The autonomic nervous system (ANS) terrifies many students because it mixes anatomy, receptors, and drug actions. The good news: once you map “who talks to whom” and link each receptor to a few predictable effects, the fog lifts. This article gives you a compact chart and a simple flowsheet you can run in your head to solve almost any ANS pharmacology question.
The 10-second ANS map
- Sympathetic (fight or flight): Short preganglionic fibers, long postganglionic fibers. Preganglionic neurotransmitter is acetylcholine (ACh) to nicotinic (Nn) receptors. Most postganglionic neurotransmitter is norepinephrine (NE) to alpha/beta receptors. Exceptions: sweat glands use ACh (M), renal vasculature uses dopamine (D1), adrenal medulla releases mostly epinephrine.
- Parasympathetic (rest and digest): Long preganglionic, short postganglionic. ACh at both synapses: nicotinic (Nn) in ganglia, muscarinic (M) at target organs.
- Somatic: One neuron from CNS to skeletal muscle using ACh at nicotinic (Nm) receptors.
The One-Page ANS Chart (text version)
- Receptor families and core effects
- Alpha1 (Gq): vascular smooth muscle constriction → ↑ peripheral resistance, ↑ BP; pupil dilator contraction → mydriasis; bladder sphincter constriction → urinary retention.
- Alpha2 (Gi): presynaptic NE release inhibition → ↓ sympathetic outflow; pancreatic β-cell inhibition → ↓ insulin; CNS effect → ↓ BP via central sympatholysis.
- Beta1 (Gs): heart rate and contractility ↑; renin release ↑ → ↑ angiotensin II.
- Beta2 (Gs): bronchodilation; vasodilation in skeletal muscle; uterine relaxation; ↑ glycogenolysis; drives K+ into cells → mild hypokalemia.
- Beta3 (Gs): lipolysis, bladder detrusor relaxation (urinary urgency relief).
- M1 (Gq): CNS/enteric; not a major drug target clinically.
- M2 (Gi): slows SA/AV node → ↓ heart rate and conduction.
- M3 (Gq): gland secretion (saliva, tears, sweat), GI motility, bladder contraction, bronchoconstriction, pupil constriction (miosis), accommodation, ↑ NO-mediated vasodilation.
- Nn: ganglia and adrenal medulla.
- Nm: neuromuscular junction of skeletal muscle.
- Typical organ targets
- Heart: Beta1 speeds and strengthens; M2 slows.
- Vessels: Alpha1 constricts most beds; Beta2 dilates skeletal muscle beds; M3 dilates via NO in intact endothelium.
- Lungs: Beta2 opens; M3 closes and increases mucus.
- Eye: Alpha1 dilates (radial muscle); M3 constricts pupil and accommodates (sphincter and ciliary); Beta increases aqueous humor; Alpha2 decreases aqueous.
- GI/Bladder: M3 increases motility and voiding; Alpha1 tightens sphincters; Beta3 relaxes detrusor.
- Metabolic: Beta2 raises glucose, shifts K+ into cells; Alpha2 lowers insulin.
- Go-to agonists (what they mimic and why we use them)
- Epinephrine (Beta1/Beta2/Alpha1): anaphylaxis (bronchodilation + raises BP), cardiac arrest.
- Norepinephrine (Alpha1 > Beta1): septic shock to raise BP; causes reflex bradycardia.
- Phenylephrine (Alpha1): nasal decongestant, mydriasis, raises BP with reflex bradycardia.
- Isoproterenol (Beta1/Beta2): increases HR, drops diastolic; testing arrhythmia susceptibility.
- Dobutamine (Beta1): acute heart failure, stress testing.
- Albuterol/salmeterol/formoterol (Beta2): asthma/COPD bronchodilation; tremor and tachycardia from systemic spillover.
- Clonidine and methyldopa (Alpha2 agonists): lower sympathetic outflow → treat hypertension; can cause sedation, dry mouth.
- Bethanechol (M): urinary retention, ileus (makes bladder and gut squeeze).
- Pilocarpine (M): glaucoma (opens trabecular outflow), xerostomia.
- Neostigmine, pyridostigmine (AChE inhibitors): increase ACh; myasthenia gravis, reverse neuromuscular blockade, treat ileus/urinary retention.
