A model illustrating cholinergic receptors on pulmonary parasympathetic nerves and airway smooth muscle. Acetylcholine (ACh) released at parasympathetic ganglion binds to nicotinic (N1) and muscarinic (M1) receptors and activates postganglionic cholinergic nerve to release ACh at neuromuscular junction. ACh at neuromuscular junctions binds to M3 receptors in the airway smooth muscle, inducing muscle contraction, and to the autoinhibitory M2 receptors located in the vagus nerve, that limits ACh release at the neuromuscular junction and decreasing smooth muscle constriction.