Streeter KA, Sunshine MD, Patel SR, Liddell SS, Denholtz LE, Reier PJ, Fuller DD, Baekey DM. Coupling multi-electrode array recordings with silver labeling of recording sites to study cervical spinal network connectivity. J Neurophysiol 2016;jn.
Mid-cervical spinal interneurons form a complex and diffuse network and may be involved in modulating phrenic motor output. The intent of the current work was to enable a better understanding of mid-cervical “network level” connectivity by pairing the neurophysiological multi-electrode array (MEA) data with histological verification of the recording locations. We first developed a method to deliver 100 nA currents to electroplate silver onto and subsequently deposit silver from electrode tips after obtaining mid-cervical (C3-5) recordings using an MEA in anesthetized and ventilated adult rats. Spinal tissue was then fixed, harvested, and histologically processed to “develop” the deposited silver. Histological studies verified that the silver deposition method discretely labeled (50 µm resolution) spinal recording locations between laminae IV-X in cervical segments C3-C5. Using correlative techniques, we next tested the hypothesis that mid-cervical neuronal discharge patterns are temporally linked. Cross correlation histograms produced few positive peaks (5.3%) in the range of 0 – 0.4 ms, but 21.4% of neuronal pairs had correlogram peaks with a lag of ≥ 0.6 ms. These results are consistent with synchronous discharge involving mono- and polysynaptic connections among mid-cervical neurons. We conclude that there is a high degree of synaptic connectivity in the mid-cervical spinal cord, and that the silver labeling method can reliably mark metal electrode recording sites and “map” interneuron populations, thereby providing a low cost and effective tool for use in MEA experiments. We suggest that this method will be useful for further exploration of mid-cervical network connectivity.
Copyright © 2016, Journal of Neurophysiology.