Sympathetic vs Parasympathetic: The Autonomic Nervous System
A&P I / AP Psychology / nursing prerequisites · 12 flashcards · 7 quiz questions · updated 2026-08-19
The autonomic nervous system runs everything you do not consciously control, and it does so with two branches that mostly oppose each other: sympathetic (fight or flight) and parasympathetic (rest and digest).
Both are active all the time. What changes is the balance — which is why the honest description is not 'one switches on' but 'one dominates'.
Organ by organ
| Organ | Sympathetic | Parasympathetic |
|---|---|---|
| Heart rate | Increases | Decreases |
| Bronchioles | Dilate | Constrict |
| Pupils | Dilate | Constrict |
| Digestion | Inhibited | Stimulated |
| Salivation | Thick and scant | Copious and watery |
| Bladder | Relaxes (retains urine) | Contracts (empties) |
| Liver | Glycogen → glucose | Glycogen storage |
| Skin blood vessels | Constrict | No major innervation |
| Sweat glands | Stimulated | No innervation |
| Adrenal medulla | Releases adrenaline | No innervation |
Anatomy: where the nerves leave the cord
- •Sympathetic is thoracolumbar: fibres leave from T1 to L2.
- •Parasympathetic is craniosacral: cranial nerves III, VII, IX and X, plus S2–S4.
- •The vagus nerve (X) alone carries about 75% of all parasympathetic fibres, reaching the heart, lungs and most of the gut.
- •Sympathetic: short preganglionic fibre, long postganglionic fibre, with ganglia in the sympathetic chain near the cord — one signal can spread widely, which is why the response feels body-wide.
- •Parasympathetic: long preganglionic fibre, short postganglionic fibre, with ganglia in or near the target organ — so effects are localised.
Neurotransmitters and receptors
- •All preganglionic neurons, in both branches, release acetylcholine onto nicotinic receptors.
- •Parasympathetic postganglionic neurons release acetylcholine onto muscarinic receptors.
- •Sympathetic postganglionic neurons release noradrenaline onto adrenergic receptors (α and β).
- •Two sympathetic exceptions worth knowing: sweat glands are cholinergic, and the adrenal medulla is stimulated directly by a preganglionic fibre, releasing adrenaline into the blood.
- •β1 receptors dominate in the heart, β2 in the bronchioles — which is why asthma inhalers target β2 and cardiac drugs target β1.
The response as a whole
Sympathetic activation is coordinated for a burst of physical effort: more oxygen in, more glucose available, blood redirected from gut and skin to skeletal muscle, sharper vision.
Parasympathetic dominance is the maintenance state: digestion and absorption running, heart rate low, energy being stored. Most of the time you are here, with sympathetic tone layered on top.
Clinical hooks that make it stick
- •Beta blockers block β1 receptors — heart rate and blood pressure fall.
- •Atropine blocks muscarinic receptors, so it raises heart rate and dries secretions before surgery.
- •Adrenaline in anaphylaxis works on several receptors at once: bronchodilation via β2, vasoconstriction via α1, increased cardiac output via β1.
- •Pupil dilation for an eye exam uses a muscarinic antagonist — removing parasympathetic constriction.
Common mistakes
- ✗Saying one branch is 'on' and the other 'off'. Both run continuously; the balance shifts.
- ✗Assuming the sympathetic branch always uses noradrenaline — sweat glands and the adrenal medulla break the pattern.
- ✗Mixing up the outflows: sympathetic is thoracolumbar, parasympathetic craniosacral.
- ✗Reversing the fibre lengths. Sympathetic preganglionic fibres are short; parasympathetic ones are long.
- ✗Forgetting that the bladder pattern is inverted relative to intuition: sympathetic retains, parasympathetic empties.
Flashcards
Tap a card to reveal the answer.
Sympathetic outflow⌄
Thoracolumbar — T1 to L2.
Parasympathetic outflow⌄
Craniosacral — cranial nerves III, VII, IX, X and S2–S4.
Which nerve carries most parasympathetic fibres?⌄
The vagus (cranial nerve X), roughly 75%.
Neurotransmitter of all preganglionic neurons⌄
Acetylcholine, acting on nicotinic receptors.
Sympathetic postganglionic neurotransmitter⌄
Noradrenaline, on α and β adrenergic receptors.
Two sympathetic cholinergic exceptions⌄
Sweat glands, and the adrenal medulla (stimulated directly by a preganglionic fibre).
Effect of sympathetic activity on the bronchioles⌄
Dilation — more airflow.
Effect of parasympathetic activity on the pupil⌄
Constriction.
What does the sympathetic branch do to the bladder?⌄
Relaxes the detrusor so urine is retained.
Fibre lengths in the sympathetic branch⌄
Short preganglionic, long postganglionic — ganglia sit near the spinal cord.
Which receptor do asthma inhalers target?⌄
β2 in the bronchioles, producing dilation.
Why does atropine raise heart rate?⌄
It blocks muscarinic receptors, removing the vagus nerve's braking effect.
Practice quiz
Answer first, then open the explanation.
1. The sympathetic division originates from which spinal segments?
- A. C1–C8
- B. T1–L2
- C. S2–S4
- D. Cranial nerves only
Show answer
B. T1–L2
Sympathetic outflow is thoracolumbar; the parasympathetic is craniosacral.2. Which neurotransmitter is released by all preganglionic autonomic neurons?
- A. Noradrenaline
- B. Acetylcholine
- C. Dopamine
- D. Adrenaline
Show answer
B. Acetylcholine
Both branches use acetylcholine at the ganglion, on nicotinic receptors.3. Parasympathetic stimulation of the digestive system:
- A. Inhibits it
- B. Stimulates it
- C. Has no effect
- D. Only affects the liver
Show answer
B. Stimulates it
Rest-and-digest increases motility and secretion.4. Which is a sympathetic effect that uses acetylcholine?
- A. Heart rate increase
- B. Bronchodilation
- C. Sweating
- D. Pupil dilation
Show answer
C. Sweating
Sweat glands are the classic cholinergic exception in the sympathetic branch.5. The vagus nerve is:
- A. A sympathetic nerve to the gut
- B. The main parasympathetic nerve, reaching the heart, lungs and gut
- C. A spinal nerve from L2
- D. Purely sensory
Show answer
B. The main parasympathetic nerve, reaching the heart, lungs and gut
Cranial nerve X carries the majority of parasympathetic output.6. Beta blockers reduce heart rate by acting on:
- A. β1 receptors
- B. β2 receptors
- C. Muscarinic receptors
- D. Nicotinic receptors
Show answer
A. β1 receptors
β1 receptors dominate in cardiac tissue; β2 is the bronchiolar target.7. Sympathetic stimulation of the bladder:
- A. Empties it
- B. Retains urine
- C. Has no effect
- D. Causes incontinence
Show answer
B. Retains urine
Sympathetic activity relaxes the detrusor and retains urine; the parasympathetic branch empties it.
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