The motor output of the Mauthner cell, a reticulospinal command neuron.

We electrically stimulated individual Mauthner (M-) cells to determine their motor contribution to C-starts of swimming goldfish. In comparison with sensory-evoked C-starts, M-reflexes triggered by electrical stimulation of single M-cells were significantly weaker and less variable. Stage 1 turns were both longer in duration and smaller in angle for the M-reflex when compared with the sensory-evoked C-start. This translates to an average reduction of 22% in angular velocity during stage 1. Likewise, during stage 2, the distance moved by the fish was reduced by 15% and the absolute value of stage 2 turning angle was reduced by 47%. In addition, the normal mechanical or neural coupling between stages 1 and 2 appeared to be altered for the M-reflex. From this and our other recent studies, we conclude that there must be two primary groups of reticulospinal neurons in the escape triggering network. The first group includes the M-cell and determines the initial left-right direction of the response and the extent of stage 1 angle. From previous EMG recordings we know that the second group of neurons can fire within 5-15 ms (average, 9 ms) after the stage 1 cells. These determine the onset time and direction of stage 2. Together the coupling of the two primary groups results in the full propulsive force and turning flexibility of the C-start.