The injection of tetraethylammonium chloride into the giant axon of the squid prolongs the action potential and eliminates most of the late current under voltage-clamp. Experiments on fibers in an external medium of high potassium ion concentration demonstrate that injected tetraethylammonium chloride causes rectification of the instantaneous current-voltage curve for potassium by excluding outward current. This interference with the flow of outward potassium ion current underlies the prolongation of the action potential seen in tetraethylammonium-injected fibers.

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