Objective Oligodendrocyte that forms the myelin in central nervous system has been described as the passive support cell to insulate the axon for decades.An individual oligodendrocyte supports 1-50 myelin sheaths and accelerates the action potential propagation by reducing the ion leakage and capacitance of cell membrane.However, recent studies indicate that oligodendrocyte depolarizes during the population activity of surrounding neurons, which suggests some potential active functions of oligodendrocyte.But the mechanism of this reaction is still elusive.Hence, we use whole cell patch recording to monitor the oligodendrocytic response to neuronal activity, which would be helpful to identify the relationship between them and clarify the oligodendrocytic effects on the propagation of action potential.Methods In order to identify oligodendrocyte in brain slice, a transgenic mouse that expresses enhanced green fluorescence protein (EGFP) driven by the 2,3-cyclic nucleotide 3-phosphodiesterase (CNP) promoter (CNP-EGFP) was used in our experiments.We performed paired whole cell recordings from a pyramidal neuron and an oligodendrocyte of layer 5 in cerebral cortex.The paired cells were filled with two different cell-impermeable fluorescent dyes through recording pipettes, which was used to trace these cells and identify the relationship between their structures.Results (1) An inward current was recorded on an oligodendrocyte when a spike was propagating on the axon wrapped by this oligodendrocyte.If the oligodendrocyte didnt connect with the paired pyramidal cell, no response was observed.(2) This current consists of two phases: a fast one and a slow one.The amplitude of the fast phase increased when the oligodendrocyte was depolarizing, while that of the slow phase decreased.(3) The blockers of voltage-gated calcium channel, ionic glutamate receptors, glutamate transporters and gap junction all had no effect on the amplitudes of these two phases.CsCl totally blocked the slow phase but had no effect on the fast one that might be mediate by the passive electrical coupling between axon and myelin.(4) Both of the two phases didnt display the short-time plasticity.Conclusions Our results give the directly evidences that oligodendrocyte insulates axon and clears the potassium released during action potential propagation.