Adrenergic Against Cholinergic: The Two Switchboards
Two simplified nerve-terminal schematics side by side, adrenergic sockets on the left and a single muscarinic socket on the right, with agonists drawn as filled keys and antagonists as outlined keys that fill the socket without turning.
| ADRENERGIC · sympathetic | CHOLINERGIC · parasympathetic | |
|---|---|---|
| Receptor | alpha-1, beta-1, beta-2 | muscarinic |
| Pupil | dilates | constricts |
| Heart rate | rises (beta-1) | falls |
| Airway | dilates (beta-2) | constricts |
| Secretions | dry mouth, dry eyes | salivation, tearing, bronchial secretions |
| Gut | motility slows | motility increases |
| Bladder | retention | emptying |
| Agonist example | albuterol, a beta-2 agonist | bethanechol, a direct-acting cholinergic |
| Antagonist example | metoprolol, a beta blocker | atropine, an anticholinergic |
| Classic overdose | tachycardia, hypertension, tremor, agitation, arrhythmia | bradycardia, wet everywhere, bronchospasm, weakness |
| What the nurse watches | heart rate, rhythm, blood pressure, tremor, glucose, potassium | heart rate, airway secretions, ability to void, mental status |
The mirror rule
Blocking one branch looks identical to stimulating the other. An anticholinergic patient and an adrenergic-stimulated patient both present dry, fast, and dilated. A cholinergic-stimulated patient and a beta-blocked patient both present slow. The distinguishing question is never which set of findings is present. It is which drug was given.
Four operations, not four hundred drugs
Every autonomic agent on the exam does exactly one of four things: stimulate sympathetic, block sympathetic, stimulate parasympathetic, block parasympathetic. Identify the operation and the entire body chart above becomes predictable without knowing anything else about the agent. That is why an unfamiliar name is not a problem here.
Watch outurinary retention from an anticholinergic is easy to miss in an older adult, who may present with new confusion or agitation rather than a complaint of not voiding. Palpate the bladder and check the last documented void.
Memory hook: SLUDGE for cholinergic excess. Salivation, Lacrimation, Urination, Defecation, Gastrointestinal upset, Emesis. If your patient is wet in six places, the parasympathetic branch is running unopposed.
Margin notedry, hot, red, blind, mad = anticholinergic.
Adrenergic agonists and antagonists given intravenously sit on the high-alert list → BOOK 05 · P9, where the atropine pairing is also given. Shock states and vasopressor selection → BOOK 09. Beta blockade inside heart failure or acute coronary syndrome → BOOK 08.
TakeawayFour operations exist, not four hundred drugs: stimulate or block, sympathetic or parasympathetic.