Sodium channels

Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na⁺) through a cell's plasma membrane. They are classified according to the trigger that opens the channel for such ions, i.e. either a voltage-change ("Voltage-gated", "voltage-sensitive", or "voltage-dependent" sodium channel also called "VGSCs" or "Nav channel") or a binding of a substance (a ligand) to the channel (ligand-gated sodium channels).

In excitable cells such as neurons, myocytes, and certain types of glia, sodium channels are responsible for the rising phase of action potentials. These channels go through 3 different states called as resting, active and inactive states. Even though the resting and inactive states wouldn't allow the ions to flow through the channels the difference exists with respect to their structural conformation.

Sodium channels are highly selective for the transport of sodium ions across cell membranes. The high selectivity with respect to the sodium ion is achieved in many different ways. All involve encapsulation of the sodium ion in a cavity of specific size within a larger molecule.

Sodium channels consist of large α subunits that associate with proteins, such as β subunits. An α subunit forms the core of the channel and is functional on its own. When the α subunit protein is expressed by a cell, it is able to form channels that conduct Na⁺ in a voltage-gated way, even if β subunits or other known modulating proteins are not expressed. When accessory proteins assemble with α subunits, the resulting complex can display altered voltage dependence and cellular localization.

The α-subunit has four repeat domains, labeled I through IV, each containing six membrane-spanning segments, labeled S1 through S6. The highly conserved S4 segment acts as the channel's voltage sensor. The voltage sensitivity of this channel is due to positive amino acids located at every third position. When stimulated by a change in transmembrane voltage, this segment moves toward the extracellular side of the cell membrane, allowing the channel to become permeable to ions. The ions are conducted through a pore, which can be broken into two regions. The more external (i.e., more extracellular) portion of the pore is formed by the "P-loops" (the region between S5 and S6) of the four domains. This region is the most narrow part of the pore and is responsible for its ion selectivity. The inner portion (i.e., more cytoplasmic) of the pore is formed by the combined S5 and S6 segments of the four domains. The region linking domains III and IV is also important for channel function. This region plugs the channel after prolonged activation, inactivating it.

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