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Ion channels are fundamental excitable elements in the membranes of neurons,and control the activities of both individual neurons and neural networks.The properties of ion channels in the soma and dendritic compartments have been intensively explored with conventional patch clamp techniques and dendritic recordings.However studies of the properties and functions of axonal ion channels have been hampered by the thin diameter of axons,which prevents accessibility of patch electrodes to their membranes.By performing whole-cell recording from the cut end of the main axon of cortical pyramidal cells,we characterized a slowly inactivating potassium current that is sensitive to alpha-dendrotoxin and low concentration of 4-AP.In pyramidal neurons,there is remarkable differential expression of this current.Depolarizing the axon,but not the soma,rapidly (< 1 ms) activated a low-threshold,slowly (several seconds) inactivating,outward current.Bath application of 4-AP,alpha-dendrotoxin and rTityustoxin-Kalpha blocks this current and causes a large increase in spike duration in the axon but only a small increase in the soma.