[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › Autonomic nervous system: overview [Pharmacology](https://durgical.com/en/pharmacology/) · Topic # Autonomic nervous system: overview The automatic wiring that controls heart rate, blood pressure, airways, gut, bladder, pupils and glands, and why so many drugs act on it. Updated 2026-11-01 The autonomic nervous system controls involuntary functions through two divisions with mostly opposite effects. The sympathetic division prepares the body for stress («fight or flight»); the parasympathetic division supports rest and digestion. Each uses a two-neuron chain with a ganglion in between, and drugs that mimic or block their transmitters (acetylcholine and noradrenaline) are among the most used in medicine. ## Key points - Sympathetic outflow is thoracolumbar; parasympathetic outflow is craniosacral (cranial nerves III, VII, IX and X, and sacral spinal cord). - All preganglionic neurons release acetylcholine onto nicotinic receptors in the ganglia. - Most sympathetic postganglionic neurons release noradrenaline; parasympathetic postganglionic neurons release acetylcholine onto muscarinic receptors. - Exceptions: sympathetic fibres to sweat glands (and some skeletal muscle vessels) release acetylcholine; the adrenal medulla acts as a modified ganglion that releases adrenaline and noradrenaline into the blood. - Many organs have dual innervation (the heart: sympathetic speeds it, vagal tone slows it); some, such as most blood vessels, are controlled mainly by the sympathetic system. ## Layout - Sympathetic: short preganglionic fibres to paravertebral chain or prevertebral ganglia; long postganglionic fibres to the organs - Parasympathetic: long preganglionic fibres to terminal or intramural ganglia near or in the organ; short postganglionic fibres - Adrenal medulla: receives preganglionic sympathetic fibres directly and secretes catecholamines as hormones ## Organ effects at a glance - Heart: sympathetic raises rate and force (beta-1); parasympathetic lowers rate (M2) - Airways: sympathetic dilates (beta-2); parasympathetic constricts and increases secretions (M3) - Eye: sympathetic dilates the pupil (alpha-1, radial muscle); parasympathetic constricts it and contracts the ciliary muscle for near vision (M3) - Gut: parasympathetic increases motility and secretion; sympathetic reduces them - Bladder: parasympathetic contracts the detrusor (M3); sympathetic relaxes it (beta-3) and contracts the internal sphincter (alpha-1) - Blood vessels: mostly sympathetic; alpha-1 constricts, beta-2 dilates vessels in skeletal muscle ## Drug groups that act here - Sympathomimetics: adrenaline, noradrenaline, phenylephrine, salbutamol - Sympatholytics: alpha blockers, beta blockers, central alpha-2 agonists such as clonidine - Parasympathomimetics: pilocarpine, bethanechol, cholinesterase inhibitors - Antimuscarinics: atropine, ipratropium, oxybutynin - Ganglion and neuromuscular drugs: see the neuromuscular junction page ## Predicting effects If you know which division dominates an organ at rest, you can predict a drug's effect. Blocking muscarinic receptors removes vagal tone and causes tachycardia, dry mouth, urinary retention and dilated pupils. Blocking beta receptors removes sympathetic drive to the heart and kidney (renin) and can tighten airways in asthma. ## Memory aid Sympathetic is the Sprint (Thoracolumbar, noradrenaline); Parasympathetic is the Pause (Craniosacral, acetylcholine). ## Test yourself Think of the answer, then tap the question to see it. ## Which transmitter and receptor are used in all autonomic ganglia? Acetylcholine acting on nicotinic receptors, in both sympathetic and parasympathetic ganglia. ## What is the main exception to noradrenaline release by sympathetic postganglionic fibres? Fibres to sweat glands (and some skeletal muscle vessels) release acetylcholine, which acts on muscarinic receptors. ## Why is the adrenal medulla called a modified sympathetic ganglion? It receives preganglionic sympathetic fibres directly and its cells release adrenaline and noradrenaline into the blood instead of having axons. ## What happens to the heart rate when muscarinic receptors are blocked at rest, and why? It rises, because resting heart rate is held down by vagal (parasympathetic) tone acting on M2 receptors. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/pharmacology/topics/autonomic-nervous-system-overview/) ## Related classes - [Beta blockers](https://durgical.com/en/pharmacology/classes/beta-blockers/) - [Antimuscarinics (anticholinergics)](https://durgical.com/en/pharmacology/classes/antimuscarinics/) - [Muscarinic (cholinergic) agonists](https://durgical.com/en/pharmacology/classes/muscarinic-agonists/) - [Vasopressors and inotropes](https://durgical.com/en/pharmacology/classes/vasopressors-and-inotropes/) - [Alpha blockers](https://durgical.com/en/pharmacology/classes/alpha-blockers/) - [Beta-2 adrenergic agonists](https://durgical.com/en/pharmacology/classes/beta-2-adrenergic-agonists/) ## Example drugs - [Atropine](https://durgical.com/en/pharmacology/atropine/) - [Pilocarpine](https://durgical.com/en/pharmacology/pilocarpine/) - [Propranolol](https://durgical.com/en/pharmacology/propranolol/) - [Metoprolol](https://durgical.com/en/pharmacology/metoprolol/) - [Salbutamol](https://durgical.com/en/pharmacology/salbutamol/) - [Phenylephrine](https://durgical.com/en/pharmacology/phenylephrine/) - [Clonidine](https://durgical.com/en/pharmacology/clonidine/) [← All of pharmacology](https://durgical.com/en/pharmacology/) Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given. Sources: [15.1 Divisions of the Autonomic Nervous System](https://openstax.org/books/anatomy-and-physiology-2e/pages/15-1-divisions-of-the-autonomic-nervous-system) (OpenStax (Rice University), 2022); [15.4 Drugs that Affect the Autonomic System](https://openstax.org/books/anatomy-and-physiology-2e/pages/15-4-drugs-that-affect-the-autonomic-system) (OpenStax (Rice University), 2022). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/pharmacology/topics/autonomic-nervous-system-overview/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/topics/autonomic-nervous-system-overview/