The goal of our research is to understand how circuits in the brainstem and spinal cord control the versatility of motor actions using the zebrafish as a model system. Our recent results have revealed that the vigor of locomotor movements are controlled by three distinct circuit modules that control selectively the activity of slow, intermediate and fast muscles. We now seek to understand the molecular mechanisms underlying the functional diversity of interneurons and motoneurons using single-cell RNAseq. Zebrafish is the only vertebrate model system in which a direct link between neuronal diversity and function could be elucidate. Thus, this project will provide novel insights into the molecular mechanisms controlling diversity of circuits for motor actions and will also serve as a guide for studies in mammalian systems.