- Go-to antagonists
- Prazosin/terazosin/tamsulosin (Alpha1 blockers): BPH urinary flow; watch for orthostatic hypotension.
- Propranolol (nonselective Beta blocker): tremor, hyperthyroidism, portal HTN; avoid in asthma.
- Metoprolol/atenolol (Beta1 selective): angina, HF, post-MI, rate control with less bronchospasm risk.
- Labetalol/carvedilol (Alpha1 + Beta blockers): hypertensive emergencies, HF; less reflex tachycardia.
- Atropine (M antagonist): bradycardia, cholinergic toxicity; causes dry mouth, blurry vision, urinary retention, hyperthermia.
- Ipratropium/tiotropium (M antagonists): COPD/asthma bronchodilators.
- Oxybutynin/solifenacin (M3 antagonists): overactive bladder.
- Rocuronium/vecuronium (Nm blockers): paralysis for surgery; reversed by neostigmine + atropine/glycopyrrolate.
- Toxicity patterns you should recognize
- Cholinergic excess (AChE inhibitor poisoning): sweating, salivation, bronchospasm, diarrhea, miosis, bradycardia. Treat with atropine (for M) plus pralidoxime early (regenerates AChE at nicotinic sites).
- Anticholinergic toxicity: “hot, dry, blind, red, mad.” Treat with supportive care; physostigmine in selected severe central cases.
- Sympathomimetic excess: tachycardia, hypertension, mydriasis, diaphoresis; unlike anticholinergic toxicity, bowel sounds are present and sweating is increased.
The flowsheet: how to solve ANS drug questions fast
- Identify the system and target: Is it sympathetic (alpha/beta) or parasympathetic (muscarinic)? Is the target heart, vessels, bronchi, eye, or glands?
- Pick the receptor:
- Cardiac rate/contractility → Beta1 vs M2.
- Bronchi → Beta2 vs M3.
- Vessels → Alpha1 constrict, Beta2 dilate (skeletal muscle), M3 dilate via NO.
- Pupil → Alpha1 dilate, M3 constrict.
- Decide direct vs indirect:
- Direct agonist/antagonist hits receptor.
- Indirect boosts or blocks transmitter (AChE inhibitors, reuptake blockers, releasers like amphetamines).
- Predict net effect using two vital signs:
- Heart rate (HR)
- Mean arterial pressure (MAP)
If MAP rises via Alpha1, the baroreflex slows HR (vagal response). If MAP falls (Beta2 vasodilation), reflex speeds HR.
- Check for special wiring: sweat glands (sympathetic but muscarinic), renal D1, adrenal medulla (releases Epi/NE).
- Apply clinical guardrails: asthma/COPD → avoid nonselective beta-blockers; BPH or glaucoma → antimuscarinics can worsen retention or angle closure; diabetes → nonselective beta-blockers blunt hypoglycemia symptoms.
Quick mental models for each receptor
- Alpha1 = squeeze: vessels, bladder sphincter, radial muscle of iris. Uses: raise BP, decongest nose, dilate pupil. Risk: ischemia, reflex bradycardia, urinary retention.
- Alpha2 = chill: presynaptic brake on NE release, CNS sympatholysis. Uses: lower BP, calm withdrawal symptoms. Risk: rebound hypertension if stopped suddenly.
- Beta1 = beat: heart contractility and rate, renin release. Uses: cardiogenic shock (dobutamine), rate control (blockers). Risk: arrhythmias, worsened HF when starting too fast.
- Beta2 = breathe and broaden: bronchi open, vessels in muscle open, uterus relaxes. Uses: asthma, tocolysis. Risk: tremor, hypokalemia, tachycardia.
- M2 = mellow the myocardium: slows SA/AV node. Uses: vagal maneuvers, blocked by atropine in bradycardia.
- M3 = moisten and move: secretions, smooth muscle tone, miosis, accommodation. Uses: dry mouth, glaucoma, ileus. Risk: bronchospasm in asthmatics, diarrhea, sweating.
- Nm = neuromuscular junction: blockade enables intubation and surgery. Reversal needs AChE inhibitor + antimuscarinic to protect the heart from bradycardia.
