Interneuronal activity patterns during Active locomotion of spinal dogfish

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Abstract
Interneuronal activity was recorded from the spinal cord of paralysed spinal dogfish ( Scyliorhinus canicula ) showing Active swimming as indicated by rhythmic activity in the motor nerves. The interneurons from which spike activity was recorded during unstimulated fictive swimming ( n = 282) were divided into three groups, according to their firing patterns. Group I units (29% ) discharged steadily (mean interspike interval 25-100 ms); their firing patterns were not or only weakly modulated in phase with the spinal cord motor output; when fish with different swimming rhythms were compared, no correlation was found between the average frequency of firing of these units and the mean cycle period of the motor rhythm ; when spinal motor output stopped, these neurons remained active. Group II units (19%) discharged throughout the entire cycle of the motor rhythm although a few became phasically active at short cycles; their firing was clearly modulated in line with the motor rhythm and during shorter cycles increased in frequency; when motor output stopped, their spike activity was strongly reduced or absent. Group III units (52 %) discharged bursts of action potentials in time with the motor rhythm, each unit firing during its own characteristic phase within the motor output cycle; their firing frequencies increased linearly with locomotor frequency; these units were silent when motor output stopped.

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