Cholinergic muscarinic receptors are a family of G protein-coupled receptors (M₁–M₅) that are activated by the neurotransmitter acetylcholine and mediate parasympathetic nervous system responses in the central and peripheral nervous systems. These receptors regulate various physiological functions, including heart rate, smooth muscle contraction, glandular secretion, pupil constriction, and cognitive processes such as learning and memory. Muscarinic receptors are important therapeutic targets for drugs used to treat conditions such as Parkinson's disease, chronic obstructive pulmonary disease (COPD), overactive bladder, glaucoma, and gastrointestinal disorders.