PROJECT SUMMARY A principle aim of the NINDS is to determine how motor control is successfully implemented by the nervous system. Locomotion and balance are complex motor functions that are largely controlled by complex microcircuits that reside outside the brain. Understanding how such microcircuits function is critical to being able to treat diseases related to age, congenital disorders, and trauma in which these circuits are impaired. This proposal will leverage advantages of a highly tractable model system, the fruit fly (Drosophila melanogaster), to elucidate the computational principles underlying the sensorimotor circuits that govern flight stabilization. Fruit flies are an excellent model system for conducting such studies for several reasons. First, through a collaborative project with the HHMI, the PI has helped develop ~220 transgenic fly lines targeting sparse populations of neurons in the fly ventral nerve cord (VNC), which can be chronically silenced, optogenetically activated, or optogenetically suppressed. Second, in experiments pioneered by the PI, we showed that the reflexive responses of the fly to yaw, pitch, and roll perturbations are described quantitatively by a proportional-integral controller?a control strategy similar to a car?s cruise control or a sophisticated thermostat. Thus, the fly?s stabilization reflexes, while complex, are well characterized. Consequently, there is an opportunity to systematically interrogate neurons in the VNC and determine their effect on a sophisticated motor behavior. Towards this end, in Aim 1 we will map the function of the motor system that actuates rapid flight stabilization in flies. Specifically, we will chronically silence or transiently manipulate individual motor neurons that innervate wing steering muscles and test control performance in free flight and under rapid mechanical perturbations. In Aim 2 we will elucidate the functional role specific mechanosensory neurons in the control reflex. Once again we will use chronic silencing or transient manipulation of individual mechanosensory neurons and test control performance in free flight and under rapid mechanical perturbations. Finally, in Aim 3 we will identify the neural architecture connecting the mechanosensory inputs to the wing muscle outputs. Specifically, we will use anterograde transsynaptic circuit tracing (trans-Tango) and ex-vivo functional imaging to identify motor and interneurons that receive direct input from the genetically-identified mechanosensory afferents. Together, these studies will enable us to determine with unprecedented detail the organization and function of these microcircuits. In turn, this knowledge will inform our understanding of design principles for sensorimotor circuits across animals.