Cat ’ s medullary reticulospinal and subnucleus

37 Animals and human beings sense and react to real/potential dangerous stimuli. However, 38 the supraspinal mechanisms relating noxious sensing and nocifensive behavior are mostly 39 unknown. The collateralization and spatial organization of interrelated neurons are 40 important determinants of coordinated network function. Here we electrophysiologically 41 studied medial medullary reticulospinal neurons (mMRF-RSNs) antidromically identified 42 from the cervical cord of anaesthetized cats and found that, 1) more than 40% (79/183) of 43 the sampled mMRF-RSNs emitted bifurcating axons running within the dorsolateral (DLF) 44 and ventromedial (VMF) ipsilateral fascicles, 2) more than 50% (78/151) of the tested 45 mMRF-RSNs with axons running in the VMF collateralized to the subnucleus reticularis 46 dorsalis (SRD) that also sent ipsilateral descending fibers bifurcating within the DLF and the 47 VMF. This percentage of mMRF collateralization to the SRD increased to more than 81% 48 (53/65) when considering the subpopulation of mMRF-RSNs responsive to noxiously 49 heating the skin, 3) reciprocal monosynaptic excitatory relationships were 50 electrophysiologically demonstrated between noxious sensitive mMRF-RSNs and SRD 51 cells, and 4) injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin 52 evidenced mMRF to SRD and SRD to mMRF projections contacting the soma and proximal 53 dendrites. The data demonstrated a SRD-mMRF network interconnected mainly through 54 collaterals of descending axons running within the VMF, with the subset of noxious 55 sensitive cells forming a reverberating circuit probably amplifying mutual outputs 56 simultaneously regulating motor activity and spinal noxious afferent input. The results 57 provide evidence that noxious stimulation positively engage a reticular SRD-mMRF-SRD 58 network involved in pain-sensory-to-motor transformation and modulation. 59