise, hypoxia and hypercapnia). Yet, little is known about how the activity in these muscles changes in response to cervical SCI. Thus, we utilized electromyography (EMG) to analyze variations in IC muscle activity and retrograde anatomical tracers to examine changes in IC neural organization in the weeks to months following cervical hemisection. Our results demonstrated that IC muscles, particularly rostral ICs (i.e. between the first few ribs), displayed substantial spontaneous recovery following cervical hemisection. In addition, this recovery may have been mediated by recruitment of cervical and thoracic interneurons promoting progressive improvements in tidal volume.;Our research focus then shifted from plasticity aiding spontaneous respiratory recovery to treatments intended to enhance it. Specifically, Chapter 5 describes a cell-based transplantation method designed to augment functional contributions of the CPP to respiratory recovery following cervical hemisection. Instead of directly altering neural regeneration, reorganization or sprouting in bulbospinal respiratory pathways, we aimed to resupply respiratory motoneurons with serotonin, an important neuromodulator lost following injury. Serotonin has the ability to increase the excitability of motoneurons, making it possible for them to fire action potentials in response to lesser synaptic input. Therefore, we hypothesized that enhancing serotonergic inputs to respiratory motoneurons below a cervical hemisection would facilitate greater activation of the CPP and improved respiratory recovery. Serotonin-expressing cells dissected from embryonic rat brainstems were transplanted intraspinally below a cervical hemisection in rats. After 6 weeks, immunohistochemistry experiments indicated that surviving grafts appeared to enhance serotonin near respiratory motoneurons. This increased serotonergic innervation resulted in augmented ipsilateral phrenic nerve activity and the production of larger tidal volumes under normal and challenged breathing conditions. These results represent a proof-of-principle that long-term serotonin supplementation to the region of respiratory motoneurons may facilitate respiratory recovery following cervical SCI.;In conclusion, the data presented in this dissertation provide the following novel findings: 1) rats utilize neural pathways associated with the CPP to spontaneously enhance tidal volume following cervical hemisection. However, utilization appears to plateau after 2 weeks and is not dependent on respiratory drive, 2) progressive increases in tidal volume observed between 2 and 8 weeks post hemisection are likely facilitated by ipsilateral inspiratory IC muscles that spontaneously recover function by 2 weeks post-hemisection, and 3) re-establishment of serotonin innervation to phrenic motoneurons via cell transplantation is associated with enhanced motor output and improved tidal volume following cervical hemisection. Accordingly, chronic serotonin supplementation may represent a promising SCI therapeutic strategy.