<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> <P> At a schematic level, the sympathetic and parasympathetic nervous systems are both organized in essentially the same way: preganglionic neurons in the central nervous system send projections to neurons located in autonomic ganglia; these neurons then send output projections to virtually every tissue of the body . In both branches the internal connections--the projections from the central nervous system to the autonomic ganglia--use acetylcholine as neurotransmitter, and the receptors it activates are of the nicotinic type . In the parasympathetic nervous system the output connections--the projections from ganglion neurons to tissues that don't belong to the nervous system--also release acetylcholine, acting on muscarinic receptors . In the sympathetic nervous system the output connections mainly release noradrenaline, although acetylcholine is released at a few points, such as the sudomotor innervation of the sweat glands . </P> <P> Acetylcholine in the serum exerts a direct effect on vascular tone by binding to muscarinic receptors present on vascular endothelium . These cells respond by increasing production of nitric oxide, which signals the surrounding smooth muscle to relax, leading to vasodilation . </P>

Where is acetylcholine released in the sympathetic nervous system
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