Muscarinic acetylcholine receptors are a type of receptor in the body that respond to the neurotransmitter acetylcholine. They are part of the autonomic nervous system and play a crucial role in various functions, including heart rate, smooth muscle contraction, and glandular secretion. These receptors are found in many tissues, including the heart, lungs, and digestive system. There are five subtypes of muscarinic receptors, labeled M1 to M5, each with distinct functions and locations. For example, M2 receptors are primarily located in the heart and help regulate heart rate, while M3 receptors are found in smooth muscles and glands, influencing contraction and secretion.