so i haven't read anything about serotonin yet (i am most interested in this because i probably have some form of depression: major depressive disorder vs dysthymia) but Laney begins talking about how the adrenergic pathways are more dominant in extroverts while the cholinergic pathways are more dominant in introverts, and this has a sensible symmetry to it, since, at least, as the peripheral nervous system is concerned, the adrenergic pathways govern the sympathetic systems, involved in the "4fs": fighting, fleeing, feeding, and, uh, mating—essentially the response to stress and activity; whereas the cholinergic pathways govern the parasympathetic system, which are often (but not always) in opposition to the sympathetic system, and basically define the resting state. of course, it gets a lot murkier in the central nervous system. acetylcholine is present in diverse tracts in the brain (the nucleus basalis and the ascending reticulating activating sytem are two that come to mind) whose functions are not as easily classifiable, and while the localization of norepinephrine seems to be more discrete (the only thing i can think of right now is the locus ceruleus, though there are others), the difference between the cholinergic pathways and the adrenergic pathways are not as dichotomous as in the peripheral nervous system, nor easily grouped as activating versus resting. (for example, in the CNS, acetylcholine actually ramps up activity—the ARAS is responsible for awareness, plus the cholinergic pathways are what makes nicotine a stimulant; in contrast, in the CNS, norepinephrine actually causes decreased blood pressure (in stark contrast to norepinephrine in the periphery), and is the rationale for using clonidine, and alpha-2 adrenergic agonist, to control hypertension.