Project 1: The Neural Basis of Muscle Action Loops Abstract The goal of Project 1 is to identify neural mechanisms of sensorimotor processing within low-level muscle action loops that control walking and flight behavior. Acting at the interface between the nervous system and the body, muscle action loops face a daunting set of demands: they must operate with both speed and precision, while retaining the flexibility and robustness necessary to function in diverse, unpredictable environments. To understand how motor control circuits balance these demands to control adaptive behavior, we will combine cell-type specific genetic tools with electrophysiology, functional imaging, and behavioral analysis in the fly. We will first investigate how activity within diverse populations of motor neurons interacts to produce muscle contraction and complex body movements. We will then ask how proprioceptive feedback signals are integrated with ongoing motor commands to adapt and refine behavioral output. Finally, we will work to understand how low-level muscle action loops interact with descending neuromodulatory signals to generate patterned motor activity. These experimental and theoretical efforts are divided into the following Specific Aims: Specific Aim 1: Investigate how motor neurons robustly control body movement. Specific Aim 2: Determine the role of proprioceptive feedback in fine-tuning precise motor output. Specific Aim 3: Identify origins and mechanisms of sensorimotor neuromodulation that contribute to flexible locomotor behavior.