Anatomical Characterization of Phrenic Afferent Projections Following Cervical Spinal Cord Injury

Abstract

The phrenic nerve has a rich component of sensory afferents, and there is some evidence that activation of these afferents can affect neuroplasticity in the phrenic motor system after cervical spinal cord injury (SCI). We are presently examining the neuroanatomical substrate that could enable phrenic afferent modulation of the spinal phrenic circuitry following high cervical SCI. Cholera toxin‐β (CT‐β) was applied to the cut end of the phrenic nerve to label spinal phrenic afferent projections in spinal‐intact adult Sprague‐Dawley rats and at 2‐8 wks‐ or 1 yr after lateralized C2 spinal hemisection (C2Hx). Cervical segments (C3‐C5) were also stained for C‐fiber projections using calcitonin gene related peptide (CGRP) and isolectin‐IB4. In spinal‐intact animals, CT‐β ‐labeled phrenic afferent projections terminated primarily in the ipsilateral mid‐cervical dorsal horn (laminae II‐III) and the intermediate grey matter (laminae IV‐V). CGRP and IB4 showed positive labeling in laminae I‐II. Coupling CT‐β labeling with transynaptic labeling of the phrenic motor circuit (via pseudorabies virus) demonstrated co‐labeling of afferent projections and spinal interneurons that are connected to the phrenic circuit. Contralaterally projecting phrenic afferents were not observed in spinal intact rats; however, axonal extensions across the midline were observed one year after C2HX. These early findings demonstrate primary afferent relationships with the spinal phrenic circuit that may play a role in post‐SCI respiratory neuroplasticity.Funding: NIH 1R01NS080180 (DDF), State of Florida Brain and Spinal Cord Injury Research Program