FICTIVE SWIMMING ELICITED BY ELECTRICAL-STIMULATION OF THE MIDBRAIN IN GOLDFISH

1. We developed a fictive swimming preparation of goldfish that will a llow us to study the cellular basis of interactions between swimming a nd escape networks in fish. 2. Stimulation of the midbrain in decerebr ate goldfish produced rhythmic alternating movements of the body and t ail similar to swimming movements. The amplitude and frequency of the movements were dependent on stimulus strength. Larger current strength s or higher frequencies of stimulation produced larger-amplitude and/o r higher-frequency movements. Tail-beat frequency increased roughly li nearly with current strength over a large range, with plateaus in freq uency sometimes evident at the lowest and highest stimulus strengths. 3. Electromyographic (EMG) recordings from axial muscles on opposite s ides 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 tai l-beat frequency and well below the frequency of brain stimulation. EM G bursts recorded from rostral segments preceded those recorded from c audal segments on the same side of the body. The interval between indi vidual 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 stimulat ion, 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 sp inal motor nerves. We characterized the motor pattern in detail to det ermine whether it showed the features present in the motor output of s wimming fish. The fictive preparations showed all of the major feature s of the swimming motor pattern recorded in EMGs from freely swimming fish. 5. The motor nerves, like the EMGs produced by stimulating midbr ain, showed rhythmic bursting at a much lower frequency than the brain stimulus. Bursts on opposite sides of the body alternated. The freque ncy of bursting ranged from 1.5 to 13.6 Hz and was dependent on stimul us 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 sa me 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 b ursts of activity in a nerve was positively and linearly correlated wi th the time between successive bursts (cycle time). Increases in cycle time were associated with increases in burst duration. Burst duration was usually a constant fraction of cycle time, with bursts occupying on average 50.6% of cycle time. 7. The rostrocaudal delay was linearly and positively correlate with cycle time. Larger cycle times were ass ociated with large delays. The delay was usually a constant fraction o f cycle time at different burst frequencies. It averaged 2.1% of a cyc le per body segment. This implies that at any point in time there is r oughly 60% of a wave of activity along the 29- to 30-segment body and tail of the fish. 8. We conclude that the motor pattern produced by mi dbrain stimulation in the paralyzed goldfish was a pattern that would produce swimming if the fish was free to move. The development and cha racterization of this fictive swimming preparation sets the stage for future studies of interactions between the spinal network for swimming and the well-studied network responsible for escape movements in gold fish.