Editing Acetylcholine (section)

===Cellular effects===
{{Main|Acetylcholine receptor}}

[[File:Cholinergic synapse.svg|class=skin-invert-image|thumb|right|Acetylcholine processing in a synapse. After release acetylcholine is broken down by the enzyme [[acetylcholinesterase]].]]
Like many other biologically active substances, acetylcholine exerts its effects by binding to and activating [[receptor (biochemistry)|receptors]] located on the surface of cells. There are two main classes of acetylcholine receptor, [[nicotinic acetylcholine receptor|nicotinic]] and [[muscarinic acetylcholine receptor|muscarinic]]. They are named for chemicals that can selectively activate each type of receptor without activating the other: [[muscarine]] is a compound found in the mushroom ''[[Amanita muscaria]]''; [[nicotine]] is found in tobacco.

[[Nicotinic acetylcholine receptor]]s are [[ligand-gated ion channel]]s permeable to [[sodium]], [[potassium]], and [[calcium]] ions. In other words, they are ion channels embedded in cell membranes, capable of switching from a closed to an open state when acetylcholine binds to them; in the open state they allow ions to pass through. Nicotinic receptors come in two main types, known as muscle-type and neuronal-type. The muscle-type can be selectively blocked by [[curare]], the neuronal-type by [[hexamethonium]]. The main location of muscle-type receptors is on muscle cells, as described in more detail below. Neuronal-type receptors are located in autonomic ganglia (both sympathetic and parasympathetic), and in the central nervous system.

[[Muscarinic acetylcholine receptor]]s have a more complex mechanism, and affect target cells over a longer time frame. In mammals, five subtypes of muscarinic receptors have been identified, labeled M1 through M5. All of them function as [[G protein-coupled receptor]]s, meaning that they exert their effects via a [[second messenger system]]. The M1, M3, and M5 subtypes are [[Gq alpha subunit|G<sub>q</sub>]]-coupled; they increase intracellular levels of [[inositol trisphosphate|IP<sub>3</sub>]] and [[calcium]] by activating [[phospholipase C]]. Their effect on target cells is usually excitatory. The M2 and M4 subtypes are [[Gi alpha subunit|G<sub>i</sub>/G<sub>o</sub>]]-coupled; they decrease intracellular levels of [[cyclic adenosine monophosphate|cAMP]] by inhibiting [[adenylate cyclase]]. Their effect on target cells is usually inhibitory. Muscarinic acetylcholine receptors are found in both the central nervous system and the peripheral nervous system of the heart, lungs, upper gastrointestinal tract, and sweat glands.