Autonomic Nervous System (ANS) - An overview of anatomy, divisions, receptors, and neurotransmitters

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What is the Autonomic Nervous System? The ANS is classically divided into two subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS), which operate independently in some functions and interact co-operatively in others. In many cases the two have "opposite" actions where one activates a physiological response and the other inhibits it. An older simplification of the sympathetic and parasympathetic nervous systems as "excitory" and "inhibitory" was overturned due to the many exceptions found. A more modern characterisation is that the sympathetic nervous system is a "quick response mobilising system" and the parasympathetic is a "more slowly activated dampening system." The catecholamines are epinephrine (adrenaline), norepinephrine (noradrenaline), and dopamine. The catecholamines are produced from tyrosine.
G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins involved in transmitting signals from a variety of different stimuli outside a cell into the inside of the cell. G proteins function as molecular switches. Their activity is regulated by factors that control their ability to bind to and hydrolyze guanosine triphosphate (GTP) to guanosine diphosphate (GDP). When they bind GTP, they are 'on', and, when they bind GDP, they are 'off'. G proteins belong to the larger group of enzymes called GTPases