Muscarinic refers to a type of receptor in the body that responds to the neurotransmitter acetylcholine. These receptors are part of the parasympathetic nervous system, which helps regulate involuntary bodily functions such as heart rate, digestion, and respiratory rate. Muscarinic receptors are found in various tissues, including the heart, smooth muscles, and glands. There are five main subtypes of muscarinic receptors, labeled M1 through M5. Each subtype has different functions and locations in the body. For example, M2 receptors are primarily found in the heart and help slow down heart rate, while M3 receptors are involved in stimulating glandular secretions and smooth muscle contraction.