The autonomic nervous system controls a huge range of processes the body runs without conscious direction — heart rate, blood pressure, digestion, pupil size — largely through two chemical messenger systems: adrenergic signalling, built around adrenaline and noradrenaline and generally associated with the body's fight-or-flight response, and cholinergic signalling, built around acetylcholine and generally associated with rest-and-digest functions. Autonomic pharmacology is largely the study of drugs that deliberately target one or the other of these two systems, and a remarkably large share of common medications, across very different conditions, turn out to be doing exactly that.
Two branches with largely opposing effects on the same organs
The adrenergic (or sympathetic) and cholinergic (or parasympathetic) branches of the autonomic nervous system frequently act on the same target organs but push them in opposite directions: adrenergic signalling tends to increase heart rate and blood pressure, dilate airways, and redirect blood flow away from digestion; cholinergic signalling tends to slow heart rate, constrict airways somewhat, and support digestive activity. A drug that either mimics or blocks one of these systems' natural signalling molecules can shift an organ's activity in a predictable direction, which is exactly the underlying logic behind an enormous range of medications treating apparently unrelated conditions.
Recognising the pattern across seemingly unrelated drug classes
A drug that blocks adrenergic receptors on the heart — a beta blocker — slows heart rate and reduces blood pressure, treating hypertension and certain heart conditions. A drug that mimics adrenergic signalling in the lungs relaxes airway muscle, treating asthma. A drug that blocks cholinergic signalling can reduce digestive spasms or dilate the pupil for an eye exam; a drug that boosts cholinergic signalling can improve muscle function in certain neuromuscular conditions. Recognising which of these two systems a given drug targets, and whether it mimics or blocks that system's natural signal, lets a student or clinician predict a huge amount about a drug's likely effects and side effects without needing to memorise each medication as an isolated, unrelated fact — the underlying logic connects a striking number of drugs that, from their names and marketed uses alone, look like they'd have nothing in common.
What we're still unsure about
The broad division of the autonomic nervous system into adrenergic and cholinergic branches, and the principle that many drugs work by targeting one of these systems, is well established pharmacology taught consistently across medical curricula. What's more genuinely complex in practice is that both systems have multiple distinct receptor subtypes distributed differently across different organs, so a drug rarely affects only the one organ it's intended to treat — predicting a drug's full range of side effects requires accounting for every receptor subtype it actually touches across the body, not just the specific one responsible for its intended therapeutic effect, which is a substantial part of why so many autonomic-targeting drugs carry long lists of possible side effects beyond their primary use.
This sits inside Autonomic Pharmacology (Adrenergic & Cholinergic), one of seven topics in Pharmacology, one of four domains in Medicine, one of seventeen subjects the app can quiz you on.