Where are muscarinic receptors found?

• A. Brain only
• B. Skeletal muscle neuromuscular junction
• C. Post-ganglionic sympathetic terminals
• D. Post-ganglionic parasympathetic terminals; smooth and cardiac muscle

Answer: (D)

Muscarinic receptors are the parasympathetic cholinergic receptors. They are G-protein–coupled receptors located on the tissues that receive signals from postganglionic parasympathetic neurons—most importantly smooth muscle, cardiac muscle, and glands. In the parasympathetic pathway, preganglionic neurons release ACh onto nicotinic receptors in the autonomic ganglia, and postganglionic neurons release ACh onto muscarinic receptors on the target organs. That’s why the receptors are found on smooth and cardiac muscle and other parasympathetic effector tissues. They are not the receptors at the skeletal neuromuscular junction (which use nicotinic receptors), and while muscarinic receptors also exist in the brain, the peripheral distribution described here best fits the target tissues of parasympathetic innervation.