Alternative neural pathways initiate fast-start responses following lesions of the mauthner neuron in goldfish
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Previous studies in adult fish show a one-to-one, time-locked, correlation between the presence of an initial action potential in the Mauthner (M-) cell and the onset of C-type fast-start responses in which the initial turn is away from the side of the active M-cell. In the present study we ask whether the M-cell is necessary for such behavioral acts.
Single M-cells in goldfish were electrolytically lesioned by passing current (7.2 μA for 20 s) from a stainless steel microelectrode situated at the M-cell initial segment. Behavioral responses were elicited by dropping a ball into the aquarium holding the fish. The behavior was filmed at 500 frames/s and responses with an initial turn opposite the side of the lesioned cell (non-Mauthner responses) were compared with responses opposite the intact cell (M-initiated responses).
Control experiments on acute preparations showed that such lesions prevent the M-cell from participating in behavioral responses because orthodromic synaptic activation of the M-axon is blocked, even though after the lesion the axon maintained resting potential and could conduct impulses to antidromic stimulation (Fig. 1). Chronic recordings from lesioned fish showed that in no case was the damaged M-cell observed to fire during any behavioral trials.
The main finding was that non-Mauthner responses were no different in mechanical performance (angular velocity, displacement speed, etc.) or response probability than M-initiated responses (Figs. 4, 10, 11). The only difference was that the non-Mauthner responses were significantly longer in response latency than the M-initiated responses (Fig. 7). Such non-Mauthner responses have not yet been observed in intact goldfish.
It is concluded that firing of the M-cell is causally related to the onset of C-type fast-start responses. But, when the M-cell is inactivated, alternative neural pathways that coexist with the M-cell can initiate the C-type fast-start behavior pattern.
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