[AhmedEzatAlzawalaty]

i want to know what makes any drug specific to a certain receptor from chemical pont of view,i wanna talk here about cholinergic drugs that i  studied ,Methacholine is specific to M2 in Heart why? what chemical basis that explains this.
Also carbachol is not specific acting on both Ng and M in GIT and eye ,bethanechol is specific to M in GIT and Eye too but not acting on Ng, i want any one to explains all this stuff based on chemical basis.
I want also to know why Physostigmine passes Blood Brain Barrier while Neostigmine which does not have CNS activity,does not.