The Autonomic Nervous System
Sympathetic vs parasympathetic, neurotransmitters, receptors and control
By the end of this unit you can…
- ✓Compare the somatic and autonomic nervous systems
- ✓Compare sympathetic and parasympathetic divisions: origin, ganglia, fiber lengths and effects
- ✓Describe the cholinergic and adrenergic neurotransmitters and receptors
- ✓Explain dual innervation, sympathetic tone and parasympathetic tone
- ✓Describe how the hypothalamus and brain stem control the ANS
Key terms
Practice →1ANS vs somatic nervous system
The autonomic nervous system (ANS) — the involuntary or visceral motor system — regulates smooth muscle, cardiac muscle and glands to maintain homeostasis, mostly without our awareness.
| Feature | Somatic | Autonomic |
|---|---|---|
| Effectors | Skeletal muscle | Cardiac muscle, smooth muscle, glands |
| Efferent pathway | One neuron from CNS to effector; thick, heavily myelinated, fast | Two-neuron chain: preganglionic (lightly myelinated) → ganglion → postganglionic (unmyelinated) |
| Neurotransmitter | ACh only | ACh (preganglionic, parasympathetic postganglionic) and NE (most sympathetic postganglionic) |
| Effect on target | Always stimulatory | Excitatory or inhibitory, depending on receptor |
| Control | Voluntary | Involuntary |
2Sympathetic vs parasympathetic
Most visceral organs receive dual innervation — fibers from both divisions, which usually have opposite effects. The parasympathetic division keeps body energy use low during non-stressful situations (“rest and digest”); the sympathetic division mobilizes the body during activity, emergency or excitement (“fight or flight” — or “E situations”: exercise, excitement, emergency, embarrassment).
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Origin | Thoracolumbar — lateral horns of T1–L2 | Craniosacral — brain stem (CN III, VII, IX, X) and S2–S4 |
| Ganglia location | Close to the spinal cord: sympathetic trunk (chain) ganglia and collateral (prevertebral) ganglia | Terminal (intramural) ganglia in or near the target organ |
| Preganglionic fibers | Short | Long |
| Postganglionic fibers | Long | Short |
| Branching | Extensive — widespread, diffuse effects | Minimal — localized, discrete effects |
| Postganglionic neurotransmitter | Mostly norepinephrine (adrenergic) | Acetylcholine (cholinergic) |
The vagus nerve (X) carries ~90% of all parasympathetic preganglionic fibers — to the heart, lungs and most abdominal organs. The pelvic splanchnic nerves (S2–S4) serve the distal large intestine, bladder and reproductive organs.
The adrenal medulla is a special sympathetic “ganglion”: preganglionic fibers synapse directly on its cells, which secrete epinephrine (~80%) and norepinephrine into the blood — prolonging and spreading the fight-or-flight response.
3Effects on organs
| Target | Sympathetic | Parasympathetic |
|---|---|---|
| Heart | ↑ rate and force | ↓ rate |
| Bronchioles | Dilate | Constrict |
| Pupils (iris) | Dilate (mydriasis) | Constrict (miosis) |
| Digestive tract | ↓ motility and secretion; sphincters close | ↑ motility and secretion |
| Blood vessels (most) | Constrict (↑ BP); skeletal muscle vessels dilate | Little or no effect (no innervation) |
| Liver | Glycogen → glucose (↑ blood glucose) | — |
| Bladder | Relax wall (retain urine) | Contract wall (urination) |
| Sweat glands | ↑ sweating (cholinergic sympathetic fibers) | — |
| Adrenal medulla | Secretes epinephrine & NE | — |
| Penis / clitoris | Ejaculation | Erection |
| Metabolic rate | Increases | — |
4Neurotransmitters & receptors
| Receptor | Binds | Location / effect |
|---|---|---|
| Nicotinic (cholinergic) | ACh | All postganglionic neurons (sympathetic and parasympathetic), adrenal medulla, skeletal muscle (NMJ) — always stimulatory |
| Muscarinic (cholinergic) | ACh | All parasympathetic targets + sympathetic sweat glands — stimulatory or inhibitory (e.g. slows the heart) |
| α (alpha) adrenergic | NE, epinephrine | Mostly stimulatory — e.g. vasoconstriction |
| β₁ (beta-1) adrenergic | NE, epinephrine | Mainly the heart — increases rate and force |
| β₂ (beta-2) adrenergic | Epinephrine | Mostly inhibitory — relaxes smooth muscle: bronchodilation, dilation of vessels to the heart and skeletal muscle |
5Interactions & control
- Sympathetic tone (vasomotor tone)
- The sympathetic division keeps blood vessels partially constricted all the time; more firing → constriction (↑ BP), less → dilation.
- Parasympathetic tone
- Dominates the heart and digestive/urinary tracts at rest — slows the heart and keeps the gut working.
- Cooperative effects
- Both divisions work together in sexual function (parasympathetic: erection; sympathetic: ejaculation) and urination.
- Unique sympathetic roles
- Thermoregulation (skin vessels, sweating), release of renin by kidneys, metabolic effects (↑ glucose, ↑ metabolic rate).
Control hierarchy: the hypothalamus is the main integration center of the ANS (links emotions from the limbic system to visceral responses; controls heart activity, BP, body temperature, water balance). Brain stem reticular formation centers (medulla) directly regulate heart rate, BP, breathing and digestion. The spinal cord handles reflexes like defecation and urination. The cortex can influence the ANS (biofeedback, meditation).