Muscarinic receptors are a type of cell membrane receptor that are activated by the neurotransmitter acetylcholine. The spelling of "muscarinic" is pronounced [mʌˈskærɪnɪk] in IPA phonetic transcription. The "musca-" prefix comes from the Latin word for "fly", referring to the origin of the compound muscarine, which was first isolated from a poisonous mushroom that attracted flies. The "-inic" suffix refers to the category of drugs that activate this type of receptor, known as "agonists". The spelling of "muscarinic receptor" reflects the compound's Latin roots and the naming conventions of pharmacology.
A muscarinic receptor refers to a specific type of receptor found primarily in the central and peripheral nervous systems. It is a G-protein-coupled receptor that mediates the effects of the neurotransmitter acetylcholine on various target tissues or cells. The term "muscarinic" originates from the fact that these receptors can be stimulated by muscarine, a natural compound with cholinergic properties.
Muscarinic receptors are classified into five subtypes, M1 to M5, each with distinct distribution patterns and functions. These receptors are present in various organs and tissues including the brain, smooth muscles, heart, glands, and the autonomic ganglia. Activation of muscarinic receptors causes a wide range of physiological responses, such as slowing heart rate, contraction of smooth muscles, stimulation of exocrine gland secretion, and modulation of neuronal activity.
Specifically, M1 receptors are mainly found in the central nervous system where they play a role in cognitive functions and memory. M2 receptors are primarily found in the heart where they regulate heart rate and contraction. M3 receptors are present in various smooth muscles and glands and are involved in the control of glandular secretions and smooth muscle contractions. M4 receptors are predominantly found in the brain and are thought to modulate neurotransmitter release. Lastly, M5 receptors are mainly found in certain regions of the brain, particularly in the limbic system, and may be involved in cognitive and emotional processes.
Overall, muscarinic receptors are crucial for the regulation of numerous physiological processes and are important targets for therapeutic drugs, particularly in the treatment of conditions such as Alzheimer's disease, overactive bladder, and certain respiratory disorders.
The etymology of the word "muscarinic receptor" can be traced back to the discovery of a chemical called muscarine in 1869 by Oscar Hesse. Muscarine is a toxin found in certain species of mushrooms, specifically Amanita muscaria, and it exhibits various physiological effects when ingested.
Later, in the 1920s, German pharmacologist Arthur Heffter identified that muscarine's effects were due to its interaction with certain receptors in the body. These receptors became known as "muscarinic receptors" after the molecule that activated them, muscarine.
The term "muscarinic" is derived from "muscarine" and the suffix "-ic", which is commonly used in chemistry to denote an acid or a substance derived from a specific source.