Although locomotion may seem effortless, it relies on spinal circuits producing extraordinarily complex patterns of muscle contractions. Graded movements occur over a wide range of amplitudes and frequencies, instantaneously adapting to changes in the environmental or inner state. Locomotion relies on oscillations generated by genetically determined circuits within the spine. Sensory information flowing in the circuits selects and shapes the pattern of motor outputs by triggering, stopping or steering locomotion. Sensory cells connect with ensembles of premotor neurons. Challenging studies in moving animals revealed that complex proprioceptive inputs were timed to the locomotor cycle. But when are mechano, chemo, and thermosensory inputs active in moving animals? And how are they integrated by spinal cell assemblies to alter motor patterns? I propose to study sensory-motor integration in a simple genetic model organism to address these crucial points. The transparency of the zebrafish larva enables measuring and manipulating sensory inputs in moving animals. Recently, we developed in vivo optogenetic approaches for probing spinal circuits. We identified a novel sensory pathway in vertebrates which interfaces spinal circuits with the cerebrospinal fluid. We demonstrated that remote activation of these inputs triggered slow locomotion. We will now combine optogenetics, population imaging, electrophysiology and quantitative behavioural analysis to dissect this novel proprioceptive function (Aim 1). We will extend our approach to other senses to examine when specific sensory inputs are recruited in the swimming cycle and how they dynamically shape motor output in moving animals (Aim 2). We will unravel how sensory inputs select motor patterns at the circuit level (Aim 3). Our studies will contribute to a fundamental question: how do circuits integrate local sensory information to shape complex output patterns?