Discharge of Mid‐Cervical Neurons Following Cervical Spinal Cord Injury

Abstract

Prior anatomical and neurophysiologic data suggest that mid‐cervical interneurons may contribute to the control of phrenic motor output after cervical spinal cord injury (cSCI). To explore this possibility, we recorded spinal interneuron bursting patterns and phrenic motor output in rats with chronic cSCI. Twelve weeks following lateral C2 hemi‐section (C2Hx), bilateral phrenic nerve activity was recorded in urethane anesthetized and mechanically ventilated rats (N=5). A multi‐electrode recording array was bilaterally inserted at C4 to enable simultaneous recording of mid‐cervical neuron discharge ipsilateral and contralateral to C2Hx. In this initial evaluation, neuronal discharge patterns were classified as tonic (i.e., active across the entire respiratory cycle) or phasic (i.e., active during only the inspiratory or expiratory phases). Spike triggered averaging of phrenic nerve activity based on discharge of recorded cells was used to identify phrenic motoneurons, all other cells were classified as interneurons for this analysis. Only interneurons (versus motoneurons) were identified in recordings ipsilateral to C2Hx, and these cells primarily burst in a tonic firing pattern (tonic: 91% and phasic: 9% of ipsilateral cells). In contrast, on the contralateral side phrenic motoneurons were readily recorded (26% of contralateral cells) and a greater proportion of phasic interneurons were detected (phasic: 26% and tonic: 48% of contralateral cells). Spike triggered averaging produced no evidence of direct (monosynaptic) coupling between interneurons and either the contra‐ or ipsilateral phrenic motoneuron pool. These preliminary findings confirm altered propriospinal neuron discharge in the ipsilateral vs. contralateral cord immediately caudal to chronic C2Hx injury. The precise role of mid‐cervical propriospinal neurons in respiratory recovery after cSCI remains unclear. Since we did not detect cells that were directly antecedent to the phrenic pool, our data suggest that following cSCI, “connectivity” between respiratory‐modulated spinal interneurons and the phrenic nucleus may involve polysynaptic pathways.Support or Funding InformationFunding: APS Undergraduate Summer Research Fellowship (SP), 1R01NS080180‐01A1 (DF), 1F32NS095620‐01 (KS), and Department of Defense W81XWH‐14‐1‐0625 (PR).