Threshold channels—a novel type of sodium channel in squid giant axon

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Abstract
Sodium channels in nerve and muscle cells are functionally similar across wide phylogenetic boundaries1 and are usually thought to represent a single, homogeneous population that initiates the .action potential at threshold and unerringly transmits it along the surface membrane. In marked contrast, many cell types are known to have several distinct potassium permeability systems2,3. Distinguishable populations of Na channels have been reported in a few cell types, however, including denervated skeletal muscle4, embryonic cardiac muscle5, Purkinje cell somata6and non-myelinated axons at low temperature7. We report here that in squid giant axon, in standard experimental conditions, there are two functionally distinct populations of Na channels. The newly discovered population accounts for only a few per cent of the total Na permeability. The channels are selectively activated by small depolarizations and have very slow closing kinetics. Because these channels activate at voltages near the resting potential and tend to stay open for long times, they must dominate behaviour of the axon membrane in the threshold region for action potential initiation.