Baroreflex: the twist that fools people
- Norepinephrine: Alpha1 raises MAP strongly; Beta1 would raise HR, but baroreflex vagal tone dominates → reflex bradycardia. That is why HR can fall despite Beta1 activity.
- Isoproterenol: Beta2 vasodilation lowers diastolic pressure; Beta1 raises HR and contractility; baroreflex also drives tachycardia → big HR rise, lower MAP.
- Phenylephrine: Pure Alpha1 raises MAP → reflex bradycardia, no direct cardiac stimulation.
- Epinephrine (dose-dependent): low dose Beta2 > Alpha1 → vasodilation, HR up; high dose Alpha1 dominates → MAP up, possible reflex bradycardia despite Beta1.
High-yield clinical patterns and pitfalls
- Glaucoma: Pilocarpine (M3 agonist) increases trabecular outflow by ciliary muscle contraction. Beta-blockers (timolol) reduce aqueous production. Alpha2 agonists (brimonidine) also reduce aqueous.
- Asthma/COPD: Beta2 agonists relieve bronchospasm fast. Antimuscarinics (ipratropium/tiotropium) reduce vagal tone and mucus. Avoid nonselective beta-blockers.
- BPH: Alpha1 blockers relax the bladder neck and prostate but can cause first-dose syncope. Tamsulosin is more uroselective (fewer BP effects).
- Overactive bladder: Antimuscarinics (oxybutynin) reduce detrusor activity; Beta3 agonists (mirabegron) relax detrusor without anticholinergic side effects.
- Bradycardia with hypotension: Atropine for vagal bradycardia; if hypotension persists, add pressors (e.g., dopamine, epinephrine) to restore perfusion.
- Organophosphate poisoning: Diarrhea, urination, miosis, bronchospasm, bradycardia, sweating. Treat quickly with atropine for muscarinic symptoms and pralidoxime to restore AChE at nicotinic sites (before aging).
- Anticholinergic vs sympathomimetic toxicity: Both have mydriasis and tachycardia. Sweating differentiates: dry skin in anticholinergic poisoning; sweaty in sympathomimetic excess.
Practice mini-cases
- Case 1: A decongestant raises BP and causes reflex bradycardia and mydriasis without cycloplegia.
- Reasoning: Alpha1 agonist (phenylephrine). Alpha1 → vasoconstriction (↑ MAP) → reflex bradycardia; radial muscle contracts (mydriasis); no effect on accommodation (that’s M3).
- Case 2: An inhaler causes tremor, mild tachycardia, and occasional hypokalemia but relieves wheeze.
- Reasoning: Beta2 agonist (albuterol). Beta2 in skeletal muscle drives K+ into cells and causes tremor.
- Case 3: Patient with angle-closure glaucoma gets blurry vision and acute eye pain after an antispasmodic for overactive bladder.
- Reasoning: Antimuscarinic (oxybutynin) causes mydriasis and decreased outflow, precipitating angle closure.
Putting it all together: a compact flowsheet you can memorize
- What changed? HR up/down? BP up/down? Bronchi open/close? Pupils big/small? Secretions wet/dry?
- Which receptor explains it?
- HR up → Beta1; HR down → M2 or reflex.
- BP up → Alpha1; BP down → Beta2 or M3 (endothelium).
- Bronchi open → Beta2; bronchi close + secretions ↑ → M3.
- Mydriasis → Alpha1; miosis → M3.
- Is baroreflex involved? If MAP shifts, expect opposite HR change unless a strong direct cardiac effect overrides.
- Any wiring exceptions? Sweat glands (sympathetic but M), adrenal medulla (releases Epi/NE), renal D1.
- Safety screen: asthma + nonselective beta-blocker? BPH + antimuscarinic? Glaucoma risk? Diabetes with hypoglycemia awareness blunted by beta-blockers?
Final takeaways
- Learn five levers: Alpha1 squeezes; Alpha2 brakes NE; Beta1 beats; Beta2 breathes/broadens; M3 moistens/moves (M2 slows heart).
- Predict the baroreflex any time vascular tone changes.
- Remember exceptions (sweat glands, adrenal medulla, renal D1) and setting-specific risks (asthma, BPH, glaucoma, diabetes).
- Use the flowsheet to move from signs → receptor → drug class → clinical action. With practice, ANS pharmacology becomes pattern recognition, not memorization.
