The classical site of muscarinic receptors is at the postganglionic parasympathetic neuroeffector junction in smooth muscle, heart, and exocrine glands. However, muscarinic receptors also exist on effectors cells, even in the absence of cholinergic innervation.Prototype agonists include muscarine and ACh.he classical site of muscarinic receptors is at the postganglionic parasympathetic neuroeffector junction in smooth muscle, heart, and exocrine glands. However, muscarinic receptors also exist on effectors cells, even in the absence of cholinergic innervation. Prototype agonists include muscarine and ACh.