Descending Premotor Inputs to Phrenic Motoneurons Across Motor Behaviors in Anesthetized and Awake Rats
Authors: Jimenez‐Ruiz Federico , Mantilla Carlos B , Khurram Obaid U , Rana Sabhya , Zhan Wen‐Zhi , Gransee Heather M , Sieck Gary C 
Published: November 2023

Neuromotor control of respiratory muscles such as the diaphragm muscle (DIAm), the main inspiratory muscle in mammals, is organized around motor units. Motor units comprise a motoneuron and the muscle fibers they innervate. Diversity in the contractile and fatigue properties of DIAm motor units permits generation of the wide range of behaviors necessary to sustain ventilation and maintain airways patent (e.g., sighs, which generate ~70% of maximal force in rats). Descending inputs to phrenic motoneurons innervating the DIAm is thought to be primarily unilateral in rats. After disrupting ipsilateral inputs by unilateral spinal cord hemisection at C2 (C2SH), latent, bilateral projections underlie recovery of ipsilateral DIAm activity. We hypothesized that DIAm EMG activity reflects bilateral descending premotor drive to motoneurons that varies across motor behaviors and conditions (anesthetized vs. awake).Using chronic DIAm EMG recordings in adult male Sprague‐Dawley rats, the bilateral vs. ipsilateral nature of premotor drive for each behavior and condition was evaluated by comparing the impact of C2SH. Three days prior to the C2SH surgery, a pair of DIAm electrodes was implanted bilaterally and DIAm EMG activity was assessed immediately prior to and after C2SH as well as at 3 days post‐C2SH in spontaneously‐breathing awake, restrained animals and following induction of ketamine‐xylazine anesthesia. DIAm EMG recordings comprised ~5 min of eupnea and exposure to hypoxia (10% O2)‐hypercapnia (5% CO2). The completeness of C2SH surgery was confirmed by the absence of ipsilateral diaphragm EMG activity during eupnea at the time of C2SH surgery and at 3 days post‐C2SH in anesthetized animals, consistent with previous studies. In addition, histological assessment of disruption of anterolateral white matter tracts was conducted to verify the C2SH injury. Ipsilateral RMS EMG amplitude in anesthetized rats pre‐C2SH was ~30% and 45% of pre‐injury anesthetized sigh levels during eupnea and hypoxia‐hypercapnia, respectively. During awake conditions pre‐C2SH, ipsilateral RMS EMG amplitude was significantly higher (~50%, 70% and 200% of pre injury anesthetized sigh levels during eupnea, hypoxia‐hypercapnia and sighs, respectively). There were no differences across diaphragm muscle sides pre‐C2SH in RMS EMG amplitude. At 3 days post‐C2SH, ipsilateral RMS EMG activity was evident in anesthetized animals during hypoxia‐hypercapnia and sighs (~10% and 50% of pre‐injury anesthetized sigh levels) and in awake animals during eupnea, hypoxia‐hypercapnia and sighs (~25%, 50% and 125% of pre‐injury anesthetized sigh levels, respectively). These findings represent considerable differences in DIAm activity across motor behaviors and anesthetized vs. awake conditions and highlight physiologically‐relevant differences in the relative contribution of ipsilateral vs. bilateral inputs to phrenic motoneurons.Support or Funding InformationNIH grant R01 HL96750.