ACTIVITIES OF SPINAL NEURONS DURING BRAIN STEM-DEPENDENT FICTIVE SWIMMING IN LAMPREY

1. We made intracellular microelectrode recordings of membrane potenti al from spinal neurons during fictive swimming elicited by brief elect rical shocks to the spinal cord in a brain stem-spinal cord preparatio n of the adult silver lamprey (Ichthyomyzon unicuspis). 2. We characte rized membrane potential activities recorded during brain stem-depende nt fictive swimming in five spinal cell types. myotomal motoneurons, l ateral interneurons (inhibitory neurons with ipsilateral descending ax ons), CC interneurons (neurons with contralateral and caudal projectin g axons), edge cells (intraspinal stretch receptors). and dorsal cells (primary mechanosensory neurons with cell bodies in the spinal cord). The membrane potential activities were compared with data from previo us reports recorded during fictive swimming in the isolated spinal cor d with fictive swimming induced by superfusion with D-glutamate. 3. Co mpared with the same cell types recorded during D-glutamate-induced fi ctive swimming in brain stem-dependent fictive swimming, the motoneuro ns and CC interneurons had significantly larger trough-to-peak amplitu des of membrane potential oscillations. whereas lateral interneurons w ere nor significantly different in amplitude. The timings of the membr ane potential oscillations and of cell spiking were nor significantly different in the two preparations. with the exception that motoneurons in brain stem-dependent fictive swimming were significantly earlier b y similar to 10% of a cycle. 4. Edge cells had only weak or no oscilla tory activities, and dorsal cells bad no detectable input during brain stem-dependent fictive swimming. These findings are similar to those in D-glutamate-induced fictive swimming. 5. These results suggest that D-glutamate-induced fictive swimming in the isolated lamprey spinal c ord produces activities in recognized neuronal types that are similar to brain stem-dependent fictive swimming. The present results also sup port the conclusion that CC interneurons and lateral interneurons may be involved in the central pattern generator for locomotion, but that dorsal cells and edge cells are not significant participants under ''f ictive'' conditions.