Figure 3: Schematic representations of the basic structure of the nicotinic acetylcholine receptor (nAChR). (A) Each receptor is composed of five subunits, each composed of four transmembrane domains (M1-M4). The M2 domains form the channel pore and contribute to channel gating when the receptor is in the inactive state. The other domains form an outer ring that encircles the M2- based pore. (B) The four transmembrane domains (M1-M4) of each subunit are joined by variable length loop regions. Two short loops link the M1 and M2 domains (intracellular loop) and the M2 and M3 domains (extracellular loop). The region joining M3 and M4 is composed of a longer, intracellular loop containing an alpha helix and has a role in channel selectivity and receptor desensitization. The N-terminal extracellular domain of each subunit contributes to the formation of the hydrophobic binding pocket [78-82].