Evidence for genetically distinct direct and indirect spinocerebellar pathways mediating proprioception.
Proprioception, the sense of limb and body position, generates a map of the body that is essential for proper motor control, yet we know little about precisely how neurons in proprioceptive pathways are wired. Defining the anatomy of secondary neurons in the spinal cord that integrate and relay proprioceptive and potentially cutaneous information from the periphery to the cerebellum is fundamental to understanding how proprioceptive circuits function. Here, we use genetic tools in both male and female mice to define the unique anatomical trajectories of long-range direct and indirect spinocerebellar pathways as well as local intersegmental spinal circuits. We find that Clarkes column (CC) neurons, a major contributor to the direct spinocerebellar pathway, has mossy fiber terminals that diversify extensively in the cerebellar cortex with axons terminating bilaterally, but with no significant axon collaterals within the spinal cord, medulla, or cerebellar nuclei. By contrast, we find that two of the indirect pathways, the spino-lateral reticular nucleus (spino-LRt) and spino-olivary pathways, are in part, derived from cervical Atoh1-lineage neurons, while thoracolumbar Atoh1-lineage neurons project mostly locally within the spinal cord. Notably, while cervical and thoracolumbar Atoh1-lineage neurons connect locally with motor neurons, no CC to motor neuron connections were detected. Silencing of caudal Atoh1- lineage neurons results in a subtle motor impairment consistent with a defect in local proprioceptive circuitry. Altogether, we define anatomical differences between long-range direct, indirect, and local proprioceptive subcircuits that likely mediate different components of proprioceptive-motor behaviors. Significance StatementWe define the anatomy of long-range direct and indirect spinocerebellar pathways as well as local spinal proprioceptive circuits. We observe that mossy fiber axon terminals of Clarkes column (CC) neurons diversify proprioceptive information across granule cells in multiple lobules on both ipsilateral and contralateral sides sending no significant collaterals within the spinal cord, medulla, or cerebellar nuclei. Strikingly, we find that cervical spinal cord Atoh1-lineage neurons form mainly the indirect spino- lateral reticular nucleus and spino-olivary tracts and thoracolumbar Atoh1-lineage neurons project locally within the spinal cord while only a few Atoh1-lineage neurons form a direct spinocerebellar tract. Altogether, we define the development, anatomical projections, and some behavioral consequences of silencing spinal proprioceptive pathways.