A neuronal circuit that generates the temporal motor sequence for the defensive response in zebrafish larvae

Animals use a precisely timed motor sequence to escape predators. This requires the nervous system to coordinate several motor behaviors and execute them in a temporal and smooth manner. We here describe a neuronal circuit that faithfully generates a defensive motor sequence in zebrafish larva. The temporal defensive escape sequence consists of an initial escape and a subsequent fast forward swim and can be initiated by unilateral stimulation of a single M-cell. The smooth transition from escape phase to fast forward swim phase is achieved by activating a neuronal chain circuit, which permits a M-cell to drive descending neurons in bilateral nMLF via activation of an intermediate excitatory loop formed by interconnected hindbrain cranial relay neurons. The sequential activation of escape command center, M-cells and a fast swim command center, neurons in bilateral nMLF via a neuronal chain circuit ensures the smooth execution of escape and swim phases in a timely manner. We propose there exists a serial model that executes a temporal motor sequence involving three different brain regions that initiates the escape behavior and triggers a subsequent fast forward swim. This model has general implications regarding the neural control of complex motor sequences.