Chapter 6: Autonomic Nervous System Regulation & Homeostasis

Key concepts include the structural differences in preganglionic and postganglionic fiber lengths, the role of varicosities in neurotransmitter release, and the specific innervation patterns of the adrenal medulla. The text provides an in-depth examination of neurochemistry, explaining how all preganglionic fibers release acetylcholine (ACh) onto nicotinic receptors, while postganglionic specificity involves ACh acting on muscarinic receptors in the PSNS and norepinephrine (NE) acting on adrenergic receptors in the SNS, with notable exceptions like sweat glands. The synthesis, storage, release, and termination of these neurotransmitters are explored, including the roles of acetylcholinesterase, monoamine oxidase (MAO), and reuptake transporters. Significant attention is given to receptor physiology, categorizing the functions of G-protein coupled receptors, including alpha 1 (vasoconstriction), alpha 2 (presynaptic inhibition), beta 1 (cardiac stimulation), and beta 2 (bronchodilation and vasodilation). The chapter further explores cotransmission and nonadrenergic noncholinergic (NANC) signaling, highlighting the role of nitric oxide in penile erection and its pharmacological modulation by phosphodiesterase inhibitors. Integrative physiology is illustrated through the dual innervation of the eye (controlling pupillary diameter and accommodation) and the unique sympathetic-only control of systemic vascular resistance. Finally, the summary encompasses critical clinical correlations and pharmacology, including the mechanism of indirect sympathomimetics like cocaine and amphetamines, the hypertensive crisis associated with MAO inhibitors and tyramine ingestion (cheese syndrome), the pathophysiology of pheochromocytoma, and the "epinephrine reversal" phenomenon. It concludes with a detailed breakdown of organophosphate poisoning, explaining the consequences of irreversible acetylcholinesterase inhibition (DUMBBELSS mnemonic) and the pharmacological rationale for treatment using atropine and pralidoxime.