▁sub unit ▁composition ▁is ▁highly ▁variable ▁across ▁different ▁t issues . ▁Each ▁sub unit ▁contains ▁four ▁regions ▁which ▁span ▁the ▁memb rane ▁and ▁consist ▁of ▁approximately ▁ 2 0 ▁am ino ▁ac ids . ▁ ▁Region ▁II ▁which ▁s its ▁closest ▁to ▁the ▁p ore ▁l umen , ▁forms ▁the ▁p ore ▁l ining . ▁ ▁Binding ▁of ▁ac et yl ch oline ▁to ▁the ▁N ▁term ini ▁of ▁each ▁of ▁the ▁two ▁alpha ▁sub un its ▁results ▁in ▁the ▁ 1 5 ° ▁rotation ▁of ▁all ▁M 2 ▁hel ices . ▁The ▁cy top las m ▁side ▁of ▁the ▁n A Ch R ▁re ceptor ▁has ▁rings ▁of ▁high ▁negative ▁charge ▁that ▁determine ▁the ▁specific ▁c ation ▁specific ity ▁of ▁the ▁re ceptor ▁and ▁remove ▁the ▁hyd r ation ▁shell ▁often ▁formed ▁by ▁ ions ▁in ▁a que ous ▁solution . ▁In ▁the ▁intermediate ▁region ▁of ▁the ▁re ceptor , ▁within ▁the ▁p ore ▁l umen , ▁val ine ▁and ▁le uc ine ▁resid ues ▁( Val ▁ 2 5 5 ▁and ▁Le u ▁ 2 5 1 ) ▁define ▁a ▁hydro ph ob ic ▁region ▁through ▁which ▁the ▁de h yd r ated ▁ion ▁must ▁pass . ▁ ▁The ▁n A Ch R ▁is ▁found ▁at ▁the ▁edges ▁of ▁j unction al ▁fol ds ▁at ▁the ▁neur om us cular ▁j unction ▁on ▁the ▁posts yn apt ic ▁side ; ▁it ▁is ▁activ ated ▁by ▁ac et yl ch oline ▁release ▁across ▁the ▁syn ap se . ▁The ▁diffusion ▁of ▁Na