In teleosts, a startle response occurs when one of a pair of Mauthner cells (Mcells) originating in the fish hindbrain is excited by sensory afferents and fires a single action potential. This action potential propagates caudally along the M-axon, which crosses the midline and continues tailward within the spinal cord. In passage, the M-spike excites (via collaterals) primary motor neurons within the spinal cord which, in turn, activate nearby axial musculature. The most obvious behavioral result of this highly conserved (at least in teleosts) and stereotypic sequence is a fast body bend toward the side of the active M-axon such that the fish forms the shape of the letter C. This is stage 1 of the teleost 'C-start', also previously called the M-reflex (for reviews, see Nissanov and Eaton, 1989; Eaton and DiDomenico, 1986; Eaton and Nissanov, 1985; Eaton and Hackett, 1984; Eaton and Bombardieri, 1978; Diamond, 1971).