Project Summary The recovery of function after damage is an important, yet elusive goal in motor control. Spinal motor neurons are particularly vulnerable to damage thanks to long axons that innervate peripheral muscles. Motor neurons also have central axon collaterals that impact motor output, but we know little of their response to injury. This is important because changes in motor neuron excitability accompany spinal injury, contributing to short-term spasticity and long-term recovery. The goal of this exploratory R21 proposal is to develop a new model system to understand how central spinal circuits reorganize after injury, beginning with an overlooked motor neuron response to injury. Like all vertebrates, zebrafish have slow, intermediate and fast types of spinal motor units defined by their target musculature and each type exhibits central axon collaterals. However, unlike all other vertebrates it is possible to track the same spinal motor neurons and their central and peripheral connections before and after injury in living, growing zebrafish. In pilot experiments using two-photon axotomies, we found that some motor neurons regrow and successfully re-innervate target musculature, while others fail to exit spinal cord and instead grow elaborate central axons and form synapses. This mirrors a phenomenon also observed in axotomized spinal motor neurons of adult cats. These observations suggest that some axotomized motor neurons are creating ectopic circuits that could impact motor control. But what conditions predict peripheral versus central regeneration and do ectopic central motor axons integrate into functional circuits? In Aim 1, we will assess the relative contribution of time of development and motor unit identity to axotomy responses. In Aim 2, we will determine if axotomized motor neurons are recruited and connected to other spinal neurons. These aims will characterize the capacity for motor neurons to form ectopic recurrent circuits and will inform studies exploring maladaptive and adaptive changes in spinal circuit structure following injury.