Patterns of synaptic inhibition in motoneurons and interneurons during fictive swimming in the lamprey, as revealed by Cl− injections

SummaryDuring fictive swimming in the isolated spinal cord of the lamprey (Ichthyomyzon unicuspis andPetromyzon marinus) the membrane potentials of motoneurons (MNs), lateral interneurons (L INs), and CC interneurons (CC INs) oscillate between a depolarised and a relatively hyperpolarised phase. After intracellular Cl− injections (usually combined with a DC hyperpolarisation) IPSP's became depolarising, and in cells which were phasically inhibited, phases of relative hyperpolarisation became phases of relative depolarisation. The peak depolarisation and/or spike burst mid point in MNs after Cl− injection occurred at a phase of 0.65 ± 0.12 (mean ±S.D.) in the cycle, with zero being the start of the ipsilateral ventral root burst. In CC INs the peak depolarisation and/or spike burst mid point after Cl− occurred significantly earlier, at a phase of 0.40 ± 0.18. L INs were also phasically inhibited with peak depolarisation and/or spike burst mid point after Cl− injection at an intermediate phase of 0.52 ± 0.21. It is concluded that the central pattern generator for fictive swimming has at least three synaptic outputs: an early excitation, and inhibition at a range of phases, which could be combinations of an early and a late inhibition.