Acetylcholine receptors

The acetylcholine receptors are of two genetically
and functionally different types. Pharmacologically
they can be differentiated according
to the effects of nicotine and muscarine.
The nicotine-sensitive acetylcholine receptor is
an ion channel for potassium and sodium. It
consists of five subunits: two !, one ", one #,
and one $ (1). Acetylcholine binds as a ligand to
the two ! subunits. Each subunit consists of
four transmembrane domains (2). Each subunit
is encoded by its own gene (3). These genes
have similar structures and nucleotide base
sequences. The ligand-gated ion channel is an
example of direct transport without an intermediate
carrier. A mutation in the second transmembrane
region has been shown to change
the ion selectivity from cations to anions