Sympathetic and parasympathetic effects on each organ !
- Table to know
- Exam priority: medium
The sympathetic system prepares the body for action; the parasympathetic one for rest, digestion and repair.
Updated 2026-11-01
Also called: Fight or flight, Rest and digest
See it in 3D: Sinoatrial (SA) node · Urinary bladder · Stomach · Main bronchi · Vagus nerve · Sympathetic trunk
What it is
Most organs receive both divisions, which usually act in opposite directions; the effect depends on the receptor on the target cell. A few targets have only one: most blood vessels, sweat glands and the adrenal medulla are sympathetic only. Knowing the receptor behind each effect lets you predict drug effects and side effects.
Organ effects and receptors
| Organ | Sympathetic (receptor) | Parasympathetic (receptor) |
|---|---|---|
| Pupil | Dilation, radial muscle (α1) | Constriction, sphincter (M3) |
| Ciliary muscle | Slight relaxation, far vision (β2) | Contraction, near vision (M3) |
| SA node and heart rate | ↑ rate (β1) | ↓ rate (M2) |
| Ventricular contractility | ↑ (β1) | Little direct effect |
| Bronchial smooth muscle | Relaxation (β2) | Constriction and secretion (M3) |
| Salivary glands | Scant, thick saliva (α1) | Abundant, watery saliva (M3) |
| Gut motility and secretion | ↓ (α2, β2) | ↑ (M3) |
| Gut sphincters | Contraction (α1) | Relaxation |
| Bladder detrusor | Relaxation (β3) | Contraction (M3) |
| Internal urethral sphincter | Contraction (α1) | Relaxation |
| Skin and splanchnic arterioles | Constriction (α1) | No innervation |
| Sweat glands | Secretion (M3, sympathetic cholinergic) | No innervation |
| Liver | Glycogenolysis and gluconeogenesis (β2, α1) | Little effect |
| Kidney juxtaglomerular cells | Renin release (β1) | No innervation |
Clinical notes
- Non-selective β-blockers can trigger bronchospasm in asthma (β2 block) and mask the tremor and palpitations of hypoglycemia.
- Anticholinergic poisoning: dry hot skin, dilated pupils, tachycardia, urinary retention, ileus, confusion.
- Organophosphate poisoning (pesticides, frequent in rural Colombia): miosis, bronchorrhea, bradycardia, salivation, diarrhea and urination.
Test yourself
Think of the answer, then tap the question to see it.
Why can a non-selective β-blocker worsen asthma while a β1-selective one is safer?
Bronchial smooth muscle relaxes through β2 receptors. Blocking β2 removes that dilation; a β1-selective agent acts mainly on the heart, though selectivity is relative.
Why does an anticholinergic drug cause dry, warm skin even though sweating is a sympathetic function?
Sympathetic fibres to sweat glands release acetylcholine onto muscarinic receptors, so muscarinic blockade stops sweating and heat loss.
What autonomic changes allow the bladder to fill without leaking?
Sympathetic β3 relaxes the detrusor and α1 contracts the internal sphincter; voiding requires parasympathetic M3 contraction of the detrusor.
Why does blood pressure fall when sympathetic tone to blood vessels drops, even though there is no parasympathetic vasodilator input to most vessels?
Vascular tone depends on continuous α1 sympathetic constriction; less sympathetic firing lets arterioles dilate passively.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- The autonomic nervous system: organization
- Adrenergic and cholinergic receptors
- The adrenal medulla and catecholamines
- Micturition and the bladder reflex
- Erection and ejaculation
Related conditions
In Biochemistry
In Pharmacology
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 15.2 Autonomic Reflexes and Homeostasis (OpenStax (Rice University), 2022); Neuroanatomy, Cranial Nerve 10 (Vagus Nerve) (StatPearls (NCBI Bookshelf), 2024); Anatomy, Head and Neck, Sympathetic Chain (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 61). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.