Department of Physical Therapy, College of Public Health and Health Professions, McKnight Brain Institute, University of Florida, Gainesville, FL 32610, USA.
In this paper we review respiratory recovery following C2 spinal cord hemisection (C2HS) and introduce evidence for ipsilateral (IL) and contralateral (CL) phrenic motor neuron (PhrMN) synchrony post-C2HS. Rats have rapid, shallow breathing after C2HS but ventilation ( logical or (E)) is maintained. logical or (E) deficits occur during hypercapnic challenge reflecting reduced tidal volume (VT), but modest recovery occurs by 12 wks post-injury. IL PhrMN activity recovers in a time-dependent manner after C2HS, and neuroanatomical evidence suggests that this may involve both mono- and polysynaptic pathways. Accordingly, we used cross-correlation to examine IL and CL PhrMN synchrony after C2HS. Uninjured rats showed correlogram peaks consistent with synchronous activity and common synaptic input. Correlogram peaks were absent at 2 wks post-C2HS, but by 12 wks 50% of rats showed peaks occurring with a 1.1+/-0.19ms lag from zero on the abscissa. These data are consistent with prolonged conduction time to IL (vs. CL) PhrMNs and the possibility of polysynaptic inputs to IL PhrMNs after chronic C2HS.