Nicotinic receptors (nAChR) are acetylcholine-activated non-selective cation channels that mediate excitatory neurotransmission. The receptor-channels are configured as hetero- or homopentamers. The human CHRNA3 and CHRNB4 genes encode a cationic-selective, ionotropic alpha3/beta 4 nicotinic acetylcholine receptor (nAChR). Channels that contain a3 subunits typically have lower Ca2+ permeability compared to those containing a7 subunits. Therefore, alpha7 nAChRs are particularly important in regulation of Ca2+-dependent signalling pathways. Tissue distribution also differs with alpha3/beta 4 receptors prominent in autonomic ganglia and alpha7 receptors widely distributed in the CNS.