Fictive swimming elicited by electrical stimulation of the midbrain in goldfish.

1. We developed a fictive swimming preparation of goldfish that will allow us to study the cellular basis of interactions between swimming and escape networks in fish. 2. Stimulation of the midbrain in decerebrate goldfish produced rhythmic alternating movements of the body and tail similar to swimming movements. The amplitude and frequency of the movements were dependent on stimulus strength. Larger current strengths or higher frequencies of stimulation produced larger-amplitude and/or higher-frequency movements. Tail-beat frequency increased roughly linearly with current strength over a large range, with plateaus in frequency sometimes evident at the lowest and highest stimulus strengths. 3. Electromyographic (EMG) recordings from axial muscles on opposite sides at the same rostrocaudal position showed that stimulation of the midbrain led to alternating EMG bursts, with bursts first on one side, then the other. These bursts occurred at a frequency equal to the tail-beat frequency and well below the frequency of brain stimulation. EMG bursts recorded from rostral segments preceded those recorded from caudal segments on the same side of the body. The interval between individual spikes within EMG bursts sometimes corresponded to the interval between brain stimuli. Thus, whereas the frequency of tail beats and EMG bursts was always much slower than the frequency of brain stimulation, there was evidence of individual brain stimuli in the pattern of spikes within bursts. 4. After paralyzing fish that produced rhythmic movement on midbrain stimulation, we monitored the motor output during stimulation of the midbrain by using extracellular recordings from spinal motor nerves. We characterized the motor pattern in detail to determine whether it showed the features present in the motor output of swimming fish. The fictive preparations showed all of the major features of the swimming motor pattern recorded in EMGs from freely swimming fish. 5. The motor nerves, like the EMGs produced by stimulating midbrain, showed rhythmic bursting at a much lower frequency than the brain stimulus. Bursts on opposite sides of the body alternated. The frequency of bursting ranged from 1.5 to 13.6 Hz and was dependent on stimulus strength, with higher strengths producing faster bursting. Activity in rostral segments preceded activity in caudal ones on the same side of the body. Some spikes within bursts of activity occurred at the same frequency as the brain stimulus, but individual brain stimuli were not as evident as those seen in some of the EMGs. 6. The duration of bursts of activity in a nerve was positively and linearly correlated with the time between successive bursts (cycle time).(ABSTRACT TRUNCATED AT 400 WORDS)