<prompt> Sodium channels are specifically blocked by:NifedipineTetrodotoxinTetraethyl leadCholine  <response>: Another means for studying the flow of ions through an individual type of channel is to block one type of channel at a time. For instance, the sodium channels can be blocked by a toxin called tetrodotoxin when it is applied to the outside of the cell membrane where the sodium activation gates are located. Conversely, tetraethylammonium ion blocks the potassium channels when it is applied to the interior of the nerve fiber.Key Concept:Tetrodotoxin TTX, one of the most potent poisons known, specifically blocks the Na channel. TTX binds to the extracellular side of the sodium channel.Reference: GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITIONINTERNATIONAL EDITION page no 67 