The g subunit is replaced by an e subunit in the adult muscle. Local anesthetics and other compounds such as phencyclidine bind to the receptor, apparantly at the site of the sodium channel and modulate the binding of acetylcholine to the active site. Local anesthetics also prevent ion conductance through a direct action at the channel. In general terms, acetylcholine binds to the a -subunits of the receptor making the membrane more permeable to cations and causing a local depolarization. The local depolarization spreads to an action potential or leads to muscle contraction when summed with the action of other receptors.
Nicotinic receptors possess a relatively low affinity for acetylcholine at rest.
At high concentrations of acetylcholine, the affinity for acetylcholine becomes higher and the receptor subsequently becomes desensitized. The subunit composition of nicotinic receptors differs in skeletal muscle, autonomic ganglia and brain. The table below lists some of the properties of receptors found in different tissues. Note that multiple subunit compositions are possible, which may permit the development of compounds selective for a particular combination. Within the CNS, the a 4 b 2 combination predominates.