Every year nearly one million Americans undergo surgery for nerve reattachment after nerve injuries, but despite continued improvements in microsurgical techniques, a majority of these patients are left with permanent motor deficits. Usually these are believed to result from poor regeneration of the peripheral nerve. However, deficits are still present when experimental nerve injuries are designed in animal models for rapid, specific and efficient nerve regeneration and muscle re-innervation. In the past we reported that structural remodeling of spinal cord circuitry after nerve lesions is in part responsible. After injury, the central synaptic branches of sensory proprioceptive axons and motor axons injured in the periphery are removed from the ventral horn of the spinal cord resulting in dysfunction of several critical motor control circuits. The mechanisms of synapse and axon removal are therefore clearly important, but unknown. Our preliminary data implicate the neuroinflammatory response that occurs inside the intact spinal cord around cell bodies of peripherally-injured motoneurons and along the central projections of peripherally-injured sensory axons. Microglia is activated in these regions and although their capacity for synapse phagocytosis has been frequently proposed, their exact function after nerve injury remains controversial. In addition, monocytes infiltrate the spinal cord and transform into macrophages. These cells were missed in previous studies because they become indistinguishable from microglia. Thus, their interactions with resident microglia and possible roles in synaptic plasticity are unknown. More recently we found that after nerve injuries triggering large synaptic circuit remodeling there is additional infiltration by T-cells. The roles T-cells play in synaptic remodeling are completely unknown. To investigate the relation between microglia and different immune cell infiltrates in relation to synapse plasticity we will use mice in which microglial cells are labeled with GFP and infiltrating immune cells by RFP and perform a number of genetic manipulations to interfere with the function of one or other cell and test possible signaling mechanisms leading to microglia activation, immune cell infiltration and synapse removal. In Aim 1 we will investigate the role of peripherally derived monocytes, macrophages and T-cells in the synaptic removal of inputs from muscle sensory afferents. In Aim 2 we will investigate whether microglia activation is necessary for recruitment of blood-derived immune cells and investigate the signals promoting these invasion. In Aim 3 we will use two- photon time-lapse imaging to directly observe and analyze the process of removal of sensory synapses and the mechanisms that facilitate specific recognition of axons injured in the peripheral nerve. Finally, in Aim 4 we will block the central neuroinflammatory response to preserve proprioceptive synaptic inputs and test patterns of muscle activation during treadmill locomotion after nerve regeneration with electromyography. The results will inform about the role of neuroinflammation and the cellular mechanisms involved in removing specific inputs from the spinal cord and will provide first insights into motor outcomes after interfering with this process.