Cardiac parameters are regulated by the neural circuits in the autonomic and central nervous systems as well as the intracardiac neurons. Although the role of the motor vagus nerve is known for heart rate, the systems-level dynamics of control are not well understood. Therefore, we aim to identify the motor vagal circuits that implement cardiac control functions, such as regulation of heart rate, cardiac output, and blood pressure, which are the crucial components of organismal homeostasis. Here, we use larval zebrafish as our model organisms due to their regenerative capability, optical transparency, and genetic accessibility. The figure below depicts transgenic zebrafish at 6dpf that express ChATa-GCaMP in the cholinergic neurons and gata1-dsred in the red blood cells. Combining optical physiology methods such as optogenetics, calcium imaging, and quantitative analysis of blood flow and heart mechanics, we identify neurons in the motor vagus nerve that support cardiac function at distinct larval stages. Neuron-mediated cardiac responses emerge at 5 days post fertilization (dpf) and continue to refine until 12 dpf. Our work sets the stage for molecular profiling of motor vagus circuits for cardiac control and subsequent mechanistic analysis of the role of cell types and molecules.Transgenic Larval Zebrafish at 6dpf (gata1-dsred= red blood cells, ChATa-GCaMP= neurons of the motor vagus)