Physiology chapters
Autonomic nervous system
How the sympathetic and parasympathetic systems act, their transmitters and receptors, how the hypothalamus and brainstem control them, the baroreceptor reflex, and what happens when autonomic control fails.
- Sympathetic prepares for "fight or flight"; parasympathetic for "rest and digest".
- All preganglionic fibres release acetylcholine on nicotinic receptors; parasympathetic postganglionic fibres use ACh on muscarinic receptors; most sympathetic postganglionic fibres use noradrenaline on α and β receptors.
- The baroreceptor reflex buffers sudden changes in blood pressure through the medulla.
On this page
Overview
The autonomic nervous system controls internal organs without conscious effort: heart rate, blood pressure, gut movement, bladder, sweating and the pupils.
It has two main divisions, which often have opposite effects:
- Sympathetic: "fight or flight". Prepares the body for stress and activity.
- Parasympathetic: "rest and digest". Conserves energy and promotes digestion.
For the anatomy of the pathways and ganglia, see the autonomic nervous system anatomy page.
Basic plan
Both divisions use two neurons: a preganglionic neuron in the brainstem or cord, and a postganglionic neuron in a ganglion.
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Origin | T1–L2 (thoracolumbar) | Cranial nerves III, VII, IX, X and S2–S4 (craniosacral) |
| Ganglia | Near the cord | Near or in the organ |
| Preganglionic transmitter | ACh (nicotinic) | ACh (nicotinic) |
| Postganglionic transmitter | Noradrenaline (mostly) | ACh (muscarinic) |
| Effect | Widespread, often all at once | More local and organ by organ |
Transmitters and receptors
Cholinergic receptors
| Receptor | Where |
|---|---|
| Nicotinic (NN) | All autonomic ganglia; adrenal medulla |
| Nicotinic (NM) | Neuromuscular junction of skeletal muscle (not autonomic) |
| Muscarinic | All parasympathetic targets; sweat glands (sympathetic cholinergic) |
Adrenergic receptors
| Receptor | Main effects |
|---|---|
| α1 | Constricts blood vessels (skin, gut, kidney); dilates the pupil (radial muscle); contracts sphincters of gut and bladder |
| α2 | On nerve endings: reduces noradrenaline release (negative feedback); reduces insulin secretion |
| β1 | Heart: increases rate, force and conduction; kidney: renin release |
| β2 | Bronchodilation; dilates muscle blood vessels; relaxes the uterus; glycogen breakdown in liver and muscle |
| β3 | Fat breakdown (lipolysis) |
β1 acts on the heart; β2 relaxes bronchi and vascular smooth muscle; α1 constricts vessels.
Sweat glands are supplied by sympathetic fibres but use acetylcholine on muscarinic receptors.
The adrenal medulla acts like a sympathetic ganglion: preganglionic fibres stimulate it to release adrenaline (about 80%) and noradrenaline into the blood.
Effects on organs
| Organ | Sympathetic | Parasympathetic |
|---|---|---|
| Eye: pupil | Dilates (α1) | Constricts |
| Eye: lens | Slight relaxation for far vision | Ciliary muscle contracts for near vision |
| Heart | Rate and force increase (β1) | Rate decreases (vagus); little effect on ventricular force |
| Bronchi | Dilate (β2) | Constrict; more mucus |
| Gut | Motility falls; sphincters contract | Motility and secretion rise; sphincters relax |
| Bladder | Detrusor relaxes (β), internal sphincter contracts (α1) | Detrusor contracts; emptying |
| Blood vessels | Most constrict (α1); muscle vessels may dilate (β2) | Few direct effects |
| Male sexual function | Ejaculation | Erection |
| Liver | Glucose release | - |
| Salivary glands | Thick, scanty saliva | Copious watery saliva |
The parasympathetic system has little control over most blood vessels. Blood pressure is controlled mainly by sympathetic tone.
Central control
- Hypothalamus: the main autonomic control centre. It controls body temperature, thirst, hunger and the stress response. The posterior hypothalamus drives sympathetic activity (heat conservation); the anterior hypothalamus drives heat loss and parasympathetic activity.
- Brainstem (medulla and pons): centres for blood pressure (vasomotor centre), heart rate, breathing, swallowing and vomiting.
- Limbic system and cortex: link emotions to autonomic responses (blushing, fainting at the sight of blood).
The baroreceptor reflex
This is the main short-term controller of blood pressure.
Receptors: stretch receptors in the carotid sinus (afferent via the glossopharyngeal nerve) and the aortic arch (afferent via the vagus).
When pressure rises:
- The arterial wall stretches; baroreceptors fire more.
- Signals reach the nucleus of the solitary tract in the medulla.
- The vasomotor centre is inhibited and vagal output increases.
- Heart rate and force fall; vessels dilate.
- Blood pressure falls back towards normal.
When pressure falls (for example on standing), the opposite happens: heart rate rises and vessels constrict.
Carotid sinus massage slows the heart by increasing vagal activity. It can stop some supraventricular tachycardias.
Baroreceptors reset within a day or two to a new pressure, so they control short-term changes, not long-term blood pressure (the kidney does that).
Other autonomic reflexes
- Pupillary light reflex: light → constriction (parasympathetic via III).
- Micturition reflex: bladder stretch → detrusor contraction (S2–S4).
- Defecation reflex.
- Bainbridge reflex: stretch of the atria increases heart rate.
Autonomic dysfunction
Orthostatic (postural) hypotension
How it happens: the baroreceptor reflex or sympathetic nerves fail, for example in diabetic autonomic neuropathy, Parkinson's disease, or with α-blocker drugs.
What you see: dizziness or fainting on standing, with a fall in blood pressure and no rise in heart rate.
Horner's syndrome
How it happens: damage to the sympathetic supply of the head.
What you see: ptosis, a small pupil and loss of sweating on that side of the face.
Denervation supersensitivity
After a nerve is cut, the organ becomes more sensitive to its transmitter because it makes more receptors.
Vasovagal syncope
How it happens: emotion or prolonged standing triggers a sudden rise in vagal activity and loss of sympathetic tone.
What you see: slow heart, low blood pressure, pallor and brief loss of consciousness.
Most questions ask which receptor mediates an effect (β1 heart, β2 bronchi, α1 vessels, muscarinic sweat glands), the transmitter at each synapse, the afferent nerves of the baroreceptors and what the reflex does when pressure rises or falls, and the causes of postural hypotension.
Quick revision
- Ganglia: nicotinic ACh. Parasympathetic targets: muscarinic ACh.
- Most sympathetic targets: noradrenaline; sweat glands: ACh.
- α1 vasoconstriction; α2 reduces transmitter release; β1 heart; β2 bronchi and muscle vessels.
- Adrenal medulla: mostly adrenaline.
- Baroreceptors: carotid sinus (IX), aortic arch (X); rise in BP → slower heart, vasodilation.
- Postural hypotension: autonomic failure.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
Which receptor is found in all autonomic ganglia?
The nicotinic acetylcholine receptor.
Which receptor causes bronchodilation?
The β2 adrenergic receptor.
What is the baroreceptor reflex?
A reflex in which stretch receptors in the carotid sinus and aortic arch detect changes in blood pressure and adjust heart rate and vessel tone to correct them.
Why do sweat glands respond to acetylcholine?
They are supplied by sympathetic postganglionic fibres that release acetylcholine on muscarinic receptors.