Plasticity and Activation of Spared Intraspinal Respiratory Circuits Following Spinal Cord Injury

52 Mid-cervical spinal interneurons form a complex and diffuse network and may be involved in53modulating phrenic motor output. The intent of the current work was to enable a better54understanding of mid-cervical “network level” connectivity by pairing the neurophysiological55multi-electrode array (MEA) data with histological verification of the recording locations. We first56developed a method to deliver 100 nA currents to electroplate silver onto and subsequently57deposit silver from electrode tips after obtaining mid-cervical (C3-5) recordings using an MEA in58anesthetized and ventilated adult rats. Spinal tissue was then fixed, harvested, and59 histologically processed to “develop” the deposited silver. Histological studies verified that the60 silver deposition method discretely labeled (50 μm resolution) spinal recording locations61between laminae IV-X in cervical segments C3-C5. Using correlative techniques, we next tested62 the hypothesis that mid-cervical neuronal discharge patterns are temporally linked. Cross63 correlation histograms produced few positive peaks (5.3%) in the range of 0 0.4 ms, but 21.4%64of neuronal pairs had correlogram peaks with a lag of ≥ 0.6 ms. These results are consistent65 with synchronous discharge involving monoand polysynaptic connections among mid-cervical66neurons. We conclude that there is a high degree of synaptic connectivity in the mid-cervical67 spinal cord, and that the silver labeling method can reliably mark metal electrode recording sites68 and “map” interneuron populations, thereby providing a low cost and effective tool for use in69MEA experiments. We suggest that this method will be useful for further exploration of mid-70 cervical network connectivity.71 New and Noteworthy72 We describe a method that reliably identifies the locations of multi-electrode array (MEA)73recording sites while preserving the surrounding tissue for immunohistochemistry. To our74knowledge, this is the first cost-effective method to identify the anatomical locations of neuronal75ensembles recorded with a MEA during acute preparations without the requirement of76specialized array electrodes. In addition, evaluation of activity recorded from silver labeled sites77revealed a previously unappreciated degree of connectivity between mid-cervical interneurons.78