properties; some inactivating extremely slowly and others inactivating extremely quickly. This difference in activation time influences the duration and rate of action potential firing, which has a significant effect on electrical conduction along an axon as well as synaptic transmission. Potassium channels differ in structure from the other channels in that they contain four separate polypeptide subunits, while the other channels contain four homologous domain but on a single polypeptide unit. Chloride (Cl−) channels Chloride channels are present in all types of neurons. With the chief responsibility of controlling excitability, chloride channels contribute to the maintenance of cell resting potential and help to regulate cell volume. Proton (H+) channels Voltage-gated proton channels carry currents mediated by hydrogen ions in the form of hydronium, and are activated by depolarization in a pH-dependent manner. They function to remove acid from cells. Phylogenetics Phylogenetic studies of proteins expressed in bacteria revealed the existence of a superfamily of voltage-gated sodium channels. Subsequent studies have shown that a variety of other ion channels and transporters are phylogenetically related to the voltage-gated ion channels, including inwardly rectifying K+ channels, ryanodine-inositol 1,4,5-triphosphate receptor Ca2+ channels, transient receptor potential Ca2+ channels, polycystin cation channels, glutamate-gated ion channels, calcium-dependent chloride channels, monovalent cation:proton antiporters, type 1, and potassium transporters. See also Potassium channel Catecholaminergic polymorphic ventricular tachycardia References External links IUPHAR-DB Voltage-gated ion channel subunits The IUPHAR Compendium of Voltage-gated Ion Channels 2005 Ion channels Electrophysiology Integral membrane proteins