Muscarinic receptors are a type of acetylcholine receptor that play a crucial role in the parasympathetic nervous system. They are G-protein coupled receptors found in various tissues, including the heart, smooth muscles, and glands. These receptors mediate several physiological responses, such as slowing the heart rate and stimulating glandular secretions. There are five known subtypes of muscarinic receptors, labeled M1 to M5. Each subtype has distinct functions and locations in the body. For example, M2 receptors are primarily found in the heart, while M3 receptors are involved in smooth muscle contraction and glandular secretion.