SpinalTRAQ: A Novel Pipeline for Volumetric Cervical Spinal Cord Analysis Identifies the Corticospinal Tract Synaptic...

The corticospinal tract (CST) is essential for forelimb-specific fine motor skills. In rodents, it undergoes extensive structural remodeling across development, injury, and disease states, with major implications for motor function. A vast body of literature, spanning numerous injury models, frequently assesses these projections. Despite this, a cohesive imaging modality for rapid, quantitative assessment of the bilateral cervical spinal cord projectome is lacking. To address this, we developed SpinalTRAQ (Spinal cord Tomographic Registration and Automated Quantification), a novel mouse cervical spinal cord volumetric reference atlas and machine learning-based analytical pipeline. Using serial two-photon tomography, SpinalTRAQ enables unbiased, region-specific quantification of fluorescently labeled CST presynaptic terminals. In healthy male mice, the CST exhibits a distinct bilateral synaptic projectome, with the densest innervation in laminae 5 and 7 on the contralateral side and lamina 7 on the ipsilateral side. We additionally observed sparse synaptic input in lamina 9, specifically axial motor neuron pools, which we found was targeted to spinal motoneurons. Following focal motor cortical stroke, the injured CST axons are depleted, and contralesional CST projections are significantly increased after 4 and 6 weeks. By 6 weeks post-stroke, ipsilateral CST synapses were increased by fivefold, with the greatest increases seen in homotopic laminae and all motor neuron pools. SpinalTRAQ offers detailed, level- and lamina-specific quantification of the bilateral cervical spinal cord synaptic projectome, revealing previously unrecognized CST connectivity and plasticity after injury.