Dissecting the connectivity map of spinal ascending projections
1 University Hospital of Cologne, Department of Neurology, Cologne, Germany
Execution and adaptation of motor behaviors require the interaction of networks of neurons in the spinal cord, sensory ganglia, and the brain. Sensory input from the skin and muscles is relayed to the dorsal horn of the spinal cord. Neurons in the dorsal and intermediate spinal cord integrate and convey the processed sensory feedback to the ventral premotor network to initiate or correct motor actions or to specialized brain areas for perception and long-term adaptation. Although the identity and post-synaptic targets of spinal ascending projection neurons have started to be characterized, we still lack a complete connectivity map of cell types and targeted regions. Understanding this map will be crucial to our understanding of how spinal neurons broadcast the sensory information for adaptive movements in response to perturbations. We engineered an intersectional genetic strategy to specifically target different classes of spinal ascending projection neurons and assess their post-synaptic targets in the brain, their refinement over development, and their contribution to adaptive behaviors. We identified expected (dorsal column nuclei) and unexpected (raphe) brain areas that receive direct spinal innervation, with some of these projections being prominent in the early postnatal stages but almost disappearing in adulthood. Our data also paint a more complex picture, with ascending projections that relay both ongoing motor actions and sensory feedback converging on common supraspinal targets. The characterization of these projections will be pivotal to our understanding of how adaptation strategies are influenced by ongoing actions and sensations.