<P> Like many other biologically active substances, acetylcholine exerts its effects by binding to and activating receptors located on the surface of cells . There are two main classes of acetylcholine receptor, nicotinic and 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 . </P> <P> Nicotinic acetylcholine receptors are ligand - gated ion channels 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 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 . </P> <P> Muscarinic acetylcholine receptors 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 receptors, meaning that they exert their effects via a second messenger system . The M1, M3, and M5 subtypes are G - coupled; they increase intracellular levels of IP and calcium by activating phospholipase C. Their effect on target cells is usually excitatory . The M2 and M4 subtypes are G / G - coupled; they decrease intracellular levels of 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 . </P> <P> Acetylcholine is the substance the nervous system uses to activate skeletal muscles, a kind of striated muscle . These are the muscles used for all types of voluntary movement, in contrast to smooth muscle tissue, which is involved in a range of involuntary activities such as movement of food through the gastrointestinal tract and constriction of blood vessels . Skeletal muscles are directly controlled by motor neurons located in the spinal cord or, in a few cases, the brainstem . These motor neurons send their axons through motor nerves, from which they emerge to connect to muscle fibers at a special type of synapse called the neuromuscular junction . </P>

Acetylcholine is the main transmitter used at mammalian
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