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 larvae. The temporally specific defensive motor sequence consists of an initial escape and a subsequent swim behavior and can be initiated by unilateral stimulation of a single Mauthner cell (M-cell). The smooth transition from escape behavior to swim behavior is achieved by activating a neuronal chain circuit, which permits an M-cell to drive descending neurons in bilateral nucleus of medial longitudinal fascicle (nMLF) via activation of an intermediate excitatory circuit formed by interconnected hindbrain cranial relay neurons. The sequential activation of M-cells and neurons in bilateral nMLF via activation of hindbrain cranial relay neurons ensures the smooth execution of escape and swim behaviors in a timely manner. We propose an existence of a serial model that executes a temporal motor sequence involving three different brain regions that initiates the escape behavior and triggers a subsequent swim. This model has general implications regarding the neural control of complex motor sequences. [Display omitted] •M-cell activation in zebrafish larvae generates defensive motor sequence•The defensive motor sequence consists of an initial escape and a subsequent swim•Transition from escape to swim relies on recruitment of neurons in nMLF•M-cells drive neurons in nMLF via activating hindbrain cranial relay neurons Xu et al. show that the temporal motor sequence for defensive response in zebrafish larvae consists of an initial escape and a subsequent swimming. This sequential movement is initiated by M-cell activation, which drives neurons in bilateral nucleus of medial longitudinal fascicle via activation of interconnected hindbrain cranial relay neurons.