Figure 22-4 Sketch of acetylcholine receptor channels (right) and tracings of cell-patch records of receptor channel openings (left). The mature, or junctional, receptor consists of two α-subunits and one each of β-, δ, and epsilon-subunits. The immature, extrajunctional or fetal form consists of two α- and one each of β, δ, and γ-subunits. These subunits are arranged around the central cation channel. The immature isoform containing the γ-subunit shows long open times and low-amplitude channel currents. The mature isoform containing the epsilon-subunit shows shorter open times and high-amplitude channel currents. Substitution of the epsilon-subunit for the γ-subunit gives rise to the fast-gated, high-conductance channel type.