Figure 1. Schematic representation of the cholinergic receptors in the plasma membrane. A. Side view of the muscarinic receptor with a G-protein complex. Note the N- and C-terminal end of the protein with its seven transmembrane segments. The acetylcholine (ACh) binding site is thought to be in the center of the receptor complex. B. Side view of the nicotinic ACh receptor. The receptor represented corresponds to the α₄β₂ which is thought to be the major brain high-affinity nicotinic receptor. Note the subunit arrangement with the axis of symmetry and the ionic pore that is formed in the center of the assembly. The ACh binding site is at the interface between two adjacent α-β subunits.