Fig. 7. A model for the function of spiral fiber neurons in wild-type and space cadet larvae. For simplicity, auditory input, spiral fiber neurons, commissural (yellow) and collateral PHP neurons (blue) are only shown on the left side. The broken line designates the midline. (A) The behavioral threshold of the Mauthner cell (M, red) is determined by the relation between excitation (sensory input, black) and inhibition (PHP neurons, blue and yellow). In wild-type fish, sensory inputs (black), e.g. from the vestibular system through the VIIIth nerve activate the Mauthner cell and PHP neurons, which in turn inhibit the Mauthner cell. At weak stimulus strengths, PHP-mediated inhibition dominates, but at higher sensory input strength, this inhibition is overcome (Faber and Korn, 1978). Spiral fiber neurons (green) are thought to produce excitatory stimuli within the Mauthner axon cap and might also activate inhibitory PHP neurons. Our results suggest that spiral fiber neurons synapse on additional hindbrain neurons. The inputs on the spiral fiber neurons are unknown. (B) In space cadet mutants, spiral fiber neurons fail to develop their normal commissural axonal trajectories and might project ipsilaterally. Absence of correct spiral fiber trajectories causes (1) reduced and (2) spurious activation of the escape response. (1) Lack of spiral fiber-mediated activation of the Mauthner cell (at the axon cap) might reduce the excitability of the Mauthner cell. (2) The absence of spiral fiber input on PHP neurons might reduce their inhibition of the Mauthner cell. As a consequence, weak (sensory) stimuli can overcome PHP-mediated inhibition, resulting in spurious activation of the escape response.