Separate brainstem circuits for fast steering and slow exploratory turns
During locomotion, trajectory changes necessitate precise tuning of descending commands to scale turning movements according to specific tasks or objectives, resulting in either rapid steering turns during prey pursuit or routine shallow turns associated with exploration. We show that these two types of turning are controlled by separate brainstem circuits that encode rapid steering turning versus slow exploratory turns. The circuit for rapid steering is widely distributed across different brainstem nuclei, involving specific excitatory V2a and inhibitory commissural V0d neurons. The steering V2a and V0d neurons are furthermore coupled via gap junctions and simultaneously recruited to ensure rapid steering through an asymmetrical recruitment of spinal motor neurons. The recruitment of these steering neurons is primarily associated with the degree of the direction change, rather than the locomotor frequency. The brainstem steering neurons are, in turn, controlled by a subset of V2a neurons in the pretectum activated by salient visual input. Conversely, the circuit controlling swim-related slow exploratory turns comprises a different set of V2a neurons localized in fewer brainstem nuclei. These findings demonstrate a modular organization of the brainstem circuits that control rapid steering and slow exploratory turning during locomotion. Overall design: CaD neurons were taken fromTg ( gly2: GFP) zebrafish brain stem, one cell from each fish, a total of eight fish