Absence of potassium conductance in central myelinated axons

Abstract

Two voltage-dependent changes in ionic permeability are responsible for the action potential in squid giant axon. The depolarization phase of the action potential is due to an initial increase in Na+ permeability and repolarization is primarily the result of a later increase in K permeability. Voltage-clamps on mammalian [rat] peripheral nodes of Ranvier implied K conductances (gk) were minimal or lacking for intact mammalian peripheral myelinated axons. Repolarization for these fibers was explained by rapid Na inactivation and large leakage current. When the myelin around these fibers was acutely disrupted, an immediate and prominent gk appeared. Following demyelination, gk blocking agents [tetraethylammonium hydrochloride and aminopyridine] reduced late outward currents that were not present in normal myelinated fibers5. K+ channels apparently were present in the axonal membrane under the myelin but are masked in normal peripheral myelinated axons. The presence or role of K+ channels in central myelinated axons was not previously investigated. gk was not detectable in mammalian dorsal column axons.