Abstract Spinal cord Injury (SCI) leads to sensorimotor paralysis for which there is currently no cure. Our previous work determined that optogenetic spinal stimulation, a relatively new method for stimulating the spinal cord, has robust neuromodulating effects that significantly increases axonal growth, neuro-vasculature and importantly, functional recovery in rats with sub-chronic cervical spinal injuries. However, recovery was less complete in rats with more severe injuries. With the overarching goal of identifying effective therapeutics for spinal injuries of varying magnitudes, the proposed study will further develop and optimize optogenetic spinal stimulation for the severely hemi-contused spinal cord. More specifically, in Aim 1 we will identify if increasing the number of weekly stimulation periods will promote significant recovery in rats with a severe cervical hemicontusion. We will increase stimulation sessions from 1x/week to 2,3 and 4x/week and investigate the effects on forelimb functional recovery of four behavioral tasks. In Aim 2, we will investigate the combinatorial effects of optogenetic spinal stimulation and Intracellular Sigma Peptide (ISP) on functional recovery in rats after severe cervical hemicontusion. ISP alleviates the inhibitory effects of chondroitin sulfate proteoglycans on axonal growth and new circuitry formation after SCI. We hypothesize that combining this peptide with optogenetic stimulation will increase the critical formation of new circuitry bridging the lesion site, helping to reconnect upstream neurons with downstream targets that will translate into improved functional recovery. This will be further explored in Aim 3, where we will identify the changes in circuitry that occur in response to optogenetic spinal stimulation alone or in combination with ISP delivery. This will be assessed by looking at spinally-evoked forelimb muscle activity in combination with the activity-sensitive viral vector CaMPARI2 to identify participating neurons, as well as additional tissue analysis using immunohistochemistry and RNAScope. Overall, the proposed study will further develop and optimize a new and powerful method for therapeutically stimulating the spinal cord and promoting significant functional recovery within the sub-chronic, severe SCI community. The findings from circuitry analysis will also provide important insights into future strategies for further improving functional recovery after SCI.