Reticulospinal Control of Rapid Escape Turning Maneuvers in Fishes

SYNOPSIS. In teleost fishes, the C-start is a rapid avoidance maneuver that steers the animal away from the strike trajectory of a predator. We review how the axial motor pattern underlying this movement is organized by the reticulospinal system. This system includes the prominent Mauthner cells and other identifiable neurons. Typical of reticulospinal cells, the Mauthner neuron has inputs from auditory, vestibular, and visual areas and has outputs to the trunk motoneurons by means of mono- and disynaptic contacts. Because reticulospinal organization and function is conserved among the vertebrates, the Mauthner network is advantageous as a model for understanding control of movement. We develop three major points. First, despite its seeming anatomical simplicity, the escape network is not a simple hierarchial system with one cell that directs the entire movement. We present evidence that the escape response is mediated by an array of neurons which include the Mauthner cells. Second, though the firing of this cell is adequate to result in a major component of the movement, the Mauthner cell is not indispensible for its execution. The network producing this movement is capable of regulating its output so that the animal produces the normal range of responses despite the absence of one of its major contributors. These findings can be accounted for by one of two models, though the most likely one involves lateral interactions, or corollary discharges, within the array. Third, our recent work also shows that the movement pattern is generated by a ballistic command which does not rely on movement-induced sensory feedback. These system properties allow the animal to produce a highly reliable and orchestrated motor command in only 15 msec from sensory stimulus to EMG.