PH2.1-8 | Autonomic & Peripheral Nervous system, Autacoids — Self-Directed Learning

A 45-year-old male farmer from rural Maharashtra is rushed to the emergency department with severe vomiting, profuse watery diarrhea, and altered sensorium. His wife notes he was spraying pesticides in the field without protective equipment. On examination, he has pinpoint pupils, heart rate 42 bpm, fasciculations over chest and limbs, and excessive salivation. A peculiar garlic-like odor is noted on his breath. His shirt is soaked with a liquid that smells of kerosene. The resident suspects organophosphate poisoning. This scenario highlights how modulation of the cholinergic system—both by toxins and therapeutic agents—can lead to life-threatening crises, and sets the stage for understanding ANS pharmacology.

ANS pharmacology is integral to everyday clinical practice. Adrenaline is the first-line drug in anaphylaxis; pilocarpine is used for glaucoma; salbutamol relieves acute asthma; beta-blockers manage hypertension and ischemic heart disease; atropine is life-saving in bradycardia and organophosphate poisoning; neuromuscular blockers facilitate surgery and mechanical ventilation. In India, with high prevalence of asthma, agricultural pesticide exposure, cardiovascular disease, and migraine, a sound grasp of these agents is essential. Rational prescribing demands knowledge of receptor selectivity, adverse effect profiles, and the interplay between the sympathetic and parasympathetic limbs to maximize efficacy and minimize harm.

The ANS comprises sympathetic (thoracolumbar, preganglionic ACh → nicotinic receptors → postganglionic norepinephrine acting on α1, α2, β1, β2, β3; adrenal medulla releases epinephrine into blood) and parasympathetic (craniosacral, preganglionic ACh → nicotinic → postganglionic ACh acting on muscarinic M1–M5). Key points: synthesis of ACh from choline and acetyl-CoA (rate-limiting step choline uptake), degradation by acetylcholinesterase; catecholamine synthesis (tyrosine → DOPA → dopamine → NE → Epi). Receptors: G-protein-coupled muscarinic (M1 neural, M2 cardiac, M3 glandular/smooth muscle) and ligand-gated ion channel nicotinic (Nn autonomic ganglia, Nm neuromuscular junction). Adrenoceptors: α1 (vascular smooth muscle contraction, pupil dilation), α2 (presynaptic inhibition, platelet aggregation), β1 (cardiac acceleration, renin release), β2 (bronchodilation, vasodilation, uterine relaxation), β3 (lipolysis). Understanding these serves as a scaffold for drug actions.