<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> <P> When a motor neuron generates an action potential, it travels rapidly along the nerve until it reaches the neuromuscular junction, where it initiates an electrochemical process that causes acetylcholine to be released into the space between the presynaptic terminal and the muscle fiber . The acetylcholine molecules then bind to nicotinic ion - channel receptors on the muscle cell membrane, causing the ion channels to open . Calcium ions then flow into the muscle cell, initiating a sequence of steps that finally produce muscle contraction . </P> <P> The autonomic nervous system controls a wide range of involuntary and unconscious body functions . Its main branches are the sympathetic nervous system and parasympathetic nervous system . Broadly speaking, the function of the sympathetic nervous system is to mobilize the body for action: the slogan often used for it is fight - or - flight . The function of the parasympathetic nervous system is to put the body in a state conducive to rest, regeneration, digestion, and reproduction: it is sometimes described using the slogans "rest and digest" or "feed and breed". Both branches use acetylcholine, but in different ways . </P>

Where is acetylcholine released from to stimulate muscle contraction