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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.
Activation of nicotinic and M1-muscarinic receptors is parasympathetic ganglia which lie in the airway wall then activate postganglionic fibers. These postganglionic fibers release acetylcholine which activate M3 muscarinic receptors which produce bronchiolar smooth muscle contraction. M2 receptors may also be present. Muscarinic antagonists attenuate pulmonary smooth muscle constriction, often resulting in bronchodilation.