Acetylcholine general discussion

In the discussion that follows we will focus mainly on norepinephrine, serotonin, and acetylcholine, the three neuromodulators most carefully studied by depression and sleep scientists. The dissociations that may occur among those three systems and other important neuromodulators like dopamine limit the value of the generalizations we can make today, but they nonetheless provide a solid grounding for the future work that will help us understand exceptions to the rules we can now lay down. 

A study of the distribution, in the mammalian central nervous system, of the various substances discussed in this review reveals that none—except y-aminobutyric acid which has general metabolic as well as humoral functions—is as widespread as acetylcholine and there is no general tendency for fibres deficient in acetylcholine to contain correspondingly large amounts of any of the other substances. This is an important point, since the non-cholinergic excitatory transmitter should be found in at least a number of acetylcholine-free nerve tracts. Instead, it appears that some areas of the nervous system contain a multiplicity of humoral substances while others have none at all. 

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