Nicotinergic acetylcholine receptors are made up of 5 subunits, symmetrically arranged around a central pore (ion channel). It is considered the best-characterized LGIC and a prototypical structure for the class of 4TM receptors. nAChRs are found mainly in postsynaptic membranes of central nervous system synapses and of the neuromuscular endplate. The physiological signal to which nAChR responds is the neurotransmitter acetylcholine, but it is also activated by nicotine. The assembly of combinations of subunits (17 distinct subunits have been identified) results in a large number of different receptors with a high variety of functional diversity as a result. 5-HT3 differ from all other 5-HT (serotonin) receptors whose actions are mediated via G proteins. Their structure and function has placed them in the Cys-loop family of cationic ligand-gated ion channels. 5-HT3 receptors are located in both the peripheral (PNS) and central (CNS) nervous systems. In the CNS, 5-HT3 receptors may play roles in a variety of functions including emesis, cognition and anxiety, whereas in the PNS they play a role in a variety of sympathetic, parasympathetic and sensory functions (e.g. signaling in gastrointestinal tract, gut motility and peristalsis).